DOWNSHIFT TIMING AND ENGINE BRAKE CONTROL FOR AUTOMATIC TRANSMISSION

Abstract: An electro-hydraulic control for an automatic transmis­ sion for an automotive vehicle, comprises a 3-2 timing valve, an overrun clutch control valve, an electromagnetic valve under the control of a computer, and a shuttle valve. The shuttle valve has a spool movable responsive to a line pres­ sure variable with the engine throttle opening degree. The spool is movable between a first position where the overrun clutch control valve is under the control of the electro­ magnetic valve and a second position where the 3-2 timing valve is under the control of the electromagnetic valve.

Description: BACKGROUND OF THE INVENTION The present invention relates to a control for an automatic transmission for an automotive vehicle having an engine, and more particularly to a control for effecting downshift timing and engine brake. Commonly, automatic transmissions are automatically shiftable between different speed ratios by switching power transmission path through activation of selected one or ones of various friction elements (clutch, brake and one-way clutch). After the automatic transmission has effected a downshift, the engine speed is subject to a rapid change due to a change in speed ratio before and after the downshift. This rapid change in engine speed causes substantially great shocks during the downshift since this downshift is effected with heavy engine load (i.e., with accelerator pedal deeply depressed). The automatic transmission uses a one-way clutch as a reaction element against a rotary member and establishes the power transmission path when the one-way clutch is activated. The one-way clutch is released to interrupt the transmission of reverse torque which the rotary member is subject to, thus preventing the occurrence of shcoks and noises that go bump. However, this means that the one-way clutch will not provide a reaction against the rotary member even if engine brake is demanded, thus failing toeffect engine brake when desired. In order to deal with the former problem, there is effected a timing control such as to delay the commencement of a downshift operation until the engine speed reaches a value required for preventing the occurrence of substantial shocks during the shifting operation, providing a so-called neutral interval. In order to deal with the latter problem, there is provided an engine brake effecting friction element as arranged in parallel to the above mentioned one-way clutch and engine brake results as desired whenever the engine brake effecting friction element is activated. According to the state of the art, as implemented in a 3-2 downshift control and an overrun clutch activation control employed by a THM-700 type automatic transmission manufactured by General Motors in the United States or as implemented in a 3-2 downshift control and a low & reverse brake activation control employed by a RN4F02A type (or RL4F02A type) automatic transaxle manufactured by Nissan Motor Company in Japan, the former control is effected by a governor pressure variable corresponding to the vehicle speed and the latter control is effected by switching a manual selector valve. However, according to the teaching of the above mentioned control arrangements, a governor pressure is still needed for effecting the timing control of the above mentioned downshift operation even though the governor pressure is no longer necessary for causing the automatic shift in the transmission as aresult of realizing electronics control for one of purposes of eliminating the use of a governor pressure valve designed to produce governor pressure. Since it is initiated on switching the manual selector valve, the engine brake control has little design freedom in implementing it in electronics control. Upon implementing these two controls electronically, therefore it is necessary to provide one solenoid which is operative responsive to vehicle speed information in order to produce a governor pressure that is used for the downshift control, and it is also necessary to provide another solenoid which is operative responsive to switching the manual selector valve in order to produce an engine brake command pressure that is used for the engine brake control. However, the use of two solenoids for the respective controls is disadvantageous in view of space and cost necessary for their installation. From GB-A 2 108 604 a control for an automatic transmission is known, comprising a shuttle valve which cooperates with a timing circuit in order to establish a servo release of a brake. In dependence of the throttle opening a governor pressure acts against a line pressure in order to select the appropriate orifices of said shuttle valve. From US-A 4 485 695 it is known to control an overrun clutch with a solenoid. Also from this document it is known to control two different valves at the same time with a solenoid. The present invention is based on the recognition of the following facts: (a) that an engine brake effecting friction element is arranged in parallel to a one-way clutch as previously mentioned and it does not contribute to power transmission during power-on running, so that the engine brake effecting friction element may be held deactivated during power-on running; (b) that the timing control of downshift is required during downshift with heavy engine load; (c) that of said both controls the engine brake control is needed during coasting of the vehicle, and the downshift timing control is needed during power-on running, that is, both of these controls have different operation ranges where they are required to be effected and thus may be carried out by the common electromagnetic valve. SUMMARY OF THE INVENTION According to the present invention, there is provided a control for an automatic transmission for an automotive vehicle having an engine, the transmission being adapted for shifting between different speed ratios and having a plurality of friction elements to effect shifting between speed ratios, the plurality of friction elements including an engine brake effecting friction element, the control comprising: a fluid source; means communicating with said fluid source for generating a line pressure which is variable with opening degree of a throttle of the engine; a downshift timing valve; a control valve for the engine brake effecting friction element; electromagnetic means for generating a control pressure; a shuttle valve communicating with said downshift timing valve, said control valve, and said electromagnetic means, said shuttle valve having valve spool movable responsive to said line pressure between a first position where said control valve is under the control of said control pressure and said downshift timing valve is not under the control of said control pressure, and a second position where said control valve is not under the control of said control pressure and said downshift timing valve is under the control of said control pressure. Another aspect of the present invention resides in the shuttle valve including means for causing said valve spool to stay in said second position thereof when said line pressure is higher than a first preset value, but causing said spool to stay in said first position thereof when said line pressure is lower than a second preset value, said first preset value being higher than said second preset value. Still another aspect of the present resides in the shuttle valve which has the spool thereof locked to the first position thereof in response to a predetermined speed range pressure. Still another aspect of the present invention resides in the electromagnetic means which switches the control pressure to a predetermined value when vehicle speed of the automotive vehicle and opening degree of the throttle satisfy a predetermined condition. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A, 1B, and 1C, when combined side by side, illustrate a electro-hydraulic control system for an automatic transmission for an automotive vehicle embodying the present invention; Fig. 2 is a schematic view showing the power train of the automatic transmission with an engine and a road wheel or the automotive vehicle; Fig. 3 is a schematic sectional view of a band brake; Fig. 4 is a simplified schematic view of part of a control system for an automatic transmission embodying a main feature of the present invention; Fig. 5 is a similar view to Fig. 4 embodying another feature of the present invention; Fig. 6 is a similar view to Fig. 4 embodying still another feature of the present invention; Fig. 7 is a simialr view to Fig. 4 embodying a further feature of the present invention; Fig. 8 is a flow chart of a control program; and Fig. 9 is a operation pattern of a coil of an electromagnetic valve. DETAILED DESCRIPTION OF THE INVENTION Referring to Figs. 1A, 1B, 1C, 2, and 3, there is described a preferred embodiment according to the present which incoporates various features which will be described later in connection with Figs. 4 to 9. Referring to Fig. 2, the power train is shown as being connected to an engine with a thottle valve which opens in degrees and load wheels although only one being diagrammatically shown. The power train is driven by the engine output shaft 1. To transmit engine torque to an input or turbine shaft 2, a torque converter 3 has a pump impeller connected to engine output shaft 1, and a turbine runner 3T connected to input shaft 2. The torque converter 3 also includes a stator 36. Pump imepeller 3P is connected to an oil pump O/P to drive the same. The power transmission further comprises a first planetary gear set 4, a second planetary gear set 5, an output shaft 6, and a various kinds of frictional elements which will be described later. Toqrue converter 3 is of a so-called lock-up torque converter including a lock-up clutch 3L which when engaged, mechanically connects the pump impeller 3P with the turbine runner 3T, thus eliminating slip taking place between them. Lock-up clutch 3L assumes a disengaged position when working fluid is supplied to a release chamber 3R and then discharged from an apply chamber 3A. Lock-up clutch 3L is disengaged when working fluid is supplied to the apply chamber 3A and then discharged from the release chamber 3R. First planetary gear set 4 is of a simple planetary gear set which comprises a sun gear 4S, a ring gear 4R, pinions 4P meshing with them, and a carrier 4P rotatably carrying the pinions 4P. Second planetary gear set 5 is of a simple planetary gear set which comprises a sun gear 5S, a ring gear 5R, pinions 5P meshing with them, and a carrier 5C rotatably carrying the pinions 5P. Carrier 4C is connectable with input shaft 2 via a high clutch H/C, and sun gear 4S is connectable with input shaft 2 via a reverse clutch R/C. Sun gear 4S is adpated to be anchored by a band brake B/B. Carrier 4C is adapted to be anchored by a low & reverse brake LR/B and its reverse rotation is prevented by a low one-way clutch LO/C. Ring gear 4R is connected integrally with carrier 5C which is drivingly connected to output shaft 6. Sun gear 5S is connected with input shaft 2. Ring gear 5R is connectable with carrier 4C via an overrun clutch OR/C. To establish a predetermined drive relation, a forward one-way clutch FO/C and a forward clutch F/C are arranged between the carrier 4C and the ring gear 5R. Engagement of the forward clutch F/C causes the forward one-way clutch FO/C to connect the ring gear 5R with the carrier 4C in the reverse rotational direction. High clutch H/C. reverse clutch R/C, low & reverse brake LR/B, overrun clutch OR/C, and forward clutch F/C are activated, i.e., engaged or applied, when supplied with hydraulic pressure. Band brake B/B is constructed as shown in Fig. 3. A housing 10 is formed with a stepped bore including three different diameter bore sections which are connected one after another as shown in Fig. 3. A stepped piston 8 is received in different diameter bore sections having the largest diameter and a diameter smaller than the largest diameter but still larger than the smallest. The stepped piston 8 defines on one side of its larger diameter piston section a second speed servo apply chamber 2S/A and on the opposite side thereof a third speed servo release chamber3S/R. Received in the bore section having the smallest diameter is a piston 9. The piston 9 defines on one side thereof a fourth speed servo apply chamber 4S/A and on the opposite side thereof where it faces the reduced diameter piston section of the stepped piston 8 a drain chamber. The piston 9 has a plunger slidably extending through the stepped piston 8, the plunger of the piston 9 serving as means for defining a passage adapted to deliver fourth speed pressure P4 to fourth speed servo apply chamber 4S/A. The plunger of piston 9 is enlarged in diameter at the remote end portion from the pistron to form a shoulder abutting with that side of the stepped piston 8 exposed to the three speed servo release chamber 3S/R. The plunger is anchored to a brake band 13. Provided within the drain chamber is a spring 11 and provided within the bore section having the largest diameter is a spring 12. Spring 11 has one end bearing against the stepped piston 8 and opposite end bearing against the piston 9 to separate them to have them assume the illustrated postion where the shoulder of the plunger is urged to abut with stepped piston 8. The spring 12 bears against the stepped piston 8 to urge the same in the release postion as illustrated in Fig. 3. With the above construction, when second speed pressure P2 is supplied to the second servo apply chamber 2S/A, the stepped piston 8 together with the piston 9 are urged to move to the right as viewed in Fig. 3 and tightens the brake band 13 to apply the band brake B/B. In this state, supplying third speed pressure to the third speed servo release chamber 36/R causes the stepped piston 8 to move to the right as viewed in Fig. 3 due to a difference in pressure acting area on theopposite sides of the stepped piston 8, thus releasing brake band 13 to release the application of band brake B/B. In this state, when fourth speed pressure P4 is supplied to the fourth speed servo apply chamber 4S/R, the piston 9 is urged to move to the left independent of the stepped piston 8, tightening the brake band 13 to apply the band brake B/B. The power train shown in Fig. 2 is shiftable into one of first, second, third, and fourth speed ratio during forward travel or into reverse when selected one or ones of friction elements B/B, H/C, F/C, OR/C, LR/B, and R/C are activated in accordance with a pattern as shown in TABLE 1 in combination with activation of friction elements FO/C and LO/C. In TABLE 1, a selected one or ones of servo chambers 2S/A, 3S/R, and 4S/A assigned with the reference character "o" are supplied with hydraulic pressure, a selected one or ones of frictional elements H/C, R/C, LR/B, and RC assigned with the reference character "o" are supplied with hydraulic pressure and activated. The reference character "x" denotes one or ones of friction elements OR/C and LR/B which are to be activated to effect engine brake running in a particular one of the forward speed ratios. When the overrun clutch OR/C is activated, forward one-way clutch FO/C arranged in parallel with the same is not in operation, while when the low & reverse brake LR/B is activated, low one-way clutch LO/C arranged in parallel with the same is not in operation. Referring to Figs. 1A, 1B and 1C, a pressure regulator valve 20 shown in Fig. 1A comprises a spool 20a biased by a spring 20a to a spring set position illustrated by the left half thereof as viewed in Fig. 1A, and a plug 20c. Pressure regulator valve 20 regulates hydraulic fluid discharged to a circuit 71 by an oil pump O/P to generate a hydraulic fluid pressure that is determined by the spring force of spring 20a and the force biased by plug 20c to spool 20b. This hydraulic pressure resulting from the pressure regulation is used as a servo actuating fluid pressure and called as "line pressure." The hydraulic fluid pressure building up in circuit 71 acts via a damping orifice 72 on a pressure acting area 20d of spool 20b to urge the same