Hydraulic control apparatus for automatic transmission

Abstract: To achieve a predetermined speed by engaging or releasing friction engagement elements of an auxiliary speed shift mechanism, a fluid pressure changeover valve is switched to the position for the communication between the regulated pressure input port and the output port, so that the regulated fluid pressure from a pressure regulating valve is supplied through second speed shift mechanism-side shift valves to the friction engagement elements of the auxiliary speed shift mechanism to be engaged or released. This position switch of the changeover valve also connects the line pressure input port to the output port, so that the line pressure is supplied to the main speed shift mechanism-side shift valves to maintain the main speed shift mechanism in a predetermined speed state. To achieve a predetermined speed by a shift in the main speed shift mechanism, the regulated pressure input port becomes connected to the output port connecting to the main speed shift mechanism-side shift valves, and the line pressure input port becomes connected to the output port.

Description: The present invention relates to a hydraulic pressure control apparatus for an automatic transmission installed in a motor vehicle and, more particularly, to a hydraulic control apparatus for an automatic transmission comprising a first speed shift (main transmission) mechanism and a second speed shift (auxiliary transmission) mechanism that enables shifts to a plurality of speeds by the combined operation of the main speed shift mechanism and the auxiliary speed shift mechanism, by using a single pressure regulating valve. A hydraulic control apparatus according to the preamble of claim 1, for an automatic transmission has been proposed in, for example, US-A-4 367 528, which controls the fluid pressure supply to a plurality of friction engagement elements for achieving shifts to a plurality of speeds by using a single impact prevention valve (pressure regulating valve), by supplying regulated fluid pressure from the impact prevention valve controlled by a single pressure regulating solenoid valve, to various hydraulic servos by way of a plurality of shift control valves. The aforementioned automatic transmission achieves four forward speeds by a three-speed main shift mechanism and an overdrive shift mechanism combined. However, the automatic transmission is disclosed without consideration of combining a main shift mechanism and an auxiliary mechanism, for example, a three-speed shift mechanism and a two-speed shift mechanism, to achieve multi-speeds, for example, six (three times two) speeds. The aforementioned automatic transmission achieves the fourth speed by shifting the overdrive shift mechanism to an overdrive state while the main shift mechanism is at the third speed. For that end, the hydraulic control apparatus of the aforementioned automatic has an arrangement where the 3-4 shift control valve for controlling the overdrive shift mechanism is disposed downstream, in fluid pressure flow, from the 2-3 shift control valve, so that it is impossible to control the 3-4 shift control valve independently. That is, the hydraulic control apparatus is unable to perform multi-speed shift operation in which the shift control of an auxiliary shift mechanism, such as an overdrive shift mechanism, can be carried out even when a main shift mechanism is at a speed other than the third speed. Another automatic transmission has recently been disclosed which employs a combination of a three-speed main shift mechanism and a three-speed auxiliary shift mechanism to achieve multi-speeds such as five forward speeds . However, this five-speed transmission may cause problems if an accelerator pedal is depressed (kick down) for an acceleration during a cruise at the fifth speed. If the kick-down results in a downshift to the third speed, the vehicle may rapidly accelerate but the engine speed will soon rise to an overspeed leading to an immediate upshift. If the kick-down results in a downshift to the fourth speed, the toque may be insufficient. In either case, the transmission fails to achieve a sufficient acceleration, failing to produce high power performance and drive feels as expected of a five-speed transmission. Accordingly, it is an object of the present invention to provide a hydraulic control apparatus for an automatic transmission that solves the aforementioned problems by employing a relatively simple hydraulic control circuit having a single pressure regulating means to control the shift operation of a first (main) shift mechanism and the shift operation of a second (auxiliary) shift mechanism independently of each other, thus enabling such a multi-speed shift operation that a shift to an intermediate speed provided between two normal adjacent speeds can be performed for, for example, an acceleration. According to the present invention, there is provided a hydraulic control apparatus for an automatic transmission, comprising: a first speed shift mechanism and a second speed shift mechanism each of which has a plurality of friction engagement elements; and a hydraulic control circuit for achieving a plurality of speeds by supplying fluid pressure to and discharging fluid pressure from a hydraulic servo unit that operates said plurality of friction engagement elements. The hydraulic control circuit comprises: a first speed shift mechanism-side hydraulic servo for controlling the friction engagement elements provided in the first speed shift mechanism; a first speed shift mechanism-side shift valve for switching over the supply and discharge