to move downwards as viewed in Fig. 1A against spring 20a. Pressure regulator valve 20 is formed with four ports 20e, 20f, 20g, and 20h which are arranged such that they are opned or closed in response to which position spool 20b assumes during its stroke. Port 20e is connected to circuit 71 and it starts to communicate with ports 20h and 20f and increase is communication as spool 20b moves donwards from the spring set position as illustrated by the left half thereof in Fig. 1A. Port 20f decreases its communication with port 20g serving as a drain port as spool 20b moves downwards from the illustrated spring set position, and it starts to communicate with port 20e when it has interrupted its communication with drain port 20g. Port 20f is connected to a volume control actuator 75 for oil pump O/P via a circuit 74 provided with a bleeder orifice 73. Oil pump O/P is of a variable volume vane pump driven by the engine and its volume decreases to become small when the pressure supplied to volume control actuator 75 exceeds a predetermined value. Plug 20c of pressure regulator valve 20 has a bottom end serving as a pressure acting area which receives a modifier pressure from a circuit 76, and has a pressure acting area 20i a reverse select pressure from a circuit 77. Thus, plug 22c applies to spool 20b an upward force resulting from forces created by reception of the pressure modifier pressure or the reverse select pressure. Initially, the pressure regulator valve 20 assumes the state as illustrated by the left half thereof as viewed in Fig. 1A. When the oil pump O/P starts discharging oil, the oil discharged flows into the circuit 71. When the spool 20b stays in the positon as illustrated by the left half thereof, the pressure rises within the circuit 71 because no oil is drained therefrom. This pressure is supplied via an orifice 72 to a pressure acting area 20d to act thereon, urging the spool 20b downwards against the spring 20a until the port 20e communicates with the port 20h. As a result, the above mentioned pressure is drained at the port 20h and thus drops, allowing the spool 20b for being pushed back by the spring 20a. Repeating these operations one after another causes the pressure regulator valve 20 to adjust the pressure within the circuit 71 (i.e., hereinafter called as a line pressure) to basically a value corresponding to the spring force of the spring 20a. The plug 20c, on the other hand, is subject to a modifier pressure acting upon the bottom end thereof and urges the spool 20h upwards after coming into abutting contact with the spool 20b as illustrated by the right half thereof as viewed in Fig, 1A in such a manner as to assist the action of the spring 20a. As will be later described, the modifier pressure appears when each of the drive ranges except the reverse drive range is selected and rises in proportion to the engine load (which corresponds to the engine output torque). Thus, the above mentioned line pressure varies such that it rises with an increase in the engine load. When the reverse drive range is selected, a reverse select pressure as high as the line pressure is supplied through a circuit 77 to the pressure regulator valve 20 to act on the plug 20c in a direction to urge the same upwards, urging the spool 20b in the direction assisting the spring force of the spring 20a after the plug 20c has been brought into abutting contact with the spool 20b. This causes the line pressure to be boosted upto a relatively high predetermined value desired for reverse drive. As the engine speed increases, the amount of oil discharged by the oil pump O/P increases. After the engine speed has exceeded a certain level, an overabundance amount of oil is discharged by the oil pump O/P, causing the pressure within the circuit 71 to rise above the value adjusted. This pressure rise causes the spool 20b to move downwards beyond the equilibrium position as illustrated by the right half thereof as viewed in Fig. 1A where the communication of the port 20f with the drain port 20g prevously established is blocked and the communication of the port 20f with the port 20h previously blocked is established, allowing the discharge of oil from the port 20e via the port 20f at a bleed 73 in a flow restricted manner, leaving a feedback pressure created within the circuit 74. This feedback pressure rises as the revolution speed of the oil pump O/P increases, causing the actuator 75 to bring down the eccentric amount (i.e., the pump capacity) of the oil pump O/P. As a result, the discharge amount of the oil pump O/P is kept constant while the revolution speed of the oil pump O/P is higher than the certain level, causing no power loss of the engine which would result if the oil pump were activated to discharge the overabundance amount of oil. The line pressure created within the circuit 71 in the above described manner is distributed through a line pressure circuit 78 to a pilot valve 26, a manual selector valve 36, an accumulator control valve 70 and a servo release pressure accumulator 66. The pilot valve 26 comprises a spool 26b biased by a spring 26a to a position as illustrated by the upper half thereof as viewed in Fig. 1A. The spool 26b has its remote end from the spring 26a exposed to a chamber 26c. The pilot valve 26 also includes a drain port 26d and is connected to a pilot pressure circuit 79 having therein a strainer S/T. The spool 26b is formed with a connecting passage 26e which allowing the transmission of the pressure from the pilot pressure circuit 79 to the chamber 26c. As the pressure within the chamber 26c rises, the spool 26b is urged for rightward movement as viewed in Fig. 1A. This rightward movement of the spool 26b allows the pilot pressure circuit 79 to switch its connection from the line pressure circuit 78 to the drain port 26d. With the pilot valve 26b held in the position as illustrated by the upper half thereof as viewed in Fig. 1A, supplying the line pressure from the circuit 78 to the pilot valve 26 causes a rise in pressure within the circuit 79. This rise in pressure is supplied via the connection passage 26e to the chamber 26c, causing the rightward movement of the spool 26b as viewed in Fig. 1A. This rightward movement of the spool 26b beyond the equilibrium state position as illustrated by the lower half thereof as viewed in Fig. 1A causes the circuit 79 to block its communication with the circuit 78 and at the same time open its communication with the drain port 26d. This results in a drop in pressure within the circuit 79, allowing the spool 26b to be pushed back by the spring 26a, causing the pressure within the circuit 79 to rise again. Thus, the pilot valve 26 reduces the line pressure from the circuit 78 down to a constant valve that is determined by the spring force of the spring 26a and outputs the results to the circuit 79 as the pilot pressure. This pilot pressure is supplied through the circuit 79 to the pressure modifier valve 22, duty solenoids 24, 34, a lock-up control valve 30, a forward clutch control valve 30, a forward clutch control valve 46, a shuttle valve 32, first, second and third shift solenoids 42, 44 and 60, and a shuttle valve 56. The duty solenoid 24 comprises a coil 24a, a spring 24d and a plunger 24b. When the coil 24a is turned ON (i.e., when electric current passes through the coil 24a), the plunger 24b is electromagnetically drawn against the spring 24d to an open position where a circuit 81 that is connected to the circuit 79 via an orifice 80 in allowed to communicate with a drain port 24c. Under the control of a computer, not illustrated, the coil 24a of the duty solenoid 24 is turned ON intermittently. The ratio of ON time to the period which in constant (i.e., duty cycle) is controlled, causing pressure within the circuit 81 to vary in dependence on the duty cycle. The duty cycle ranges from 0 % to 100 % such that during operation with each of the drive ranges except the reverse drive range being selected, the duty cycle is decreased as the engine load (for example, the engine throttle opening degree) increases, and the duty cycle is set as 100 % when the reverse drive range is selected. When the duty cycle is set as 100 %, the control pressure within the circuit 81 is zero, whereas when it is set as 0 %, the control pressure takes the maximum value. The pressure modifier valve 22 comprises a spring 22a and a spool 22b which is biased downwards as viewed in Fig. 1A by means of the spring force of the spring 22a and a control pressure from the circuit 81. The pressure modifier valve 22 is provided with an outlet port 22c connected to the circuit 76, an inlet port 22d connected to the pilot pressure port 79, and a drain port 22e. The spool 22b has its remote end from the spring 22a exposed to a chamber 22f which the circuit 76 is connected to. The arrangement is such that upon the spool assuming a position as illustrated by the left half thereof as viewed in Fig. 1A, the port 22c becomes out of communication with the ports 22d and 22e. In the pressure modifier valve 22, the spool 22b is urged downwards, as viewed in Fig. 1A, upon being subject to the spring force by the spring 22a and the control pressure supplied thereto from the circuit 81, whereas it is urged upwards upon being subject to the output pressure from the outlet port 22c. The spool 22b assumes the eqilibrium position where the forces acting on the spool 22b balance. If the upward force due to the output pressure acting on the spool 22b is insufficient for opposing to the downward forces due to the spring force and the control pressure acting on the spool 22b, the spool 22b tends to move downwards beyond the equilibrium position as illustrated by the left half thereof as viewed in Fig. 1A. This downward movement of the spool 22b uncovers the port 22d to allow it to communicate with the port 22c, allowing the supply of pilot pressure from the circuit 79 to the port 22c, thus causing a rise in the output pressure. If the upward force due to the output pressure acting on the spool 22b is excessively large for opposing to the downward forces, the spool 22b moves upwards toward a position as illustrated by the right half thereof asviewed in Fig. 1A. In this position of the spool 22b, the port 22c is allowed to communicate with the drain port 22e, causing a drop in the output pressure. After repeating these operations, the pressure modifier valve 22 adjusts the output pressure from the port 22c to a value corresponding to the sum of the spring force of the spring 22a and the force due to the contro, pressure from the circuit 81, and supplies the output pressure, as a modifier pressure, through the circuit 76 to the plug 20c of the pressure regulator valve 20. With this modifier pressure resulting from amplification of the control pressure by the spring force of the spring 22a, the line pressure is controlled such that it rises as the engine load increases during operation with each of the drive ranges except the reverse drive range being selected because the control pressure rises as the engine load increases although it is zero during operation with the reverse drive selected. The torque converter regulator valve 28 comprises a spring 28a, and a spool 28b biased by the spring to a position as illustrated by the right half thereof as viewed in Fig. 1A. It is formed with ports 28c and 28d which are kept communicating with each other during the stroke of the spool 28b between the position an illustrated by the left half thereof and the position as illustrated by the right half thereof as viewed in Fig. 1A. As it moves upwards from the position as illustrated by the left half thereof as viewed in Fig. 1A, the spool 28b begins covering the port 28d and uncovering the port 28e, reducing the degree of the communication of the port 28c with the port 28d and increasing the degree of communication of the port 28c with the port 28e. In order to control the stroke of the spool, the spool 28b has its remote end from the spring 28a exposed to a chamber 28f and it has formed therethrough a connecting passage with which the chamber 28f communicates always with the port 28c. The port 28c is connected to predetermined portions to be lubricated via a relief valve 82 and also to the lock-up control valve 30 via a circuit 83. The port 28d is connected via a circuit 84 to the pressure regulator valve 20 at the port 20h. The port 28e is also connected to the lock-up control valve 30 via a circuit 85. This circuit 85 is provided with an orifice 86 and it has a portion thereof between the orifice 86 and the port 28e connected to the circuit 83 via an orifice 87 and also to an oil cooler 89 and parts 90 to be lubricated via a circuit 88. Initially, the torque converter regulator valve 28 assumes the position as illustrated by the right half thereof as viewed in Fig. 1A. Under this condition, oil supplied thereto from the port 20h of the pressure regulator valve 20 via the circuit 84 is allowed to pass through the circuit 83 to the torque converter 3 in the manner described later, causing a torque converter supply pressure to build up at the port 28d. This torque converter supply pressure is supplied to the chamber 28f via the connection passage 28g, urging the spool 28b upwards, as viewed in Fig. 1A against the spring 28a. When the spool 28b moves upwards beyond the position as illustrated by the left half thereof as viewed in Fig. 1A, it begins to uncover the port 28e, allowing the discharge of oil through the port 28e and circuit 88. This causes the torque converter supply pressure to be adjusted to a value determined by the spring force of the spring 28a. The oil discharged from the circuit 88 is directed toward the lubrication parts 90 after being subject to cooling by the oil cooler 89. If the torque converter supply pressure exceeds the above mentioned value as a result of the above mentioned pressure regulation, the relief valve 82 opens to relieve the excessive pressure toward the lubrication parts so as to avoid deformation of the torque converter 3. The lock-up control valve 30 comprises a spool 30a and a plug 30b which are arranged along an axis. When the spool 30a assumes a position as illustrated by the right half thereof as viewed in Fig. 1A, the circuit 83 is allowed to communicate with a circuit 91 leading to a torque converter release chamber 3R, while when it moves downward to a position as illustrated by the left half thereof as viewed in Fig. 1A, the circuit 83 is now allowed to communicate with the circuit 85. When the spool 30a moves donwards beyond the position as illustrated by the left half thereof as viewed in Fig. 1A, the circuit 91 is allowed to communicate with a drain port 30c. In order to control the stroke of the spool 30a, the spool 30a has its remote end from the plug 30b exposed to a chamber 30d, and the plug 30b has its remote end from the spool 30a exposed to a chamber 30e which the pressure within the circuit 91 is supplied thereto via an orifice 92. There is provided a circuit 91 extending from a torque converter apply chamber 3A which is connected to the circuit 85 at a portion between the orifice 86 and the lock-up control valve 30. The plug 30b is subject to a downward force resulting from the pilot pressure acting thereon from the circuit 79 via an orifice 94, preventing pulsation of the spool 30a. In the lock-up control valve 30, the stroke of the spool 30a is controlled by pressure supplied to a chamber 30d. When this pressure is sufficiently high, the spool 30a assumes a position as illustrated by the right half thereof as viewed in Fig. 1A. In the position of the spool 30a, the oil from the circuit 83 which is under pressure regulation by the torque converter regulator valve 28 passes through the cicuit 91, release chamber 3R, apply chamber 3A, circuit 93 and circuit 85 to the circuit 88 where it is discharged. Under this condition, the torque converter 3 performs power transmission in its converter state. As the pressure within the chamber 30d drops, the spool 30a is urged for downward movement, as viewed in Fig. 1A, by means of the plug 30b due to pressure acting thereon via the orifices 92 and 94. When, during this downrard movement, the spool 30a moves downward beyond the position as illustrated by the left half thereof as viewed in Fig. 1A, the oil under regulation from the circuit 83 pauses through the circuits 85, 93, apply chamber 3A, release chamber 3R and circuit 91 to the drain port 30c Thus, the torque converter performs the power transmission in a slip state with a rate controlled in proportion to a drop in the pressure witrhin the chamber 30d of the torque converter 3. Further drop in the pressure within the chamber 30d causes further downward movement of the spool 30a beyond this state. This causes the circuit 91 to fully communicate with the drain port 30c, bringing down the pressure within the release chamber 3R to zero, allowing the torque converter 3 to perform the power transmission in a lock-up state. The shuttle value 32 is designed to effect stroke control of the lock-up control valve 30 as well as that of the forward clutch control valve 46 later described. It includes a spool 32b biased by a spring 32a to a position as illustrated by the lower half thereof as viewed in Fig. 1A. This spool 32b is movable to a position as illustrated by the upper half thereof as viewed in Fig. 1A in response to pressure within a chamber 32c. When the spool 32b assumes the position as illustrated by the lower half thereof as viewed in Fig. 1A, the shuttle valve 32 allows the circuit 95 from the chamber 30d to communicate with the pilot pressure circuit 79 and a circuit 96 from a chamber 46a of the forward clutch control valve 46 to communicate with a circuit 97 from a duty solenoid 34. When the spool 32b moves to the position as illustrated by the upper half thereof, the shuttle valve 32 allows the circuit 95 to communicate with the circuit 97 and the circuit 96 to communicate with the circuit 79. The duty solenoid 34 comprises a coil 34a, a spring 34d and a plunger 34b biased to a close position by the spring 34d. When the coil 34a is turned ON (i.e., when electric current passes through the coil 24a), the plunger 34b is electromagnetically drawn against the spring 34d to an open position where a circuit 79 that is connected to the circuit 79 via an orifice 98 is allowed to communicate with a drain port 34c. Under the control of a computer, not illustrated, the coil 34a of the duty solenoid 34 is turned ON intermittently. The ratio of ON time to the period which is constant (i.e., duty cycle) is controlled, causing pressure within the circuit 97 to vary in dependence on the duty cycle. In the case where the shuttle valve 32 assumes the position as illustrated by the upper half thereof and the control pressure within the circuit 97 is used to effect the stroke control of the lock-up control valve 30, the duty cycle of the solenoid 34 is determined as follows. That is, the duty cycle should be 0 % to allows the control pressure within the circuit 97 to increase as high as the pilot pressure within the cicuit 79 when the engine operates with heavy load at low speeds where the torque multiplying function of the torque converter 3 and the torque variation absorbing function thereof are required. Under this condition, the control pressure supplied to the chamber 30d urges the spool 30a to the position as illustrated by the right half thereof, rendering the torque converter 3 to operate in the torque converter state as desired. As the requirement degree of the above mentioned two functions imposed on the torque converter 3 decreases, the duty cycle should be increased to bring down the control pressure, rendering the torque converter 3 to operate in the desired slip state. When the engine operates with light load at high speeds where the above mentioned functions of the torque converter are not required, the duty cycle should be 100 % to bring down the control pressure to zero, allowing the torque converter 3 to operate in the lock-up state. In the case where the shuttle valve 32 assumes the position as illustrated by the lower half thereof as viewed in Fig. 1A and the control pressure within the circuit 97 is used to effect the stroke control of the forward clutch control valve 46, the duty cyle of the solenoid 34 is determined in such a manner later described that N-D select shock is alleviated and creep is prevented. The manual selector valve 36 comprises a spool 36a which is movable depending on manual select operation of a driver to a park (P) range, a reverse (R) range, a neutral (N) range, a forward automatic drive (D) range, a forward second speed engine brake (II) range, a forward first speed ratio engine brake (I) range. Selecting one of the above mentioned ranges causes the line pressure line pressure circuit 78 to communicate the corresponding one of the output ports 36 D, 36 II, 36 I, and 36 R in accordance with the pattern shown by the following table. In the above TABLE, the reference character "o" denotes the particular port which communicates with the line pressure circuit 78, while the other ports which are not denoted by this reference character are drained. The first shift valve 38 comprises a spring 38a and a spool 38b biased by the spring 38a to a position as illustrated by the left half thereof as viewed in Fig. 1B. This spool 38b assumes a position as illustrated by the right half thereof as viewed in Fig. 1A when a pressure is supplied to a chamber 38c. When the spool 38b assumes the position as illustrated by the left half thereof as viewed in Fig. 1B, the first shift valve 38 allows a port 38d to communicate with a drain port 38e, a port 38f to communicate with a port 38g, and a port 38h to communicate with a port 38i. When the spool 38b assumes the position as illustrated by the right half thereof as viewed in Fig. 1B, the first shift valve 38 allows the port 38d to communicate with a port 38j, the port 38f to communicate with a port 38k, and the port 38h to communicate with a port 38l. The second shift valve 40 comprises a spring 40a and a spool 40b biased by the spring 40a to a position as illustrated by the left half thereof as viewed in Fig. 1B. This spool 38b assumes a position as illustrated by the right half thereof an viewed in Fig. 1A when a pressure is supplied to a chamber 40c. When the spool 40b assumes the position as illustrated by the left half thereof as viewed in Fig. 1B, the second shift valve 40 allows a port 40d to communicate with a drain port 40e, a port 40f to communicate with a port 40g, and a port 40h to communicate with a drain port 40i via an orifice. When the spool 40b assumes the position as illustrated by the right half thereof as viewed in Fig. 1B, the second shift valve 40 allows the port 40d to communicate with the port 40j, the port 40f to communicate with the port 40e, and the port 40h to communicate with a port 40k. The spool positions of the first and second shift valves 38 and 40 are controlled by a first shift solenoid 42 and a second shift solenoid 44, respectively. These shift solenoids comprise coils 42a and 44a, respectively, plungers 42b and 44b, respectively, and springs 42d and 44d, respectively. The first shift solenoid 42 is connected to the pilot pressure circuit 79 via an orifice 99, and it blocks communication between a circuit 100 leading to a chamber 38c and a drain port 42c when the coil 42a is turned ON (i.e., when the current passes through the coil 42a), allowing a control pressure within the circuit 100 to increase as high as the pilot pressure, urging the first shift valve 38 to move to the position as illustrated by the right half thereof as viewed in Fig. 1B. The second shift solenoid 44 is connected to the pilot pressure circuit 79 via an orifice 101, and it blocks communication between a circuit 102 leading to a chamber 40c and a drain port 44c when the coil 44a is turned ON (i.e., when the current passes through the coil 44a), allowing a control pressure within the circuit 102 to increase as high as the pilot pressure, urging the second shift valve 40 to move to the position as illustrated by the right half thereof as viewed in Fig. 1B. The first to fourth speed ratios are established depending on various combinations of ON and OFF of the shift solenoids, i.e., various combinations of upshift and downshift positions of the shift valves 38 and 40, in accordance with the pattern shown in the following TABLE 3. In the above TABLE, the reference character "o" represents the state where the shift valve is in the position as illustrated by the right half thereof as viewed in Fig. 1B, while the reference character "x" represents the state where the shift valve is in the position as illustrated by the left half thereof as viewed in Fig. 1B. The ON and OFF of the shift solenoids 42 and 44 are determined by the computer, not illustrated, versus vehicle speed and engine load in accordance with a predetermined shift pattern so as to establish an appropriate speed ratio for the vehicle speed and the engine load. The forward clutch control valve 46 comprises a spool 46b. For preventing the pulsation of the spool 46b, the pilot pressure supplied to the forward clutch control valve 46 from the circuit 79 via an orifice 103 acts on the spool 46b in a donward direction as viewed in Fig. 1A. The spool 46b ia subhect to another downward force, too, created by the actuating pressure for the forward clutch F/C supplied, as a feedback pressure, to the clutch control valve 46 from a circuit 105 via an orifice 104 to act on the spool 46b. The spool 46b will move to a position where the above mentioined downward forces balance with an upward force due to the pressure within the chamber 46a. When the spool 46b assumes the position as illustrated by the right half thereof as viewed in Fig 1A, the circuit 105 is allowed to communicate with a drain port 46c, whereas when it assumes the position as illustrated by the left half thereof, the circuit 105 is allowed to communicate with a circuit 106. The circuit 105 is provided with a one-way orifice 107 which has a throttle effect only upon the hydraulic pressure directed toward the forward clutch F/C. The circuit 106 is connected to the port 36 D of the manual selector valve 36. The 3-2 timing valve 48 comprises a spring 48a and a spool 48b biased toward a position as illustrated by the left half thereof as viewed in Fig. 1C where a port 48c is allowed to communicate with a port 48d having an orifice 48f. When a pressure within a chamber 48e is high enough to urge the spool 48b to the position as illustrated by the right half thereof as viewed in Fig. 1C, the communication between the ports 48c and 48d is blocked. The 4-2 relay valve 50 comprises a spring 50a and a spool 50b biased by the spring 50a to a position as illustrated by the left half thereof as viewed in Fig. 1B where a port 50c is allowed to communicate with a drain port50d having an orifice therein. When a pressure within a chamber 50e is high enough to urge the spool 50b to a position as illustrated by the right half thereof as viewed in Fig. 1B, the port 50c is allowed to communicate with a port 50f. The 4-2 sequence valve 52 comprises a spring 52a and a spool 52b biased by the spring 52a to a position as illustrated by the right half thereof as viewed in Fig. 1B where a port 52c is allowed to communicate with a drain port 52d having therein an orifice. When a pressure within a chamber 52e is high enough to urge the spool 52b to a position as illustrated by the left half thereof as viewed in Fig. 1B, the port 52c is allowed to communicate with a port 52f. The I range pressure reduction valve 54 comprises a spring 54a and a spool 54b biased by the spring 54a to a position as illustrated by the right half thereof as viewed in Fig. 1C. It is formed with ports 54c and 54d which are allowed to communicate with each other when the spool 54b assume the position as illustrated by the right half thereof. It is also formed with a drain port 54e which begins to communicate with the port 54c when the spool 54b has moved upwards beyond a position as illustrated by the left half thereof as viewed in Fig. 1C where the spool 54d completely covers the port 54d. The spool 54b has its remote end from the spring 54a exposed to a chamber 54f connected to the port 54c via an orifice 108. Initially, the I range pressure reduction valve 54 assumes the position as illustrated by the right half thereof as viewed in Fig. 1C where supplying the port 54d with a pressure causes the pressure to appear, as an output pressure, at the port 54c. This output pressure acts via the orifice 108 upon the bottom end, as viewed in Fig. 1C of the spool 54b, urging the spool 54b for upward movement as the output pressure rises. When, during this upward movement, the spool 54b moves upwards beyond the position as illustrated by the left half thereof as viewed in Fig. 1C, the fort 54c begins to communicate with the drain port 54e, causing a drop in the output pressure from the port 54c. This pressure drop causes downward movement of the spool 54b. When the spool moves downwards beyond the position as illustrated by the left half thereof as viewed in Fig. 1C, the port 54c begins to communicate with the port 54d, causing an increase in the outoput pressure from the port 54c. Repeating thses operations results in providing the output pressure from the port 54c having a constant value that is determined by the spring force of the spring 54a. The shuttle valve 56 comprises a spring 56a and a spool 56b biased by the spring 56a to a position as illustrated by the left half thereof as viewed in Fig. 1C. The spool 56b keeps on staying in this position as long as a chamber 56g is supplied with a pressure. Under a condition where there is no supply of pressure to the chamber 56g, when an upward force due to the pressure from the port 56c and applied to the spool 56b is higher than a predetermined value, the spool 56b is urged to move upwards to a position as illustrated by the right half thereof as viewed in Fig. 1C. In the position as illustrated by the left half thereof as viewed in Fig. 1C, a port 56d is allowed to communicate with a circuit 109 from a third shift solenoid 60, and a port 56e is allowed to communicate with a drain port 56f. In the position as illustrated by the right half thereof as viewed in Fig. 1C, the port 56d is allowed to communicate with the pilot pressure circuit 79 and the port 56e is allowed to communicate with the circuit 109. The third shift solenoid 60 comprises a coil 60a, a plunger 60b and a spring 60d. When the coil 60a is turned ON (i.e., when the current passes through the coil 60a), the plunger 60b is urged against the spring 60d to assume a position where the circuit 109 connected via an orifice 110 to the pilot pressure circuit 79 is prevented from communicating with a drain port 60c, causing a control pressure within the circuit 109 to increase as high as the pilot pressure. ON and OFF of the third shift solenoid 60 are determined by the computer, not illustrated, in a manner later described. The overrun clutch control valve 58 comprises a spring 58a and a spool 58b biased by the spring 58a to a position as illustrated by the left half thereof as viewed in Fig. 1C. This spool 58b moves to a position as illustrated by the right half thereof as viewed in Fig. 1C when a chamber 58c is supplied with a pressure. In the position as illustrated by the left half thereof as viewed in Fig. 1C, the spool 58b allows a port 58d to communicate with a drain port 58e, and a port 58f to communicate with a port 58g. In the position as illustrated by the right half thereof as viewed in Fig. 1C, the spool 58b allows the port 58d to communicate with a port 58h and the port 58f to communicate with the drain port 58e. The overrun clutch pressure reduction valve 62 comprises a spring 62a and a spool 62b biased by the spring 62a to a position as illustrated by the left half thereof as viewed in Fig. 1C. This spool 62b is held in this position under a downward force, as viewed in Fig. 1C, created when a port 62c is supplied with a pressure. Under a condition where there is no pressure inflow from a port 62c, supplying a port 62d with hydraulic pressure causes an increase in output pressure from a port 62e. This output pressure is fed back to a chamber 62f, urging the spool 62b for upward movement as viewed in Fig. 1C. When the output