of fluid pressure with respect to the first speed shift mechanism-side hydraulic servo in accordance with various speed signals; a second speed shift mechanism-side hydraulic servo for controlling the friction engagement elements provided in the second speed shift mechanism; a second speed shift mechanism-side shift valve for switching over the supply and discharge of fluid pressure with respect to the second speed shift mechanism-side hydraulic servo in accordance with various speed signals; a fluid pressure source for generating a line pressure; a pressure regulating valve for regulating the line pressure from the fluid pressure source; and a fluid pressure changeover valve for selectively supplying the line pressure and the fluid pressure from the pressure regulating valve to the first speed shift mechanism-side shift valve and the second speed shift mechanism-side shift valve. The terms "first speed" and "second speed" mentioned above and in the appended claims do not necessarily refer to the actual 1st or 2nd speed, but they merely means "one speed" and "another speed", respectively. With this construction, since the plural friction engagement elements of the first and second speed shift mechanisms can be controlled by the single pressure regulating valve, the construction of the hydraulic control circuit can be simplified. It also becomes possible to selectively supply the regulated fluid pressure from the pressure regulating valve and the line pressure to the first and second speed shift mechanism-side shift valves by using the fluid pressure changeover valve, so that the first speed shift mechanism and the second speed shift mechanism can be controlled independently of each other. Thereby the invention is able to achieve many speeds, for example, intermediate speeds between normally achieved speeds, improving drive feels. Preferably the present invention further has the following construction. The fluid pressure changeover valve comprises: a regulated pressure input port for inputting the fluid pressure from the pressure regulating valve, a line pressure input port for inputting the line pressure from the fluid pressure source, a first output port for outputting to the first speed shift mechanism-side shift valve, and a second output port for outputting to the second speed shift mechanism-side shift valve. The fluid pressure changeover valve changes over a position where the regulated pressure input port and the first output port communicate with each other and the line pressure input port and the second output port communicate with each other, and a position where the regulated pressure input port and the second output port communicate with each other and the line pressure input port and the first output port communicate with each other. In the control of the fluid pressure supply to the friction engagement elements, it is normally preferable to control the engaging pressure by the regulated fluid pressure from the pressure regulating valve until the torque capacity needed by the friction engagement elements is reached, and after the engagement is completed, supply the line pressure thereto in order to achieve sufficiently large torque capacity. With aforementioned preferred construction of the invention, since the line pressure can be supplied to a predetermined friction engagement element of the first or second speed shift mechanism, it becomes possible to smoothly control the engaging pressure by the regulated fluid pressure from the pressure regulating valve and provide sufficiently large torque capacity by the line pressure. The invention may have another preferred construction as follows. The first speed shift mechanism receives, as input, drive force from an engine and achieves a plurality of reduced speeds, and the second speed shift mechanism receives, as input, output from the first speed shift mechanism. The second speed shift mechanism-side shift valve supplies the fluid pressure from the fluid pressure source to the hydraulic servo that operates a friction engagement element that is engaged when the second speed shift mechanism is at a predetermined speed. At least when the second speed shift mechanism completes shifting to a predetermined speed, the fluid pressure changeover valve switches from the position where the fluid pressure from the pressure regulating valve is supplied to the second speed shift mechanism-side shift valve, to the position where the line pressure from the fluid pressure source is supplied to the second speed shift mechanism-side shift valve. The friction engagement elements of the second speed shift mechanism receive larger load torques than the friction engagement elements of the first speed shift mechanism since the second speed shift mechanism receives rotations speed-reduced by the various speeds of the first speed shift mechanism. With the aforementioned preferred construction, since the friction engagement elements to be engaged for a predetermined speed of the second speed shift mechanism are constantly supplied with the line pressure from the fluid pressure source after the shift control is completed, the invention provides sufficiently large torque capacities for the friction engagement elements that engage and receive large load toques during a predetermined speed of the second speed shift mechanism, and eliminates the need to increase the control gain of the pressure regulating valve for a high regulated pressure output comparable to the line pressure, enabling an improvement in the pressure regulation precision of the pressure regulating valve. It is also preferred that the first speed shift