pressure attains a value corresponding to the spring force of the spring 62a, the spool 62b assumes a position as illustrated by the right half thereof as viewed in Fig. 1C where the communication between the ports 62d and 62e is blocked. As a result, the overrun clutch reduction valve 62 effects pressure reduction to adjust the output pressure from the port 62e at a constant value that is determined by the spring force of the spring 62a. The second speed ratio servo apply pressure accumulator 64 comprises a stepped piston 64a biased by a spring 64b to a position as illustrated by the left half thereof as viewed in Fig. 1B. The stepped piston 64a has a shoulder thereof exposed to a chamber 64c open to the ambient atmosphere. It has a small diameter piston end and a large diameter piston end exposed to sealed chambers 64d and 64e, respectively. The third speed ratio servo release pressure accumulator 66 comprises a stepped piston 66a biased by a spring 66b to a position as illustrated by the left half thereof as viewed in Fig. 1C. The stepped piston 66a has a shoulder exposed to a chamber 66c which the line pressure circuit 78 is connected to. It has a small diameter piston end a large diameter piston end exposed to sealed chambers 66d and 66e, respectively. The fourth speed ratio servo apply pressure accumulator 68 comprises a stepped piston 68a biased by a spring 68b to a position as illustrated by the left half thereof as viewed in Fig. 1C. The stepped piston 68a has a shoulder exposed to a sealed chamber 68c. It has a small diameter piston end and a large diameter piston end exposed to sealed chambers 68d and 68e, respectively. The accumulator control valve 70 comprises a spring 70a and a spool 70b biased by the spring 70a to a position as illustrated by the left half thereof as viewed in Fig. 1A. The spool 70b has its remote end from the spring 70a exposed to a chamber 70c which the control pressure is supplied to from the circuit 81. In its position as illustrated by the left half thereof as viewed in Fig. 1A, the spool 70b allows an outlet port 70d to communicate with a drain port 70e. When the spool 70b moves upwards to a position as illustrated by the right half thereof as viewed in Fig. 1A in response to a rise in the control pressure supplied to the chamber 70c, the spool 70b allows the outlet port 70d to communicate with the line pressure circuit 78. The outlet port 70d is connected to the chambers 64d and 68c of the accumulators 64 and 68, respectively, via a circuit 111, and it is also connected to a chamber 70f receiving therein the spring 70a. The spool 70b of the accumulator control valve 70 is urged to move upwards, as viewed in Fig. 1A, further beyond a position as illustrated by the right half thereof as viewed in Fig. 1B in response to the control pressure supplied to the chamber 70c upon selecting one of the drive ranges excluding the reverse drive. In this position, the line pressure from the circuit 78 is allowed to output to the circuit 111. When the pressure within the circuit 111 increases to a value corresponding to the above mentioned control pressure, the spool 70b assumes the position as illustrated by the right half thereof as viewed in Fig. 1A. In this manner, the hydraulic pressure within the circuit 111 is adjusted to the value corresponding to the control pressure. Since, as previously described, the control pressure increases in proportion to engine load during operation with one or drive ranges excluding the reverse drive range, the hydraulic pressure within the chambers 64d and 68c of the accumulators 64 and 68 supplied thereto from the circuit 111 will increase in proportion to engine output torque. Upon selecting the reverse drive range, the control pressure is zero so that no hydraulic pressure is output to the circuit 111. Hereinafter, the hydraulic fluid pressure network is further described. A circuit 106 extending from the port 36 D of the manual selector valve 36 has middle portions connected to the port 38g of the first shift valve 38 and the port 40g of the second shift valve 40. It has a branch circuit 112 connected to the port 56c of the shuttle valve 56 and also to the port 58g of the overrun clutch control valve 58. The port 38f of the first shift valve 38 is connected via a circuit 113 to the port 50f of the 4-2 relay valve 50 and it is also connected via a one-way orifice 114 to the accumulator's chamber 64e and the second speed ratio servo apply chamber 2S/A, and the port 50f is connected via the circuit 115 to the chamber 32c of the shuttle valve 32, too. Furthermore, the port 38h of the first shift valve 38 is connected via a circuit 116 to the chamber 50e of the 4-2 relay valve 50 and the port 58h of the overrun clutch control valve 58, while the port 50c of the 4-2 relay valve 50 is connected via a circuit 117 to the port 40k of the second shift valve 40. The ports 38k and 38l of the first shift valve 38 and the port 40f of the second shift valve 40 are connected via a circuit 118 to the high clutch H/C. The circuit 118 is provided with one-way orifices 119 and 120 arranged as directed to the opposite directions. Separated from the circuit 118 at a portion between these orifices 119 and 120 and the high clutch H/C is a branch circuit 121 which is connected via a one-way orifice 122 to the third speed ratio servo release chamber 3S/R and the accumulator chamber 66e. There is a circuit 123 connected to the circuit 121 bypassing the orifice 122, and the 3-2 timing valve 48 forms part of this circuit 123 with their ports 48c and 48d connected within the circuit 123. Separated from the circuit 121 at a portion between the one-way orifice 122 and the third speed ratio servo release chamber 3S/R is a circuit 124 which is connected to the chamber 52e of the 4-2 sequence valve 52. The ports 52c and 52f of the 4-2 sequence valve 52 are connected to the port 38i of the first shift valve 38 and the port 40h of the second shift valve 40, respectively. The first shift valve 38 has its port 38j connected via a circuit 125 to the port 40d of the second shift valve 40, and its port 38d connected via a circuit 126 to one of two inlet ports of a shuttle ball 127. The other inlet port of the shuttle ball 127 is connected to a circuit 128 which is connected at one end to the port 36 R of the manual selector valve 36 where the bore mentioned circuit 77 is connected and which is connected on the other end to the reverse clutch R/C and the accumulator chamber 68d via a one-way orifice 129. The shuttle ball 127 has its outlet port connected via a circuit 130 to the low & reverse brake LR/B. The port 40j of the second shift valve 40 is connected via a circuit 131 to the port 54c and the chamber 54f of the I range pressure reduction valve 54. The port 54d of the I range pressure reduction valve 54 is connected via a circuit 132 to the port 36 I of the manual selector valve 36. The shuttle valve 56 has its port 56e connected via a circuit 133 to the chamber 48e of the 3-2 timing valve 48, and its port 56d connected via a circuit 134 to the chamber 58c of the overrun clutch control valve 58. The port 58d of the overrun clutch control valve 58 is connected via a circuit 135 to the accumulator chamber 66d, and also to the accumulator chamber 68e and the fourth speed ratio servo apply chamber 4S/A via a one-way orifice 136. The port 58f of the overrun clutch control valve 58 is connected via a circuit 137 to the port 62f of the overrun clutch reduction valve 62. The overrun clutch pressure reduction valve 62 has its port 62e connected via a circuit 138 to the overrun clutch OR/C. A check valve 139 is connected between the circuits 137 and 138. The port 62c of the overrun clutch pressure reduction valve 62 is connected via a circuit 140 to the port 36 II of the manual selector valve 36 and the chamber 56g of the shuttle valve 56. The operation of the control system is hereinafter described. The pressure regulator valve 20, pressure modifier valve 22, and duty solenoid 24 operate in the previously described manner so that the oil discharged by the oil pump O/P is regulated to provide the line pressure that rises in proportion to the engine output torque during the drive ranges excluding the reverse drive range and that is kept constant during the reverse drive range, and this line pressure is supplied to the circuit 78. This line pressure reaches the pilot valve 26, manual selector valve 36, accumulator control valve 70, and accumulator 66, keeping the accumulator 66 at the position as illustrated by the right half thereof as viewed in Fig. 1B. During the drive ranges excluding the reverse drive range, the accumulator control valve 70 supplies, via the circuit 111, the chambers 64d and 68c of the accemulators 64 and 68 with an accumulator backup pressure (i.e., the line pressure) that is variable in proportion to the engine output torque, urging these accumulators to assume the positions as illustrated by the right halves thereof as viewed in Figs. 1B and 1C, respectively. During the reverse drive, the accumulator control valve 70 brings down the accumulator backup pressure to zero, allowing the accumulators 64 and 68 to assume the positions as illustrated by the left halves thereof as viewed in Figs. 1B and 1C, respectively. The pilot valve 26 always outputs the constant pilot pressure to the circuit 79. P, N RANGE When the driver places the manual selector valve 36 at the P range or N range, all of the ports 36 D, 36 II, 36 I and 36 R of the manual selector valve 36 serve as drain ports. Since the line pressure does not come out of thses ports, the forward clutch F/C, high clutch H/C, band brake B/B, reverse clutch R/C, low & reverse brake LR/B, and overrun clutch OR/C are all held deactivated because they are activated on the line pressure coming out of these ports of the manual selector valve 36. This renders the power train shown in Fig. 2 in the neutral state where the power transmission is impossible. D RANGE When the driver places the manual selector valve 36 at the D range, the automatic shift is effected in the following manner: (First Speed Ratio) With the manual selector valve 36 placed at the D range, the line pressure from the circuit 78 is supplied to the port 36 D as shown by the TABLE 2. As a D range pressure, the line pressure from the port 36 D is supplied via the circuit 106 to the port 38g of the first shift valve 38, the port 40g of the second shift valve 40 and the forward clutch control valve 46, and it is also supplied via the circuit 112 to the port 56c of the shuttle valve 56 and the port 58g of the overrun clutch control valve 58. When the vehicle is at a standstill with the manual selector valve 36 placed at the D range, the computer causes the first shift solenoid 42 and the second shift solenoid 44 to be turned ON, causing the first shift valve 38 and the second shift valve 40 to assume the positions as illustrated by the right halves thereof, respectively, as viewed in Fig. 1B. As a result, the high clutch H/C is allowed to communicate via the circuit 118 and the port 40f with the drain port 40e, and thus it is deactivated. The band broke B/B is deactivated because the second speed ratio servo apply chamber 2S/R, the third speed ratio servo release chamber 3S/R and the fourth speed ratio servo apply chamber 4S/A communicate with the same drain port 40e. The second speed ratio servo apply chamber 2S/A is allowed to communicate via the circuit 113, the port 38f, the port 38k, the circuit 118 and the port 40f with the drain port 40e. The third speed ratio servo release chamber 3S/R is allowed to communicate via the circuit 121, the circuit 118 and the port 40f with the drain port 40e. The fourth speed ratio servo apply chamber 4S/A is allowed to communicate with the same drain port 40e in the manner hereinafter described. As long as the engine output torque is higher than a certain level, since it rises in proportion to the engine output torque, the D range pressure (i.e., line pressure) supplied to the port 56c of the shuttle valve 56 keeps the spool 56b at the position as illustrated by the right half thereof as viewed in Fig. 1C where the pilot pressure within the circuit 79 is admitted via the circuit 134 to the overrun clutch control valve 58, urging this valve 58 to the position as illustrated by the right half thereof as viewed in Fig. 1C. The overrun clutch control valve 58 is kept at the position as illustrated by the right half thereof as viewed in Fig. 1C even when the engine output torque is not higher than the certain level and the shuttle valve 56 stays in the position as illustrated by the left half thereof as viewed in Fig. 1C because unless there is no engine brake command later described, the third shift solenoid 60 is turned ON under the control of the computer in order to cause the control pressure directed to the overrun clutch control valve 58 via the circuits 109 and 134 as high as the above mentioned pilot pressure. Under these conditions, therefore, the fourth speed ratio servo apply chamber 4S/A is allowed to communicate via the circuit 135, the ports 58d, 58h, the circuit 116, the ports 38h, 38l, the circuit 118 and the port 40f with the drain port 40e. Besides, the reverse clutch R/C is drained via a circuit 128 at the port 36 R, and thus it is deactivated. The low & reverse brake LR/B is drained in the manner described hereinafter and thus deactivated. That is, the shuttle ball 127, which has its outlet port connected to the circuit 130 communicating with the low & reverse brake LR/B, has one of its two inlet ports connected to the circuit 128 that is drained as described and the other inlet port connected to the circuit 126 that is in turn connected via the ports 38d, 38j, the circuit 125, the ports 40d, 40j, and the circuit 131 to the I range pressure reduction valve 54. Since it is not subject to pressure from the port 36 I of the manual selector valve 36 and thus left in the position as illustrated by the right half thereof as viewed in Fig. 1C, the low & reverse brake LR/B is drained at the port 36 I via the circuit 132 and deactivated. The overrun clutch OR/C is allowed to communicate via the circuit 138, check valve 139 and the port 58f with the drain port 58e since the overrun clutch control valve 58 is held in the position as illustrated by the right half thereof as viewed in Fig. 1C for the previously mentioned reason, and thus it is deactivated. As previously described, there exists no hydraulic pressure within the circuit 113 that leads to the second speed ratio servo apply chamber 2S/A so that the shuttle valve 32 stays in the position as illustrated by the lower half thereof as viewed in Fig. 1A because the chamber 32c is connected to the circuit 113 via the circuit 115. With the shuttle valve 32 in the position as illustrated by the lower half thereof, the pilot pressure within the circuit 79 is supplied via the circuit 95 to the chamber 30d, urging the lock-up control valve 30 to the position as illustrated by the right half thereof as viewed in Fig. 1A, rendering the torque converter 3 operable in the converter state. In this position, the shuttle valve 32 allows the supply of the control pressure within the circuit 97 to the chamber 46a via the circuit 96. With the duty cycle of the solenoid 34 appropriately controlled, this control pressure is adjusted such that the forward clutch control valve 46 is controlled in the following manner. That is, 100 % is set as the duty cycle of the solenoid 34 to cause the control pressure supplied to the chamber 46a to be zero unless manual operation for starting the vehicle is performed even under the D range condition. (The manual operation for starting the vehicle involves depression of an accelerator pedal with a foot brake released when the vehicle speed is zero.) This causes the forward clutch control valve 46 to assume the position as illustrated by the right half thereof as viewed in Fig. 1A where the D range pressure supplied to the circuit 106 is blocked and thus prevented from raching the forward clutch F/C, leaving the forward clutch F/C deactivated. Under this condition, since the high clutch H/C, band brake B/B, reverse clutch R/C, low & reverse brake