mechanism achieve the highest speed and the speed next to the highest by engaging or releasing a predetermined friction engagement element of the first speed shift mechanism, and that at least after the shift to the highest speed is completed, the fluid pressure changeover valve be held in the same position as assumed during the shift to the highest speed. Normally, the highest speed and the second highest speed are frequently used in automatic transmissions. It is therefore desirable to achieve no-time-lag shifts between these speeds. With this preferred construction, since the highest speed is achieved by controlling the engaging pressure on a predetermined friction engagement element (for example, a direct clutch C2), and the second highest speed is achieved by releasing the predetermined friction engagement element while maintaining the fluid pressure changeover valve in the same position as assumed for the shift to the highest speed (for example, the position for supplying the fluid pressure from the pressure regulating valve to the first speed shift mechanism-side shift valve), it is possible to control the releasing pressure without having to switch the fluid pressure changeover valve in position, thus correspondingly reducing the time lag. This invention may have a further preferred construction as follows. When speed is to be shifted from a first speed to a second speed by releasing at least a predetermined friction engagement element of the second speed shift mechanism from engagement while a predetermined friction engagement element of the first speed shift mechanism is maintained in engagement: the first speed shift mechanism-side shift valve maintains the predetermined friction engagement element of the first speed shift mechanism in engagement by supplying fluid pressure to the hydraulic servo that operates the predetermined friction engagement element, during the operation of the shift from said first speed to said second speed; and the second speed shift mechanism-side shift valve changes over the supply of the fluid pressure from the pressure regulating valve to the hydraulic servo that operates the predetermined friction engagement element of the second speed shift mechanism and the discontinuation thereof, during the operation of the shift from said first speed to said second speed; and the fluid pressure changeover valve supplies the first speed shift mechanism-side shift valve with the fluid pressure from the pressure regulating valve raised within a predetermined time, after a predetermined time following the discontinuation of supply of the fluid pressure from the pressure regulating valve by the second speed shift mechanism-side shift valve, at the time of completion of the shift from said first speed to said second speed. For the regulated pressure control of the releasing pressure on the friction engagement elements (for example, the fourth brake B4) of the second speed shift mechanism, the releasing pressure control is normally performed by changing the holding pressure on a predetermined friction engagement element (for example, the second brake B2) of the first speed shift mechanism to the line pressure using the fluid pressure changeover valve, and by supplying the regulated pressure from the pressure regulating valve to the hydraulic servo of the friction engagement element (B4) of the second speed shift mechanism. However, because the regulated pressure from the pressure regulating valve is substantially reduced at the end of the shift, there is a danger that if the fluid pressure changeover valve is switched in position in this state, the holding pressure on the predetermined friction engagement element (B2) of the first speed shift mechanism may become a low pressure. With the aforementioned preferred construction, the communication between the fluid pressure source and the hydraulic servo that operates the predetermined friction engagement element of the second speed shift mechanism is discontinued by the second speed shift mechanism-side shift valve blocking the fluid pressure supply from the fluid pressure source to the hydraulic servo, and after the fluid pressure from the pressure regulating valve is raised for a predetermined time length, the fluid pressure supply passage is connected to the hydraulic servo of the predetermined friction engagement element of the first speed shift mechanism. The invention thus prevents a low pressure on the predetermined friction engagement element. The present invention preferably further comprises: a first fluid passage for supplying the fluid pressure from the pressure regulating valve to the fluid pressure changeover valve; a second fluid passage for supplying the fluid pressure from the fluid pressure changeover valve to at least one of the first speed shift mechanism-side shift valve and the second speed shift mechanism-side shift valve; and a third fluid passage that connects the first fluid passage to the second fluid passage without going through the fluid pressure changeover valve, the third fluid passage being provided with connecting means for connecting the first fluid passage and the second fluid passage to each other when the fluid pressure in the second fluid passage becomes less than the fluid pressure in the first fluid passage. It is normally necessary to supply the line pressure to the friction engagement element that is engaged by the engagement control of the pressure on the friction engagement element using the pressure regulating valve, so as to provide the friction engagement element with a sufficiently large torque capacity. However, during the positional switching of the fluid pressure changeover valve for the pressure changeover, the second fluid passage may become disconnected from the fluid pressure source. With aforementioned preferred construction, if the fluid pressure in the second fluid passage becomes lower than the fluid pressure in the first fluid passage during the positional switching of the fluid pressure changeover valve, a third fluid passage is opened by the connecting means to connect the first fluid passage and the second fluid passage bypassing the fluid pressure changeover valve. The invention thus prevents a low pressure in the second fluid passage. In addition, the invention may further comprise: a predetermined first: speed shift mechanism-side shift valve for achieving a low speed of the first speed shift mechanism; a predetermined second speed shift mechanism-side shift valve for achieving a predetermined speed of the second speed shift mechanism; and a solenoid valve for controlling the changeover of the second speed shift mechanism-side shift valve. In this construction, the solenoid valve is disposed in a fluid passage that connects the fluid pressure source and the second speed shift mechanism-side shift valve in communication. The predetermined first speed shift mechanism-side shift valve is disposed in said fluid passage, the predetermined first speed shift mechanism-side shift valve closing said fluid passage when a low speed is maintained in the first speed shift mechanism. When the first speed shift mechanism is in a low speed state, the friction engagement elements of the second speed shift mechanism engaged for a high speed of the second speed shift mechanism receive increased loads because the first speed shift mechanism in such a state inputs increased torques to the second speed shift mechanism. However, the aforementioned preferred construction of the invention prevents the second speed shift mechanism from shifting to a higher speed during a low speed of the first speed shift mechanism, by discontinuing the source pressure on the solenoid valve for switching the position of the shift valve that controls the shift of the second speed shift mechanism (for example, to the second and third speed , or the third speed) . The invention thus increases the service life of the high-speed friction engagement elements of the second speed shift mechanism. In the aforementioned construction, it is preferred that the predetermined first speed shift mechanism-side shift valve in the fluid passage be provided between the fluid pressure source and the solenoid valve. In a construction in which the first speed shift mechanism-side shift valves are disposed in fluid passages between the solenoid valves and the second speed shift mechanism-side shift valves, the flow loss would become large if the connection between the fluid pressure source and the drain fluid passage should be established by the solenoid valve. However, with the aforementioned preferred construction of the invention, when one of the solenoid valves is in the position for the connection between the fluid pressure source and the drain fluid passage, the corresponding one of the first speed shift mechanism-side shift valves disconnects the fluid pressure source from the drain fluid passage to reduce flow loss. Preferably the invention further comprises: changeover means for changing the connection of a fourth fluid passage that is provided between the fluid pressure source and the pressure regulating valve and that connects to an input port of the pressure regulating valve, over to a line pressure supply passage extending from the fluid pressure source and to a drain fluid passage; a fifth fluid passage for, when the fluid pressure changeover valve is in a predetermined position, connecting to a regulated pressure passage extending from the pressure regulating valve to supply the regulated pressure from the pressure regulating valve to one of the first speed shift mechanism-side shift valve and the second speed shift mechanism-side shift valve via the fluid pressure changeover valve; a sixth fluid passage for, when the fluid pressure changeover valve is in the predetermined position, connecting to the fourth fluid passage to supply the line pressure from the fluid pressure source to the other one of the first speed shift mechanism-side shift valve and the second speed shift mechanism-side shift valve via the fluid pressure changeover valve; and a seventh fluid passage provided with connecting means for connecting the regulated pressure passage and the sixth fluid passage to each other when the fluid pressure in the regulated pressure passage is higher than the fluid pressure in the sixth fluid passage, the seventh fluid passage being capable of connecting the regulated pressure passage to the sixth fluid passage by the connecting means. It is necessary to maintain friction engagement elements in the released state, for example, in the P range. However, the fluid pressure drainage may not be reliably achieved in an automatic transmission that controls the supply and discontinuation of fluid pressure to the hydraulic servos that operate the friction engagement elements by using the pressure regulating valve, because the fluid pressure from the hydraulic servos may be maintained at low pressures depending on the conditions of the pressure regulating valve (the relation between its spool and spring). With the aforementioned preferred construction, the regulated fluid pressure from the pressure regulating valve is conducted to the sixth fluid passage by the connecting means, such as a check valve, and to the fourth fluid passage by the