LR/B, and overrun clutch OR/C are deactivated, too, as previously described, the automatic transmission stays in neutral unless the manual operation for starting the vehicle is performed even when the D range is selected in the automatic transmission. As a result, the occurrences of creep and select shock (i.e., N-D select shock) are prevented. Upon performing the manual operation for starting the vehicle, the computer starts decreasing the duty cycle of the solenoid 34 at a gradual rate until it finally become zero. This causes the gradual increase in the control pressure supplied to the chamber 46a until finally it increases to the same level as the pilot pressure. This gradual increase in the control pressure causes the spool 46b of the forward clutch control valve 46 to switch its position from the position as illustrated by the right half thereof to the position as illustrated by the left half thereof at the corresponding gradual rate, causing a gradual increase in hydraulic pressure supplied to the forward clutch F/C through the circuit 105 until the hydraulic pressure therein becomes as high as the D range pressure (i.e., line pressure) from the circuit 106. Thus, the forward clutch F/C is gradually activated until the automatic transmission establishes the first speed ratio in cooperation with the activations of the forward one-way clutch FO/C and low one-way clutch LO/C as shown in the TABLE 1, allowing the vehicle to move from a standstill. During this vehicle's starting operation, since the working hydraulic pressure of the forward clutch F/C gradually rises, the activation of the forward clutch F/C progresses at a predetermined speed in cooperation with the throttle effect due to the one-way orifice, thus preventing start-up shock. (Second Speed Ratio) When the vehicle has reached the running state where the second speed ratio is to be established as a result of an increase in the vehicle speed, the computer switches the state of the first shift solenoid 42 to the OFF state in accordance with the pattern shown in TABLE 3, causing the spool 38b of the first shift valve 38 to switch to the position as illustrated by the left half thereof as viewed in Fig. 1B. As a result, the first shift valve 38 now allows the circuit 126 to communicate with the drain port 38e, keeping on draining the circuit 126, and now allows the port 38h to communicate with the port 38i to keep on draining the circuit 116 via the ports 38h, 38i, and the port 52c of the 4-2 sequence valve 52 (This valve assumes the position as illustrated by the right half thereof as viewed in Fig. 1B under the condition where the third speed ratio servo release chamber 3S/R is not supplied with hydraulic pressure.) at the drain port 52d. On the other hand, the first shift valve 38 allows the circuit 113 to communicate with the circuit 106, supplying the second speed ratio serve apply chamber 2S/A with the D range pressure via the circuit 113, activating the band brake B/B. With the forward clutch F/C kept activated and the forward one-way clutch being activated, the activation of the band brake B/B causes the automatic transmission to shift to the second speed ratio as readily be understood from TABLE 1. During this upshift operation from the first to second speed ratio, the hydraulic fluid pressure supplied to the second speed ratio servo apply chamber 2S/A is increased at a gradual rate owing to the fluid flow restriction due to the one-way orifice 114 and gradual upward movement, as viewed in Fig. 1B, of the accumulator piston 64a in response to the pressure build-up in the accumulator chamber 64e, thus alleviating shocks inherent with this shifting operation. Besides, the effect of this shift shock alleviation is secured because the backup pressure within the chamber 64d acting upon the accumulator piston 64a is proportional to the engine output torque. Since, as readily understood from TABLE 1, the D range pressure is supplied to the second speed ratio servo apply chamber 2S/A not only upon selecting the second speed ratio, but also selecting the third speed ratio and the fourth speed ratio, the shuttle valve 32 keeps on staying in the position as illustrated by the upper half thereof as viewed in Fig. 1A because the D range pressure is supplied via the circuit 115 to the chamber 32c. This allows the supply of the pilot pressure from the circuit 79 to the chamber 46a of the forward clutch control valve 46, keeping the forward clutch control valve 46 in the position as illustrated by the left half thereof as viewed in Fig. 1A. Since the forward clutch control valve 46 does not effect pressure regulation when it assumes this position, the forward clutch F/C is kept in the fully activated state upon selecting any one of the second speed to fourth speed ratios. On the other hand, the chamber 30d of the lock-up control valve 30 is supplied with the control pressure from the circuit 97 so that the state of the torque converter 3 can be switched among the converter state, slip control state, and lock-up state in accordance with a predetermined pattern matched to the operating conditions by controlling the duty cycle of the duty solenoid 34 under the control of the computer (Third Speed Ratio) When, subsequently, the vehicle has attained the running state where the third speed ratio is to be established, the computer switches the state of the second shift solenoid 44 to the OFF state, too, in accordance with the pattern shown in TABLE 3, causing the spool 40b of the second shift valve 40 to switch to the position as illustrated by the left half thereof as viewed in Fig. 1B. As a result, the D range pressure admitted to the port 40g is now supplied via the port 40f and circuit 118 to the high clutch H/C for activation thereof, In the process, the hydraulic fluid passes through the one-way orifice 120 unrestricted and then through the one-way orifice 119 restricted. On the other hand, this hydraulic fluid pressure passes through the branch circuit 121 off the circuit 118 and through the one-way orifice 122 unrestricted to the third speed ratio servo release chamber 3S/R, deactivating the band brake B/B. The hydraulic fluid pressure supplied to the third speed ratio servo release chamber 3S/R is applied to the chamber 52e of the 4-2 sequence valve 52 via the circuit 124, urging the spool 52b for upward movement toward the position as illustrated by the left half thereof. Even though this upward movement causes the port 52c to communicate with the port 52f after separating it from the drain port 52d, since the second shift valve 40 connects this port 52c to the drain port 40i, the circuit 116 continues to be drained. Thus, deactivation of the band brake B/B with the high clutch H/C and the forward one-way clutch FO/C held activated causes the automatic transmission to shift to the third speed ratio. During this upshift operation from the second speed ratio to the third speed ratio, the hydraulic fluid pressure supplied to the high clutch H/C and the third speed ratio servo release chamber 3S/R gradually increases owing to the fluid flow restriction due to the one-way orifice and gradual upward movement, as viewed in Fig. 1B, of the accumulator piston 66a against the line pressure within the chamber 66c, thus preventing the occurrence of shocks inherent with this shifting operation. (Fourth Speed Ratio) When, subsequently, the vehicle has reached the running state where the fourth speed ratio is to be established, the computer switches the state of the first shift solenoid 42 to the ON state in accordance with the pattern shown in TABLE 3, causing the spool 38b of the first shift valve 38 to switch to the position as illustrated by the position as illustrated by the right half thereof as viewed in Fig. 1B. This causes the first shift valve 38 to switch the port connection such that even though it disconnects the circuit 113 leading to the second speed ratio servo apply chamber 2S/A from the D range pressure circuit 106, it allows the circuit 113 to communicate via the port 38k with the circuit 118 in order to continue the supply of the D range pressure to the second speed ratio servo apply chamber 2S/A, whereas even though it disconnects the circuit 126 from the drain port 38e, it allows the circuit 126 to communicate via the port 38j with the circuit 125 that communicates with the drain port 40e in order to cause the circuit 126 to conrtinue to be drained. In this position, the first shift valve 38 allows the circuit 116 to communicate with the circuit 118 via the ports 38h and 38l, supplying the D range pressure via the circuit 118, circuit 116, ports 58h and 58d, circuit 135 and one-way orifice 136 to the fourth speed ratio servo apply chamber 4S/A, thus switching the state of the forward clutch F/C to the activated state. Thus, activating the band brake B/B with the forward clutch F/C and high clutch H/C held activated causes the automatic transmission to shift to the fourth speed ratio. During this upshift operation from the third to fourth speed ratio, the fourth speed ratio select hydraulic fluid pressure (i.e., the highest speed ratio select pressure) supplied to the fourth speed ratio servo apply chamber 4S/A increases at a gradual rate owing to the fluid flow restriction due to the one-way orifice 114 and gradual upward movement, as viewed in Fig. 1C, of the accumulator piston 68a in response to the pressure build-up in the accumulator chamber 68c, thus alleviating shocks inherent with this shifting operation. Besides, the effect of this shift shock alleviation is secured because the backup pressure within the chamber 68c acting upon the accumulator piston 68a is proportional to the engine output torque. Besides, this pressure (i.e., the fourth speed ratio select pressure) supplied to the fourth speed ratio servo apply chamber 4S/A passes to the chamber 66d of the accumulator 66. This results in switching the capacity of the accumulator 66 to a value meeting the demand for the upshift from the second speed ratio to the fourth speed ratio, this value being different from the capacity required for the upshift from the second speed ratio to the third speed ratio. As a result, shift shock alleviation takes place effectively even during the upshift from the second speed ratio to the fourth speed ratio junping the third speed ratio. (4-3 Downshift) When, during running with the fourth speed ratio, the vehicle has reached the running state where the third speed ratio is to be established, the computer switches the state of the first shift solenoid 42 to the OFF state in accordance with the pattern shown in TABLE 3, causing the first shift valve 38 to switch to the position as illustrated by the left half thereof as viewed in Fig. 3B to have it assume the same position as it assumes upon selecting the third speed ratio. As a result, the hydraulic pressure supplied to the fourth speed ratio servo apply chamber 4S/R is depressurized quickly because the hydraulic fluid passes through the one-way orifice 136 unrestricted to be discharged at the drain port 40i, causing the downshift to the third speed ratio to take place. (4-2 Downshift) When, during running with the fourth speed ratio, the vehicle has reached the running state where the second speed ratio is to be established, the computer switches the state of the first shift solenoid 42 to the OFF state, thereby to switch the first shift valve 38 to the position as illustrated by the left half thereof as viewed in Fig. 1B, and it switches the state of the second shift solenoid 44 to the ON state, thereby to switch the second shift valve 40 to the position as illustrated by the right half thereof as viewed in Fig. 1B. Even though this movement of the first shift valve 38 switches the connection of the circuit 113 leading to the second speed ratio servo apply chamber 2S/A to the circuit 106 from the circuit 118, the first shift valve 38 continues to supply hydraulic pressure to the second speed ratio servo apply chamber 2S/A. The switching of the second shift valve 40 results in disconnecting the circuit 118 from the D range pressure circuit 106 and connecting it to the drain port 40e. As a result, the activating pressure supplied to the high clutch H/C is eliminated by discharging the hydraulic fluid via the circuit 118 at the drain port 40e after having passed through the one-way orifice 119 unrestricted and being subject to flow restriction by the one-way rifice 120, and the pressure within the third speed ratio servo release chamber 3S/R is eliminated by discharging the hydraulic fluid via the circuit 121 and then through the same path at the drain port 40e after having being subject to flow restriction by the one-way orifice 122. The pressure within the third speed ratio servo release chamber 3S/R is delivered via the circuit 124 to the 4-2 sequence valve 52. This 4-2 sequence valve 52 is responsive to this pressure and assumes the position as illustrated by the left half thereof as viewed in Fig. 1B where the port 52c that is connected to the circuit 116 via the ports 38i and 38h is disconnected from the drain port 52d and allowed to communicate with the port 52f until this pressure is substantially eliminated. This prevents the pressure within the fourth speed ratio servo apply chamber 4S/A connected to the circuit 116 from being drained and thus it is maintained until the third speed ratio servo release chamber 3S/R is drained. During this transition, the pressure within the fourth speed ratio servo apply chamber 4S/R is supplied via the circuit 116 to the 4-2 relay valve 50, holding this valve to the position as illustrated by the right half thereof as viewed in Fig. 1B. This allows the circuit 113 leading to the second speed ratio servo apply chamber 2S/A to communicate with the fourth speed ratio servo apply chamber 4S/A via the ports 50f, 50c, circuit 117, ports 40k, 40h, 52f, 52c, 38i, 38h, circuit 116, ports 58h, 58d, and circuit 135, thus keeping the pressure within the fourth speed ratio servo apply chamber 4S/R. When the pressure within the third speed ratio servo release chamber 3S/R is drained, the 4-2 sequence valve 52 shifts to the position as illustrated by the right half thereof as viewed in Fig. 1B where the pressure within the fourth speed ratio servo apply chamber 4S/A connected to the circuit 116 is drained at the drain port 52d. The draining of the pressure within the circuit 116 causes the 4-2 relay valve 50 to shift to the position as illustrated by the left half thereof as viewed in Fig. 1B, allowing the pressure within the circuit 117 to be drained at the drain port 50d. Thus, during this 4-2 downshift operation, the pressure within the fourth speed ratio servo apply chamber 4S/A is drained after the pressure within the high clutch H/C has been drained so that the 4-2 downshift skipping the third speed ratio is assured by preventing the former pressure from being drained prior to the drainage of the latter pressure which would cause the 4-3-2 downshift. (3-2 Downshift) When, during running with the third speed ratio, the vehicle has reached the running state where the second speed ratio is to be established, the computer switches the state of the second solenoid 44 to the ON state to cause the second shift valve 40 to shift to the position as illustrated by the right half thereof as viewed in Fig. 1B in accordance with the pattern shown in TABLE 3. Even though this switches the connection of the port 40h from the drain port 40i to 40k, the fourth speed ratio servo apply chamber 4S/A is kept in the depressurized state regardless of the position of the 4-2 sequence valve 52 because, during running with the third speed ratio, the circuit 116 connected to the fourth speed ratio servo apply chamber 4S/A in depressurized to cause the 4-2 relay valve 50 to assume the position as illustrated by the left half thereof where the circuit 117 is connected to the drain port 50d and thus the port 52f connected to the circuit 117 is caused to serve as a drain port. The above mentioned shift of the second shift valve 40 causes the circuit 118 to communicate with the drain port 40e, draining the pressure within the high clutch H/C and the pressure within the three speed ratio servo release chamber 3S/R in the same manner as described in connection with the 4-2 downshift. This results in a downshift from the third speed ratio to the second speed ratio. As will be described hereinafter, the timing at which the pressure within the third speed ratio servo release chamber 3S/R varies depending on the operating conditions of the engine such that smmoth 3-2 downshift takes place. That is, when the engine output torque is below a predetermined value, the D range pressure (i.e., line pressure) applied to the port 56c of the shuttle valve 56 is low corresponding in magnitude to the engine output torque, allowing the shuttle valve 56 to assume the position as illustrated by the left half thereof as viewed in Fig. 1C, allowing the chamber 48e of the 3-2 timing valve 48 to communicate via the circuit 133 and port 56e with the drain port 56f, thus allowing the 3-2 timing valve 48 to assume the position as illustrated by the left half thereof as viewed in Fig. 1C Thus, when the engine output torque is low, the third speed ratio servo release chamber 3S/R is depressurized quickly because the hydraulic fluid is discharged not only through the one-way orifice 122, but also through the orifice 48f, When the engine output torque is above the predetermined level, the D range pressure (i.e., line pressure) applied to the port 56c of the shuttle valve 56 is high corresponding to the increased engine output torque, urging the shuttle valve 56 to the position as illustrated by the right half thereof as viewed in Fig. 1C, connecting the circuit 133 to the circuit 109, thus rendering the 3-2 timing valve 48 shiftable in accordance with the control pressure produced within the circuit 109. The computer switches the state of the third shift solenoid 60 to the ON state when the engine output torque is above the predetermined level and the vehicle speed is higher than a predetermined speed, causing the control pressure to increase to a value as high as the pilot pressure. This causes the 3-2 timing valve 48 to shift to the position as illustrated by the right half thereof as viewed in Fig. 1C, depressurizing the third speed servo release chamber 3S/R slowly by discharging the hydraulic fluid through the one-way orifice 122 only. (2-1 Downshift) When, during running with the second speed ratio, the vehicle reaches the running state where the first speed ratio is to be established, the computer switches the state of the shift solenoid 42 to the ON state, thus switching the first shift valve 38 to the position as illustrated by the right half thereof as viewed in Fig. 1B in accordance with the pattern shown in TABLE 3. This causes the circuit 133 leading to the second speed ratio servo apply chamber 2S/A to disconnected from the D range pressure circuit 106, but to be connected to the circuit 118 via the ports 38f and 38k. Since the circuit 118 is connected to the drain port 40e by the second shift valve 40, the pressure within the second speed ratio servo apply chamber 2S/A is depressurized quickly by dischaging the hydraulic fluid passing through the one-way orifice 114, thus effecting a quick downshift from the second speed ratio to the first speed ratio. (Overdrive Inhibition) When the driver turns ON an OD inhibitor switch, not illustrated, disposed within the reach of the driver, wishing the engine brake to be effected with the third speed ratio and wishing no upshift to the fourth speed ratio (overdrive), the output of the OD switch causes the computer to set the ON/OFF state of the first and second shift solenoids 42 and 44 in accordance with the pattern shown in TABLE 3 so as to provide a speed ratio suitable for the driving state such that the fourth speed ratio will not be established. In this case, the automatic transmission is allowed to perform automatic shift between the first, second and third speed ratios in the similar manner as in the D range. With the third speed ratio to be selected, the computer switches the state of the third shift solenoid 60 to the OFF state, bringing down the control pressure within the circuit to zero. When, under this condition, the engine torque is low (this is the case where the engine brake is needed) and accordingly the D range pressure (i.e., line pressure) applied to the port 56c of the shuttle valve 56 is low, thus failing to urge the shuttle valve 56 to the position as illustrated by the right half thereof as viewed in Fig. 1C, leaving it in the position as illustrated by the left half thereof as viewed in Fig. 1C, the control pressure within the circuit 109 brought down to zero is admitted to the overrun clutch control valve 58 via the circuit 134. However, this admission of the control presure to the chamber 58v does not cause the overrun clutch control valve 58 to shift from the position as illustrated by the left half thereof as viewed in Fig. 1C. Since the overrun clutch control valve 58 remains in the position as illustrated by the left half thereof, the D range pressure within the circuit 112 is now supplied via the circuit 137 and the overrun clutch pressure reduction valve 62 to the overrun clutch OR/C, thus activating the same. This activation of the overrun clutch OR/C causes the transmission to effect the engine brake running with the third speed ratio as will be understood from the TABLE 1. During this engine broke running, since there is no hydraulic pressure at the port 36 II of the manual selector valve 36, the overrun clutch pressure reduction valve 62 effects pressure regulation to reduce the activating pressure supplied to the overrun clutch OR/C, matching the capacity thereof to a value required, thus causing substantial reduction in shocks upon shifting to engine brake operation. When the engine output torque is high or large (this is the case where engine brake is not required) and thus the D range pressure applied to the port 56c of the shuttle valve 56 is high enough to urge the shuttle valve 56 to assume the position as illustrated by the right half thereof, allowing the pilot pressure within the circuit 79 to be admitted via the circuit 134 to the overrun clutch control valve 58 to urge the same to the position as illustrated by the right half thereof as viewed in Fig. 1C. This position of the overrun clutch control valve 58 causes the circuit 138 to be drained always, thus rendering the overrun clutch OR/C deactivated, causing no engine brake to be effected. The hydraulic fluid to be discharged from the overrun clutch OR/C in this case is quickly discharged at the drain port 58e via the check valve 139. II Range When the driver places the spool 36a of the manual selector valve 36 at the II range, wishing the engine brake running with the second speed ratio, the manual valve allows the line pressure within the circuit 78 to output from the port 36 II, too, in accordance with the pattern shown in TABLE 2. Under this condition, the pattern of distribution of the pressure from the port 36 D is quite the same as it is upon selecting the D range. The computer switches the states of the first and second shift solenoids 42 and 44 in accordance with the pattern shown in TABLE 3 for selecting the first speed ratio or the second speed ratio, thus causing the automatic transmission to shift between the first speed ratio and the second speed ratio. The pressure (II range pressure) from the port 36 II of the manual selector valve 36 reaches the port 62c of the overrun clutch pressure reduction valve 62 via the circuit 140. The II range pressure from the circuit 140 reaches also to the chamber 56g of the shuttle valve 56, locking this valve to the position as illustrated by the left half thereof as viewed in Fig. 1C. With the shuttle valve 56 locked to this position, the control pressure within the circuit 109 is supplied via the circuit 134 to the chamber 58c of the overrun clutch control valve 58. In this state, the computer brings down the control pressure to zero during running with the second speed ratio by switching the state of the third shift solenoid 60 to the OFF state, causing the overrun clutch control valve 58 to assume the position as illustrated by the left half thereof as viewed in Fig. 1C. This causes the D range pressure within the circuit 112 to be supplied to the overrun clutch OR/C via the overrun clutch pressure reduction valve 62 and the circuit 137, thus causing the automatic transmission to effect engine brake with the second speed ratio. Under this condition, the overrun clutch pressure reduction valve 62 does not effect pressure reduction since it is locked as mentioned above, allowing the capacity of the overrun clutch OR/C to increase to a value matched to the requirement, thus preventing engine brake from becoming ineffective owing to the shortage in capacity. Durring running with the first speed ratio selected, the computer turns ON the third shift solenoid 60, allowing an inacrease in the control pressure to the level as high as the pilot pressure, causing the overrun clutch control valve 58 to stay in the position as illustrated by the right half thereof as viewed in Fig. 1C. This results in discharge of the hydraulic fluid from the overrun clutch OR/C via the check valve 139 and port 58f at the drain port 58e, deactivating the overrun clutch OR/C, thus establishing the same state as is the case for running with the first speed ratio during the D range. I Range When the driver places the spool 36a of the manual selector valve 36 wishing the engine brake running with the first speed ratio, the line pressure within the circuit 78 is allowed to output from the ports 36 D, 36 II and 36 I in accordance with the pattern shown in TABLE 2. The pattern of distribution of the pressure from the port 36 D is quite the same as it is upon selecting the D range. The computer switches the states of the first and second shift solenoids 42 and 44 in accordance with the pattern shown in TABLE 3 for selecting the first speed ratio or the second speed ratio, thus causing the automatic transmission to shift between the first speed ratio and the second speed ratio. There are cases where the second speed ratio is established despite the fact the I range is selected because it is necessary to prevent the occurrence of the overrun of the engine which would be caused if the first speed ratio were established immediately after selecting the I range. In this situation, the second speed ratio is established and maintained until the vehicle speed drops sufficiently so that the donwshift to the first speed ratio does not cause any excessive rotation of the engine. The pressure from the port 36 II of the manual selector valve 36 is supplied to the shuttle valve 56 and the overrun clutch pressure reduction valve 62 in the same manner as the case upon selecting the II range, thereby to hold these valves to the position as illustrated by the left halves thereof, respectively. This allows the control pressure within the circuit 109 to switch the state of the overrun clutch control valve 58. With the I range selected, the computer turns OFF the third shift solenoid 60 to bring down the control pressure to zero, allowing the overrun clutch control valve 58 to assume the position as illustrated by the left half thereof as viewed in Fig. 1C, thus allowing the D range pressure within the circuit 112 to keep on activating the overrun clutch OR/C. The pressure from the port 36 I of the manual selector valve 36 arrives via the circuit 132 at the I range pressure reduction valve 54 where it is reduced to the certain value in the manner descrived before and output to the circuit 131. The pressure within the circuit 131 is allowed to appear in the circuit 125 because the second shift valve 40 is held in the position as illustrated by the right half thereof regardless of which the first speed ratio or the second speed ratio is established. The first shift valve 38, on the other hand, stays in the position as illustrated by the left half thereof as viewed in Fig. 1B during running with the second speed ratio in accordance with the pattern shown in TABLE 3 where it cuts off the pressure within the circuit 125 and drains the pressure within the circuit 126 at the drain port 38e. As a result, the circuit 130 leading to the low & reverse brake LR/B is allowed to communicate via the shuttle ball 125 and the circuit 126 with the drain port 38e, causing deactivation of the low & reverse brake LR/B. This allows the engine brake running with the second speed ratio owing to the activation of the overrun clutch OR/C. The first speed ratio is established after selecting the I range upon the vehicle speed dropping low enough not to cause excessive rotation of the engine. With this first speed ratio, the first shift valve 38 is in the position as illustrated by the right half thereof as viewed in Fig. 1B where the circuit 125 is allowed to communicate with the circuit 126, thus supplying the pressure within the circuit 125 via the circuit 126, shuttle ball 127 and circuit 130 to the low & reverse brake LR/B to activate the same. This results in establishment of engine brake running with the first speed ratio because the overrun clutch OR/C is activated. Durring engine brake running with the first or second speed ratio, the overrun clutch pressure reduction valve 62 is locked to the position as illustrated by the left half thereof as viewed in Fig. 1C, causing no pressure reduction to take place. This causes the capacity of the overrun clutch OR/C to be maintained sufficiently high enough to the value demanded, thus preventing the engine brake from becoming ineffective due to the slip therein. Since during engine brake running with the first speed ratio the pressure admitted to the low & reverse brake LR/B is reduced to the certain value by means of the I range pressure reduction valve 54, the capacity of the low & reverse brake is adjusted to the value demanded, thus allowing the downshift to the first speed ratio to take place without substantial shocks. R Range When the driver places the spool 36a of the manual selector valve 36 at the R range wishing the reverse travel, the line pressure within the circuit 78 is allowed to output from the port 36 D only in accordance with the pattern shown in TABLE 2. The pressure from the port 36 R (reverse select pressure) is supplied via the circuit 128 and the one-way orifice 129 where it is subject to fluid flow restriction to the reverse clutch R/C to activate the same and at the same time it is supplied to the chamber 68d of the accumulator 68. The pressure within the circuit 128 is supplied via the circuit 130 to the low & reverse brake LR/B to activate the same pushing through the shuttle ball 127. As a result, the automatic transmission selects the reverse drive as shown in TABLE 1. During this transition, the pressure supplied to the reverse clutch R/C increases at a gradual rate to cause the engagement of the reverse clutch R/C to progress at a predetermined gradual rate because the hydraulic fluid to be supplied to the reverse clutch R/C is restricted by the one-way orifice 129 and then pusshes down the stepped piston 68a of the accumulator upon selecting the R range (this stepped piston being shifted to the position as illustrated by the left half thereof as viewed in Fig. 1C upon placing the spool 36a of the manual selector valve 36 at the R range), thus alleviating shocks (N-R select shock) taking place upon selecting the R range from the N range. When the driver switches the spool 36a of the manual selector valve 36 from the R range to another drive range wishing to suspend the reverse travel, the port 36D becomes a drain port to allow the discharge of the pressure from the reverse clutch R/C. Such discharge of pressure is effected quickly because the hydraulic fluid passes through the one-way orifice 129 without receiving any flow restriction. Thus, the low & reverse brake LR/B is deactivated without any delay. Referring to Fig. 4, there is shown a simplified hydraulic circuit embodying the present invention as applied to an automatic transmission which includes as friction elements a forward clutch F/C, a band brake B/B, a high clutch H/C and an overrun clutch (an engine brake effecting friction element) OR/C. The first speed ratio