fluid pressure changeover valve (b → 21d → 21c → a → d 2 → 45 → c → 21f → 21e → i), and the fourth fluid passage is drained by the changeover means (for example, a manual valve). The invention thus ensures the drain of the fluid pressure from the fifth fluid passage (b) connecting to the hydraulic servos. The invention may have a further preferred construction as follows. When a first friction engagement element of the first speed shift mechanism and a second friction engagement element of the second speed shift mechanism are maintained in engagement, the hydraulic control apparatus shifts from a first speed to a second speed by releasing a third friction engagement element of the first speed shift mechanism from engagement. During the operation of the shift from said first speed to said second speed, the second speed shift mechanism-side shift valve maintains the second friction engagement element of the second speed shift mechanism in engagement by supplying fluid pressure to a second hydraulic servo that operates the second friction engagement element. During the operation of the shift from said first speed to said second speed, the first speed shift mechanism-side shift valve changes over the supply of the fluid pressure from the pressure regulating valve to a third hydraulic servo that operates the third friction engagement element of the first speed shift mechanism, and the discontinuation thereof, and also changes over the supply of the fluid pressure from the pressure regulating valve to a first servo that operates the first friction engagement element of the first speed shift mechanism, and the supply of the line pressure to the first hydraulic servo. The hydraulic control apparatus comprises a changeover valve for selectively supplying the first hydraulic servo with the fluid pressure from the first speed shift mechanism-side shift valve and the fluid pressure supply to the second hydraulic servo, the changeover valve supplying the first hydraulic servo with the fluid pressure supply to the second hydraulic servo when the fluid pressure from the first speed shift mechanism-side shift valve becomes less than the fluid pressure supply to the second hydraulic servo. In cases, such as the control of the shift from the highest speed down to the next speed, where the releasing pressure on a predetermined (third) friction engagement element (for example, C2) of the first speed shift mechanism is to be controlled, it is necessary to maintain a predetermined (second) friction engagement element (for example, C3) of the second speed shift mechanism in engagement and maintain another (first) friction engagement element (for example, B2) of the first speed shift mechanism in engagement. In this control operation, the first friction engagement element (B2) is maintained in engagement by supplying thereto the line pressure via the first speed shift mechanism-side shift valve, but after the shift, a low pressure may occur to the first hydraulic servo (B-2) for operating the first friction engagement element (B2) because the switching of the first speed shift mechanism-side shift valve after the shift connects the hydraulic servo (B-2) to a fluid passage whose pressure has been reduced by pressure regulation. However, the aforementioned preferred construction of the invention comprises a changeover valve (39) for selectively supplying the first hydraulic servo (B-2) with the fluid pressure from the first speed shift mechanism-side shift valve or the fluid pressure supply to the second hydraulic servo (C-3). The changeover valve (for example, a two-way check valve) supplies the first hydraulic servo (B-2) with the fluid pressure supply to the second hydraulic servo (C-3) if the fluid pressure from the first speed shift mechanism-side shift valve becomes lower than the fluid pressure supply to the second hydraulic servo (C-3). Thus, the invention prevents a low pressure on the first hydraulic servo even if the hydraulic servo is connected to the regulated pressure fluid passage by the switching of the first speed shift mechanism-side shift valve after a shift operation. The operation of the hydraulic control circuit for an automatic transmission of the invention will be briefly described. To achieve a predetermined speed by engaging or releasing the friction engagement elements of the first speed shift mechanism (2), the fluid pressure changeover valve (21) is switched to the position where the regulated pressure input port (21c) and the first output port (21d) are connected in communication. In this position, the regulated pressure from the pressure regulating valve (20) is supplied by the first speed shift mechanism-side shift valves (23, 25) to the hydraulic servo of the friction engagement element of the first speed shift mechanism (2) to be engaged or released, thus smoothly controlling the engagement or release of the friction engagement element. Since the line pressure input port (21e) and the second output port (21f) are connected in communication in this case, the line pressure is supplied to the second speed shift mechanism-side shift valves (26, 27), so that the second speed shift mechanism (5) is maintained at a predetermined speed. To achieve a predetermined speed by engaging or releasing the friction engagement elements of the second speed shift mechanism (5), the fluid pressure changeover valve (21) is switched to the position where the regulated pressure input port (21c) and the second output port (21f) are connected in communication. In this position, the regulated pressure from the pressure regulating valve (20) is supplied by the second speed shift mechanism-side shift valves (26, 27) to the hydraulic servo of the friction engagement element of the second speed shift mechanism (2) to be engaged or released, thus smoothly controlling the engagement or release of the friction engagement element. Since the line pressure input port (21d) and the first output port (21d) are connected in communication in this case, the line pressure is supplied to the first speed shift mechanism-side shift valves (23, 25), so that the first speed shift mechanism (2) is maintained at a predetermined speed. Thus the first speed shift mechanism (2) and the second speed shift mechanism (5) are able to shift to desired speeds separately without affecting each other. For example, a shift of the second speed shift mechanism (5) to the 1st speed while the first speed shift mechanism is maintained to the highest speed (3rd speed) will achieve a 3rd speed low that is lower than the 3rd speed achieved by the combination of the 2nd speed of the first speed shift mechanism (2) and the 2nd speed of the second speed shift mechanism (5). A shift of the second speed shift mechanism (5) to the 2nd speed while the first speed shift mechanism (2) is maintained to the highest speed will achieve a 4th speed low that is lower than a 4th speed achieved by the combination of the 2nd speed of the first speed shift mechanism (2) and the highest speed (3rd speed) of the second speed shift mechanism (5). The reference numerals in the parentheses are given merely for the purpose of reference to the accompanying drawings and do not limit the construction of the invention. The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment with reference to the accompanying drawings, wherein: Fig. 1 is a skeleton diagram of an automatic transmission comprising a main speed shift mechanism and an auxiliary speed shift mechanism; Fig. 2 is an illustration of a hydraulic control circuit of the automatic transmission, showing the left-hand half thereof; Fig. 3 is an illustration of the hydraulic control circuit of the automatic transmission, showing the right-hand half thereof connecting to what is shown in Fig. 2 by line A-A; Fig. 4 shows the operations of the solenoid valves, clutches and brakes of the automatic transmission; Fig. 5 is a main flowchart illustrating the 2nd-to-3rd shift control; Fig. 6 is a flowchart illustrating the solenoid timing control at the start of the 2nd-to-3rd shift control; Fig. 7 is a flowchart illustrating the solenoid timing control at the end of the 2nd-to-3rd shift control; Fig. 8 is a timing chart of the 2nd-to-3rd shift control; Fig. 9 is a main flowchart illustrating the 3rd-to-2nd shift control; Fig. 10 is a flowchart illustrating the solenoid timing control at the start of the 3rd-to-2nd shift control; Fig. 11 is a flowchart illustrating the solenoid timing control at the end of the 3rd-to-2nd shift control; Fig. 12 is a timing chart of the 3rd-to-2nd shift control; Fig. 13 is amain flowchart illustrating the 5th-to-3rd low shift control; Fig. 14 is a flowchart illustrating the solenoid timing control at the start of the 5th-to-3rd low shift control; Fig. 15 is a flowchart illustrating the solenoid timing control at the end of the 5th-to-3rd low shift control; Fig. 16 is a timing chart of the 5th-to-3rd low shift control; Fig. 17 is a main flowchart illustrating the 5th-to-4th low shift control; Fig. 18 is a flowchart illustrating the solenoid timing control at the start of the 5th-to-4th low shift control; Fig. 19 is a flowchart illustrating the solenoid timing control at the end of the 5th-to-4th low shift control; and Fig. 20 is a timing chart of the 5th-to-4th low shift control. A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. Referring to Fig. 1, a 5-speed automatic transmission 1 comprises a torque converter 4, a 3-speed main speed shift mechanism 2, a 3-speed auxiliary speed shift mechanism 5 and a differential 13 which are joined together and contained in an integrally built case. The torque converter 4 comprises a lockup clutch 4a and, therefore, transmits torque from an engine crank shaft to the main speed shift mechanism 2 by the fluid provided in the torque converter 4 or the mechanical connection achieved by the lockup clutch. The integral case rotatably supports a first shaft 3 (specifically, an input shaft 3a), a second shaft 6 (counter shaft 6a) disposed parallel to the first shaft 3, and a third shaft 14 (right and left shafts 14l, 14r). A valve body is disposed on the outer face of the case. The main speed shift mechanism 2 has a simple planetary gear unit 15 composed of a simple planetary gear set 9 and a double-pinion planetary gear set 7. The simple planetary gear set 9 is made up of a sun gear S formed of a long gear, a ring gear R1, and a carrier CR supporting pinions P1 meshed with the sun gear S and the ring gear R1. The pinions P1 are formed of long pinions. The double-pinion planetary gear set 7 is made up of the same sun gear S, a ring gear R2, and the same carrier CR, which supports the same pinions P1 meshed with the sun gear S and pinions P2 meshed with the ring gear R2 so that the pinions P1 and the pinions P2 mesh. The input shaft 3a cooperatively connected to the engine crank shaft by the torque converter 4 can be connected to the ring gear R1 of the simple planetary gear set 9 by a first (forward) clutch C1, and to the common sun gear S thereof by a second (direct) clutch C2. The sun gear S can be stopped directly by a first brake B1, and it can also be stopped by a second brake B2 by way of a first one-way clutch F1. The ring gear R2 of the double-pinion planetary gear set 7 can be