is selected by activating the forward clutch F/C. An upshift to the second speeed ratio is effected by activating the band brake B/B, and an uposhift from the second speed ratio to the third speed ratio is effected by deactivating the band brake B/B and activating the high clutch H/C. The activation or deactivation of the band brake B/B is performed by a band servo B/S having a servo apply chamber S/A and a servo release chamber S/R. Supplying the servo apply chamber S/A with hydraulic fluid pressure causes the activation of the brake B/B, while supplying the servo release chamber S/R with hydraulic fluid pressure causes the deactivation of the band brake B/B regardless of the presence or absence of hydraulic fluid pressure within the servo apply chamber S/A because of the difference in pressure acting areas formed on a servo piston. If the overrun clutch OR/C is activated during running with the first or second or third speed ratio being established, the automatic transmission effects engine brake during running with the speed ratio established. Comparing Fig. 4 with Figs. 1A, 1B and 1C, it will be readily understood, a manual selector valve 36, a pilot valve 26, and a 3-2 timing valve 48 are identical in construction to their counterparts. Referring to Fig. 4, a shuttle valve 56A, an overrun clutch control valve 58A, a first shift valve 38A (i.e., a 1-2 shift valve) and a second shift valve 40A (i.e., a 2-3 shift valve) are similar to their counterparts shown in Figs. 1A, 1B and 1C. Therefore, like reference numerals are used to denote like parts throughout Figs. 1A, 1B and 1C and Fig. 4. Referring to Fig. 4, a line pressure circuit 78 receives a line pressure generated by a line pressure regulator as shown in Fig. 1A and denoted by the reference numeral 20. Alternatively, the line pressure as described in the publication entitled "AUTOMATIC TRANSAXLE RN4F02A TYPE, RL4F02A TYPE SERVICE MANUAL" (A261C06) issued by NISSAN MOTOR COMPANY LIMITED may be used. This line pressure is variable with throttle opening degree and vehicle speed such that its stays relatively low during coasting of the vehicle although it rises to a relatively high value during power-on running of the vehicle. The 1-2 shift valve 38A includes a spring 38a biasing a spool 38b to a downshift position thereof as illustrated by the right half thereof as viewed in Fig. 4 where a port 38f is allowed to communicate with a drain port 38k. When a control pressure, as high as a pilot pressure, is supplied through a circuit 100 to a chamber 38c, the spool 38b is urged to an upshift position thereof as illustrated by the left half position thereof as viewed in Fig. 4 where the port 38f is allowed to communicate with a port 38g. The 2-3 shift valve 40A includes a spring 40a biasing a spool 40b to a downshift position thereof as illustrated by the left half thereof as viewed in Fig. 4 where a port 4c is allowed to communicate with a drain port 40e. When a control pressure, as high as the pilot pressure, is supplied through a circuit 102 to a chamber 40c, the spool 40b is urged to an upshift position thereof as illustrated by the right half thereof as viewed in Fig. 4 where the port 40f is allowed to communicate with a port 40g. The shuttle valve 56A includes a spool 56b and a spring 56a biasing the spool 56b to a position as illustrated by the left half thereof as viewed in Fig. 4 where a port 56e is allowed to communicate with a drain port 56f and at the same time a port 56d is allowed to communicate with a circuit 109. When the spool 56b is urged by a pressure within a chamber 56c against the spring 56a to assume a position as illustrated by the right half thereof as viewed in Fig. 4, the port 56e is allowed to communicate with the circuit 109 and at the same time the port 56d is allowed to communicate with a pilot pressure circuit 79. The overrun clutch control valve 58A includes a spool 58b and a spring 58a biasing the spool 58b to a position an illustrated by the left half thereof as viewed in Fig. 4. The spool 58b is urged by a pressure supplied to a chamber 58c for upward movement to a position as illustrated by the right half thereof as viewed in Fig. 4. When the spool 58b is in the position as illustrated by the left half thereof as viewed in Fig. 4, the valve 58A allows a port 58f to communicate with a port 58g while when it is in the position as illustrated by the right half thereof as viewed in Fig. 4, it allows the port 58f to communicate with a drain port 58e. The pilot pressure circuit 79 is connected to the shuttle valve 56A at one port thereof and has the branch circuit 109 connected to the shuttle valve 56A at another port thereof. The branch circuit 109 is connected to the pilot pressure circuit 79 via an orifice 110 and has a drain port 60c, disposed downstream of the orifice 110. A solenoid 60A is arranged to close or open the drain port 60c and has a coil 60a, a plunger 60b and a spring 60d such that when the coil 60a is turned OFF (i.e., when no current passes through the coil 60a), the plunger 60b closes the drain port 60c under the spring force of the spring 60d, allowing the pressure within the branch circuit 109 to become as high as the pilot pressure within the circuit 79. When the coil 60a is turned ON (i.e., when current passes through the coil 60a), the plunger 60b is urged against the spring 60d for downward movement, as viewed in Fig. 4, opening the drain port 60c draining the pressure within the branch circuit 109. The port 56e of the shuttle valve 56A is connected via a circuit 133 to the chamber 48e, while the port 56d in connected via a circuit 134 to the chamber 58c. An outlet port 36 D of a manual selector valve 36 is connected via a circuit 106 to the forward clutch F/C, to the port 38g of the 1-2 shift valve 38A and also to the port 40g of the 2-3 shift valve 40A. The circuit 106 is also connected via a circuit 112 to the port 58g of the overrun clutch control valve 58A and to the port 56c of the shuttle valve 56A. The port 38f of the 1-2 shift valve 38A is connected via a circuit 113 to the servo apply chamber S/A, the port 40f of the 2-3 shift valve 40A is connected via a circuit 118 to the high clutch H/C. This port 40f is connected via the circuit 118 and a branch circuit 121 to the servo release chamber S/R. There is provided a one-way orifice 122 in the branch circuit 121. Bypassing this one-way orifice 122, the 3-2 timing valve is arranged to selectively establish a circuit 123 having one end connected to the circuit 118 and its opposite end connected to the circuit 121. The remaining port 58f of the overrun clutch control valve 58A in connected via a circuit 137 to the overrun clutch OR/C. The operation of this embodiment is as follows: When a driver places the spool 36a of the manual selector valve 36 at the D range position, the line pressure from the circuit 78 is allowed to output from the output port 36D. Thus, the line pressure is supplied to the circuits 106 and 112. Under this condition, when the pressure within the circuit 100 and that within the circuit 102 are low, the spools 38b and 40b of the 1-2 shift valve 38A and 2-3 shift valve 40A assume their downshift positions, respectively, the hydraulic pressure within the servo apply chamber S/A is drained at the drain port 38k, and at the same time the hydraulic fluid pressure within the servo release chamber S/R and within the high clutch H/C is drained at the drain port 40e. As a result, the band brake B/B and high clutch H/C are deactivated, and the forward clutch F/C is activated to cause the automatic transmission to establish the first speed ratio. Subsequently, when the control pressure within the circuit 100 rises and becomes as high as the pilot pressure, the 1-2 shift valve 38A assumes the upshift position as illustrated by the left half thereof as viewed in Fig. 4. This causes the line pressure within the circuit 106 to pass through the circuit 113 to the servo apply chamber S/A to activate the band brake B/B, causing the automatic transmission to shift to the second speed ratio. When the control pressure within the circuit 102 is also rises and becomes as high as the pilot pressure, the 2-3 shift valve 40A assumes the upshift position as illustrated by the right half thereof as viewed in Fig. 4. This causes the line pressure within the circuit 106 to pass through the circuit 118 to the high clutch H/C on one hand, and to pass through the circuits 121 to the servo release chamber S/R on the other hand. As a result, the band brake B/B is deactivated and the high clutch H/C is activated, and the automatic transmission effects shifting upto the third speed ratio. When, subsequently, the pressure within the circuit 102 drops to cause the spool 40b of the 2-3 shift valve 40A to move downwards to the downshift position as illustrated by the left half thereof as viewed in Fig. 4, the port 40f is allowed to communicate with the drain port 40e, draining the pressure supplied to the high clutch H/C at the drain port 40e, deactivating the high clutch H/C. At the same time, this causes the pressure supplied to the servo release chamber S/R to be drained at a predetermined rate which will be described later in detail, allowing the gradual activation of the band brake B/B. Thus, with the appropriate donwshift timing, the automatic transmission downshifts from the third speed ratio to the second speed ratio. The line pressure arrives via the circuit 112 at the chamber 56c of the shuttle valve 56A, causing the spool 56b to stroke. As previously mentioned, the line pressure is relatively low during vehicle's coasting and causes the spool 56b of the shuttle valve 56A to assume the position as illustrated by the left half thereof as viewed in Fig. 4 when the line pressure is lower than a preset line pressure value. The line pressure is relatively high during power-on running of the vehicle and causes the spool 56b to assume the position as illustrated by the right half thereof as viewed in Fig. 4 when the line pressure is higher than the preset value. Describing the operation when the shuttle valve 56A has its spool 56b assume the position as illustrated by the left half thereof, the port 56d is allowed to communicate with the circuit 109, rendering the overrun clutch control valve 58A operable under the control of the solenoid 60A. When the solenoid coil 60a is turned ON in response to the presence of an engine brake command from the driver via closure of an OD switch or the presence of an automatic engine brake command as a result of judgement of the gradient of slope where the vehicle is travelling through, the plunger 60b opens the drain port 60c, bringing down the control pressure supplied to the chamber 58c to zero, causing the spool 58b of the overrun clutch control valve 58A to assume the position as illustrated by the left half thereof as viewed in Fig. 4. In this position of the spool 58b, the line pressure arrives via the circuits 112 and 137 at the overrun clutch OR/C, activating the same. Thus, under this condition, when the driver wishes engine brake to be effected, the engine brake takes place during operation with the first speed ratio or second speed ratio or third speed ratio in automatic shift range. During coasting of the vehicle, since the shuttle valve 56A has its spool 56b assume the position as illustrated by the left half thereof as viewed in Fig. 4 to cause the chamber 48e of the 3-2 timing valve 48 to be drained at the drain port 56f, the spool 48b of the 3-2 timing valve 48 stays in the position as illustrated by the left half thereof as viewed in Fig. 4. With the spool 48b of the 3-2 timing valve 48 assuming the position as illustrated by the left half thereof as viewed in Fig. 4, the pressure supplied to the servo release chamber S/R is quickly drained at the drain port 40e via the circuit 123 bypassing the one-way orifice 122, completing the downshift from the third speed ratio to the second speed ratio quickly without the downshift timing control. This causes no inconvenience because such downshift timing control is not necessary during coasting of the vehicle. When the shuttle valve 56A has its spool 56b assume the position as illustrated by the right half thereof as viewed in Fig. 4 during power-on running of the vehicle, the port 56e is allowed to communicate with the circuit 109, rendering the 3-2 timing valve 48 operable under the control of the solenoid 60A. The control of the solenoid coil 60a is such that it is turned OFF when the vehicle speed is higher than a predetermined value. This causes the pilot pressure to be admitted to the chamber 48e via the circuit 133, thus urging the 3-2 timing valve 48 to assume the position as illustrated by the right half thereof as viewed in Fig. 4. Thus, the 3-2 timing valve 48 blocks the communication via the circuit 123 and thus it causes the discharge of fluid pressure from the servo release chamber S/R to occur at a controlled rate by flow restriction by the one-way orifice 122, causing the downshift from the third speed ratio to the second speed ratio under the control of the before mentioned downshift timing. This shifting operation is completed when the engine speed has increased to a value suitable for a vehicle speed and a gear ratio supposed to be accomplished after the shifting, alleviating substantial shift shock. During power-on running, since the shuttle valve 56A has its spool 56b assume the position as illustrated by the right half thereof as viewed in Fig. 4, the pilot pressure arrives via the circuit 134 at the chamber 58c, keeping the overrun clutch control valve 58A at the position as illustrated by the right half of its spool. With this position of the valve 58A, the circuit 24 is allowed to communicate with the drain port 58e, leaving the overrun clutch OR/C deactivated, causing no engine brake to be effected. This does not cause any inconvenience because during power-on running it is not necessary to effect engine brake and the activation and deactivation of the overrun clutch OR/C (engine brake effecting friction element) does not play any role in the transmission of power. In the case where he/she wishes engine brake to be effected with the transmission locked to the second speed ratio or the first speed ratio, the driver places the spool 36a of the manual selector valve 36 at the II range position or the I range position. When the spool 36a of the manual selector valve 36 assumes the II range position, the line pressure from the circuit 78 is supplied also to the port 36 II as will be understood from the TABLE 2, and when the spool 36a of the manual selector valve 36 is shifted to the I range, the line pressure from the circuit 78 is supplied also to the port 36 I. The line pressure from the port 36 II arrives via a circuit network, not illustrated, at predetermined portions, causing the forward clutch F/C and band brake B/B to be activated to lock the transmission to operate with the second speed ratio, and causing the overrun clutch OR/C to be activated to keep engine brake effective. The line pressure from the port 36 I arrives via a circuit network, not illustrated, at predetermined portions, prohibiting activations of all of the friction elements except the forward clutch F/C to lock the transmission to operate with the first speed ratio as long as vehicle speed satisfies a predetermined condition (viz., the second speed ratio is established before the first speed ratio is established when the vehicle speed is high enough to cause the engine to overrun), causing the overrun clutch OR/C to be activated to keep engine effective. When the driver places the spool 1a of the manual selector valve 36 at the R range position for reverse drive, the line pressure from the circuit 78L is supplied only to the port 36 R as shown in TABLE 2. The line pressure from the port 36 R arrives at the appropriate friction elements, not shown, which are to be activated for establishing the reverse drive. Referring to Fig. 5, a hydraulic control system illustrated herein is substantially the same as previously described one in connection with Fig. 4 except that a shuttle valve 56 has replaced the shuttle valve 56A. The shuttle valve 56 includes a spool 56b and a spring 