stopped by a third brake B3 and also by a second one-way clutch F2. The carrier CR is firmly connected to a counter drive gear 8. The auxiliary speed shift mechanism 5 comprises.an output gear 16, a first simple planetary gear set 10 and a second simple planetary gear set 11 which are disposed in that order toward the rear along the axis of the counter shaft 6a forming the second shaft 6. The counter shaft 6a is rotatably supported on the integral case by bearings. The first and second simple planetary gear sets are of Simpson type. The ring gear R3 of the first simple planetary gear set 10 is connected to a counter driven gear 17 meshed with the counter drive gear 8. The sun gear S3 is rotatably supported by the counter shaft 6a. The pinions P3 are supported by a carrier CR3 formed by a flange firmly connected to the counter shaft 6a. The carrier CR3, supporting the pinions P3 also on the other side, is connected to an inner hub of a UD direct clutch C3. The sun gear S4 of the second simple planetary gear set 11 is connected to the sun gear S3 of the first simple planetary gear set. The ring gear R4 of the second simple planetary gear set 11 is connected to the counter shaft 6a. The UD direct clutch C3 is disposed between the carrier CR3 of the first simple planetary gear set and the coupled sun gears S3, S4, and the coupled sun gears S3, S4 can be stopped by a fourth brake B4 formed by a band brake. The carrier CR4 of the second simple planetary gear set supporting the pinions P4 can be stopped by a fifth brake B5. The differential 13 is disposed on the third shaft 14 formed of a front axle. The differential 13 has a ring gear 10 meshed with the output gear 16, and transmits rotations from the ring gear to right and left wheel axles 141, 14r. A hydraulic control circuit used in the 5-speed automatic transmission will be described with reference to Figs. 2 and 3, which shows the halves of one circuit drawing for limited area of the sheets, and the drawings of Fig. 2 and 3 connect to each other at (1) and (1), (2) and (2), (3) and (3), (4) and (4), (5) and (5), (6) and (6), (7) and (7), (8) and (8), (9) and (9). In the drawings, No. 1, No. 2, No. 4, No. 5 denote on-off controlled solenoid valves; and SLU, SLS, SLT denote linear solenoid valves. The solenoid valve No. 1 is normally open; the solenoid valve No. 2 is normally closed; the solenoid valve No. 3 is normally closed; the solenoid valve No. 4 is normally open; and the solenoid valve No. 5 is normally closed. The linear solenoid valve SLT is operated corresponding to the throttle opening based on the accelerator pedal operation by an operator. Figs. 2 and 3 further show a shift pressure control valve (pressure regulating valve) 20, a pressure relay valve (fluid pressure changeover valve) 21, a solenoid relay valve 22, a first (M1) shift valve 23 for the main speed shift mechanism, a second (M2) shift valve 25 for the main speed shift mechanism, a first (U1) shift valve 26 for the auxiliary speed shift mechanism, and a second (U2) shift valve 27 for the auxiliary speed shift mechanism. The shift pressure control valve 20 has a control fluid chamber 20a that receives a control fluid pressure from the linear solenoid valve SLS so that the shift pressure control valve 20 suitably regulates the line pressure from a port 20c of the shift pressure control valve and outputs the regulated pressure from an output port 20b. The control fluid chamber 21a of the pressure relay valve 21 receives an on-off fluid pressure from the solenoid valve No. 5 so that the pressure relay valve 21 correspondingly connects a line pressure input port 21b receiving the line pressure P L or a regulated pressure port 21c receiving the regulated pressure from the output port 20b to a main speed shift mechanism (M) -side output port 21d, and correspondingly connects the regulated pressure input port 21c or another line pressure input port 21e to an auxiliary speed shift mechanism (U)-side output port 21f. Figs . 2 and 3 further show a hydraulic pump 29, a primary regulator valve 30, a manual valve 31, a solenoid modulator valve 32, a B-4 control valve 33, a B-5 control valve 35, a 3-4 timing valve 36, a DR check valve 37, a B-2 check valve 39, and a B-4 check valve 40. Figs. 2 and 3 further show hydraulic servos C-1, C-2, C-3 for the corresponding clutches C1, C2, C3, and hydraulic servos B-1, B-2, B-3, B-4 for the corresponding brakes, an accumulator 41 for the servo C-1, and an accumulator 42 for the servo C-2. The operation of this embodiment will be described. First, the operation of the mechanical sections of the 5-speed automatic transmission will be described with reference to Figs. 1 and 4. For the first (1st) speed in the D (drive) range, the forward clutch C1 engages and the second one-way clutch F2 and the fifth brake B5 operate so that the ring gear R2 of the double-pinion planetary gear set and the carrier CR4 of the second simple planetary gear set are maintained in stoppage. In this state, the rotation of the input shaft 3a is transmitted to the ring gear R1 of the simple planetary gear set by the forward clutch C1. Since the ring gear R2 of the double-pinion planetary gear set is held in the stopped state, the common carrier CR rotates in the forward direction at a considerably reduced speed while the sun gear S idles in the reverse direction. Thus the main speed shift mechanism 2 is in the first speed state. The reduced rotation is then transmitted to the ring gear R3 of the first simple planetary gear set of the auxiliary speed shift mechanism 5 by the counter gears 8, 17. Since the carrier CR4 of the second simple