56a biasing the spool 56b to a position as illustrated by the left half thereof as viewed in Fig. 5 where a port 56e is allowed to communicate with a drain port 56f and at the same time a port 56d is allowed to communicate with a circuit 109. When the spool 56b is urged by a pressure within a chamber 56p against the spring 56a to assume a position as illustrated by the right half thereof as viewed in the Fig. 5, the port 56e is allowed to communicate with the circuit 109 and at the same time the port 56d is allowed to communicate with a circuit 79. In order to assure the upward movement of the spool 56b by the pressure within the chamber 56p, a land 56i of the spool 56b which is arranged to cover or uncover the drain port 56f is designed and arranged as to be always in sliding contact with a land guide 56j formed between the drain port 56f and the chamber 56p. The spool 56b also includes another land 56k that is smaller in diameter than the land 56i and disposed remoter from the spring 56a than the land 56i is, and it also includes still another land 56l that is smaller in diameter than the land 56k and is disposed remoter from the spring 56a than the land 56k is. There are formed land guides 56m and 56n corresponding to these lands 56k and 56l. The arrangement of the land guides 56m and 56n is as follows: That is, when the spool 56b stays in the position as illustrated by the left half thereof, the land 56k is slidably fit with the guide 56m, and when the land 56k becomes out of engagement with the guide 56m during the upward movement of the spool 56b, the land 56l is brought into slidable fit with the guide 56n. Thus, when the spool 56b assumes the position as illustrated by the right half thereof as viewed in Fig. 5, the land 56l is slidable fit with the guide 56n and the downward movement of the spool 56b causes the land 56k to come into slidable fit with the guide 56m when the land 56l becomes out of engagement with the guide 56n. As a result, with the land 56k slidably fit with the guide 56m, the spool 56b is subject to an upward force as viewed in the Fig. 5 which is expressed by the product of a difference in pressure acting area between the lands 56i and 56k and the pressure within the chamber 56p, while with the land 56l slidably fit with the guide 56n, the spool 56b in subject to an upward force as viewed in Fig. 5 which is expressed by the product of a difference in pressure acting area between the lands 56i and 56l and the pressure within the chamber 56p. In order to make the feature of this shuttle valve clear, the operation of this valve is described hereinafter. When it assumes the position as illustrated by the left half thereof as viewed in Fig. 5, the spool 56b is subject to the above mentioned upward force as viewed in Fig. 5 resulting from the product of the line pressure within the chamber 56p and the difference in pressure acting area between the lands 56i and 56k. When, in this state, the line pressure rises, this upward force increases to cause the spool 56b for upward movement from the position as illustrated by the left half thereof as viewed in Fig. 5. During this upward movement, when it passes a position where the land 56k disengages from the guide 56m and the land 56l becomes slidably fit with the guide 56n, the spool 56b begins to be subject to the before mentioned upward force resulting from the product of the line pressure and the difference in pressure acting area between the lands 56i and 56l. Since this latter pressure acting area difference is greater than the former, the spool 56b quickly assumes the position as illustrated by the right half thereof as viewed in Fig. 5 after passing the above mentioned position. Thereafter, as long as the line pressure stays higher than this pressure value (which is called hereinafter as a first preset line pressure value) where the spool 56b passes the above mentioned point during its upward movement, the spool 56b is kept at the position as illustrated by the right half thereof as viewed in Fig. 5. As, in this state, the line pressure supplied to the chamber 56p drops, the upward force resulting from the product of the line pressure and the pressure acting area difference between the lands 56i and 56l decreases, thus allowing the downward movement of the spool 56b from the position as illustrated by the right half thereof under the action of the spring 56a. When, during this downward movement, it passes a position where the land 56l disengages from the guide 56n and the land 56k become slidably fit with the guide 56m, the spool 56b begins to be subject to the upward force resulting from the product of the line pressure supplied to the chamber 56p and the pressure acting area difference between the lands 56i and 56k. Since this pressure acting area difference is smaller than the former, the spool 56b quickly assumes the position as illustrated by the left half thereof as viewed in Fig. 5 after passing this position. Thereafter, as long as the line pressure stays lower than the line pressure value (which is hereinafter called as a second preset line pressure value) at which the spool 56b passes this position during the downward movement thereof, the spool 56b is kept at the position as illustrated by the left half thereof as viewed in the Figure. The above mentioned first preset line pressure value is higher than the second preset line pressure value, thus providing the movement of the spool 56b with the hysteresis. The shuttle valve 56 just described above is substantially the same as the shuttle valve 56 shown in Fig. 1C. Referring to Fig. 6, a hydraulic control system illustrated herein is substantially the same as the before described one in connection with Fig. 4 except that a chamber 56g of a shuttle valve 56A having received therein a spring 56a is connected to a port 36 II of a manual selector valve 36 via a circuit 140. This arrangement is particularly necessary in the case where line pressure is boosted when the spool 36a of the manual selector valve 36 is placed at II range. The increased line pressure is necessary during operation with the manual selector valve 36 placed at II range because sufficiently large hydraulic pressure is needed by a band brake B/B under this condition to maintain grip strong enough to hold fast a rotary member that is subject to reverse torque. The advantage according to this arrangement will be clarified from the following desctiprion: When the driver places the spool 36a of the manual selector valve 36 at II range wishing engine brake running, the line pressure is boosted to meet the above mentioned demand. In response to this increase in line pressure, the spool 56b of the shuttle valve 56A tends to move upwards toward the position as illustrated by the right half thereof as viewed in Fig. 6 even though the vehicle is coasting, creating the concern that the engine brake is not effected under this condition. This, however is prevented in the following manner: During operation with II range selected, the manual selector valve 36 allows the line pressure to be supplied also to the port 36 II as will be understood from TABLE 2. The line pressure from the port 36 II is supplied via the circuit 140 to the chamber 56g of the shuttle valve 56A. This assists the action of the spring 56a in biasing the spool 56b to lock the same into the position as illustrated by the left half thereof as viewed in Fig. 6, keeping the overrun clutch control valve operable under the control of the solenoid 60A. During the operation with II range selected, the coil 60a of the solenoid 60A is left turned OFF, keeping the overrun clutch OR/C activated, thus assuring effective engine brake. The line pressure from the port 36 II arrives via a circuit network, not illustrated, at predetermined portions, causing the forward clutch F/C and band brake B/B to be activated to lock the transmission to operate with the second speed ratio, thus effecting engine brake during operation with the second speed ratio without effecting any upshifting to the third speed ratio. During this engine brake operation, the capacity of the band brake B/B tends to drop because the band brake works in the trailing direction upon receiving reverse torque from the wheels. This drop in the capacity is compensated for by the above mentioned increase in the line pressure, thus preventing engine brake from becoming ineffective due to slip which otherwise would take place in the band brake B/B. During this operation, since the shuttle valve 56A is locked to the position as illustrated by the left half thereof as viewed in the Fig. 6, the 3-2 timing valve 48 is kept at the position as illustrated by the left half thereof as viewed in Fig. 6 where the downshift timing control is not carried out. This does not cause any inconvenience because during the operation with II range the third speed ratio is not selected and thus the 3-2 downshift will not take place. Referring to Figs 7, 8 and 9, there will be described control strategy of a coil 60a of a solenoid 60A. Referring to Fig. 7, a circuit 109 is directly connected to a chamber 48e of a 3-2 timing valve 48 and a shuttle valve is not illustrated for ease of explanation. As shown in Fig. 7, ON/OFF control of a solenoid 60A is carried out by a controller 200. Input signals to the controller 200 include a signal from a 3-2 shift sensor 202 that detects a 3-2 downshift event, a signal from a vehicle speed sensor 204, and a signal from a throttle opening degree sensor 206. Upon receiving the signal from the 3-2 shift sensor 202, the controller 200 initiates execution of a control program shown in Fig. 8. The controller 200 reads vehicle speed detected by the sensor 204 at a step 208, and then throttle opening degree detected by the sensor 206 at a step 210. At the subsequent step 212, the controller 200 retrieves a prestored ON/OFF operation range table data as shown in Fig. 9 to find out whether the solenoid coil 60a is to be turned ON or OFF versus vehicle speed and engine throttle openind degree. If the operation conditions demand the solenoid coil 60a be turned ON, the controller 200 proceeds to a step 214 where the solenoid coil 60a is turned ON, while if the operation conditions demand the solenoid coil 60a be turned OFF, the controller 200 proceeds to a step 216 where the solenoid coil 60a is turned OFF. The above mentioned cycle of these steps is repeated whenever the 3-2 downshift event occurs during a predetermined period of time including the time from the commencement of the shifting operaion to the end thereof. When the operation conditions fall in the solenoid OFF range on the righthand side as viewed in Fig. 9, since the solenoid coil 60a is turned OFF, the pressure within the chamber 48e of the 3-2 timing valve 48 is as high as the pilot pressure, urging the 3-2 timing valve 48 to the position as illustrated by the right half thereof as viewed in Fig. 7. In this position, the 3-2 timing valve 48 blocks communication via the circuit 123 so that the depressurization of the pressure within the servo release chamber S/R is effected under the flow restriction provided by the one-way orifice 122 at a controlled rate. The depressurization of the hydraulic pressure from the high clutch H/C, on the other hand, is carried out quickly so as to establish a temporal neutral state allowing the engine to race to increase its speed. When the engine speed reaches the engine speed value to be established upon completion of the shifting operation, the depressurization of the servo release chamber S/R is completed. Then, the automatic transmission establish the second speed from the neutral state, completing the downshift from the third speed ratio to the second speed ratio without any substantial shocks. When the operation conditions demand the solenoid coil 60a be turned ON, the pressure supplied to the chamber 48e drops to zero, allowing the 3-2 timing valve 48 to assume the position as illustrated by the left half thereof as viewed in Fig. 7. In this position, the 3-2 timing valve 48 allows the communication via the circuit 123, allowing the quick discharge of hydraulic fluid therethrough in effecting depressurization of the servo release pressure, providing a short neutral interval. The setting is made such that this neutral interval matches a period of time required for the engine to increase its speed a value supposed to be established upon completion of the shifting operation under this operating conditions, so that the 3-2 downshift is carried out without any substantial shocks. Although, in this example, the switching of the 3-2 timing valve 48, i.e., ON/OFF of the solenoid coil 60a, is carried out in response to vehicle speed and engine throttle opening degree, other factors, such as engine temperature, road slope gradient, engine output torque, which are closely related to an interval required as the above mentioned neutral interval may be used in effecting the above mentioned control. In this case, the above mentioned operation and effect results in more complete manner. The present specification includes subject-matter which is claimed in EP-A- 0318062 and EP-A 0321992

Claims: A control for an automatic transmission for an automotive vehicle having an engine, the transmission being adapted for shifting between different speed ratios and having a plurality of friction elements (B/B, H/C, F/C, OR/C, LR/B, R/C, FO/C, LO/C) to effect shifting between speed ratios, the plurality of friction elements (B/B, H/C, F/C, OR/C, LR/B, R/C, FO/C, LO/C) including an engine brake effecting friction element, (OR/C) the control comprising: a fluid source (O/P); means (78) communicating with said fluid source for generating a line pressure which is variable with opening degree of a throttle of the engine; a timing valve (48); a control valve (58;584A) for the engine brake effecting friction element (OR/C); electromagnetic means (60;60A) for generating a control pressure; a shuttle valve (56;56A) communicating with said timing valve (48), said control valve (58;58A), and said electromagnetic means (60;60A), said shuttle valve (56;56A) having valve spool (56b) movable responsive to said line pressure between a first position where said control valve (58;58A) is under the control of said control pressure and said timing valve (48) is not under the control of said control pressure, and a second position where said control valve (58;58A) is not under the control of said control pressure and said timing valve (48) is under the control of said control pressure. A control as claimed in claim 1, wherein said shuttle valve (56; 56A) including means (56a) for causing said valve spool (56b) to stay in said second position thereof when said line pressure is higher than a first preset value, but causing said spool to stay in said first position thereof when said line pressure is lower than a second preset value, said first preset value being higher than said second preset value. A control as claimed in claim 1, further comprising means (36) for generating a predetermined speed range pressure when said predetermined speed range is selected. A control as claimed in claim 3, wherein said line pressure generating means (O/P) communicates with said predetermined speed range pressure generating means (36) and urged to boost said line pressure in response to said predetermined speed range pressure. A control as claimed in claim 4, wherein said shuttle valve (56;56A) communicates with said predetermined speed range pressure generating means (36) and has the spool (56b) thereof locked to said first position thereof in response to said predetermined speed range pressure. A control as claimed in claim 1, wherein said electromagnetic means (60;60A) is operable responsive to vehicle speed of the automotive vehicle and opening degree of the throttle of the engine. A control as claimed in claim 6, wherein said electromagnetic means (60;60A) switches said control pressure to a predetermined value when the vehicle speed and the opening degree of the throttle satisfy a predetermined condition.

Inventors: HAYASAKI, KOICHI, SUGANO, KAZUHIKO
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1986
Main classification: F16H61
IPC codes: F16H61, F16H59