planetary gear set of the auxiliary speed shift mechanism 5 is stopped by the fifth brake B5 thus establishing the first speed state of the auxiliary speed shift mechanism 5, the auxiliary speed shift mechanism 5 further reduces the reduced rotation from the main speed shift mechanism 2 and outputs the further reduced rotation from the output gear 16. For the second (2nd) speed, the second brake B2 (and the first brake B1) comes to operate in addition to the forward clutch C1. Furthermore, the second one-way clutch F2 is disengaged and the first one-way clutch F1 engages instead while the fifth brake B5 is maintained in operation. In this state, the common sun gear S is stopped by the second brake B2 and the first one-way clutch F1 (and the first brake B1), so that the rotation of the ring gear R1 of the simple planetary gear set transmitted from the input shaft 3a by the forward clutch C1 rotates the carrier CR at a reduced speed in the forward direction while idly rotating the ring gear R2 of the double-pinion planetary gear set in the forward direction. The reduced rotation is then transmitted to the auxiliary speed shift mechanism 5 by the counter gears 8, 17. The main speed shift mechanism 2 is thus in the second speed state while the auxiliary speed shift mechanism 5 is in the first speed state due to the engagement of the fifth brake B5. The combination of the second speed state and the first speed state achieves the second speed of the entire automatic transmission 1. For the third (3rd) speed, the forward clutch C1, the second brake B2 and the first one-way clutch F1 (and the first brake B1) are maintained in engagement, but the fifth brake B5 is released from engagement and the fourth brake B4 is engaged. That is, the state of the main speed shift mechanism 2 remains unchanged, so that rotation is transmitted to the auxiliary speed shift mechanism 5 by the counter gears 8, 17 in the same manner as in the second state. In the auxiliary speed shift mechanism 5, the rotation from the ring gear R3 of the first simple planetary gear set is outputted as second speed rotation from the carrier CR3 because the sun gear S3 is fixed. The combination of the second speed of the main speed shift mechanism 2 and the second speed of the auxiliary speed shift mechanism 5 achieves the third speed of the entire automatic transmission 1. For the fourth (4th) speed, the main speed shift mechanism 2 remains in the same state as for the second and third speeds, where the forward clutch C1, the second brake B2 and the first one-way clutch F1 (and the first brake B1) are engaged, but the auxiliary speed shift mechanism 5 releases the fourth brake B4 and engages the UD direct clutch C3. In this state, the ring gear R3 of the first simple planetary gear set and the sun gears S3, S4 are coupled to achieve locked-up rotation in which the planetary gear sets 10, 11 rotate together. The combination of the second speed of the main speed shift mechanism 2 and the locked-up rotation (third speed) of the auxiliary speed shift mechanism 5 results in the fourth speed rotation of the entire automatic transmission outputted from the output gear 16. For the fifth (5th) speed, the forward clutch C1 and the direct clutch C2 engage to transmit the rotation of the input shaft 3 to both the ring gear R1 of the simple planetary gear set and the sun gear S. The main speed shift mechanism 2 thus undergoes locked-up rotation where its gear units rotate together. The auxiliary speed shift mechanism 5 remains in the locked-up rotation state, where the UD direct clutch C3 is engaged. The combination of the third speed (locked-up rotation) of the main speed shift mechanism 2 and the third speed (locked-up rotation) of the auxiliary speed shift mechanism 5 results in the fifth speed rotation of the entire automatic transmission outputted from the output gear 16. Furthermore, this automatic transmission provides intermediate speeds that are put in effect during downshifts for acceleration and the like, that is, a third speed low (that is, an intermediate speed between the third and second speeds, referred to as "2.5th speed") and a fourth speed low (that is, an intermediate speed between the fourth and third speeds, referred to as "3.5th speed"). For the 2.5th speed, the forward clutch C1 and the direct clutch C2 engage (the second brake B2 also engages but the braking effect is canceled by the one-way clutch F1). Thus the main speed shift mechanism 2 assumes the third speed state, where the planetary gear unit 15 is locked up. The auxiliary speed shift mechanism 5 engages the fifth brake B5, assuming the first speed state. With the combination of the third speed state of the main speed shift mechanism 2 and the first speed state of the auxiliary speed shift mechanism 5, the automatic transmission 1 achieves the intermediate speed whose gear ratio is set between those of the second and third speeds. For the 3.5th speed, the main speed shift mechanism 2 assumes the third speed (locked-up) state as for the 2.5th speed, where the forward clutch C1 and the direct clutch C2 are engaged. On the other hand, the auxiliary speed shift mechanism 5 assumes the second speed state, where the fourth brake B4 is engaged and the sun gear S3 of the first simple planetary gear set 10 is fixed. With the combination of the third speed state of the main speed shift mechanism 2 and the second speed state of the auxiliary speed shift mechanism 5, the automatic transmission 1 achieves the intermediate speed whose gear ratio is set between those of the third and fourth speeds. For the R (reverse) range, the automatic transmission 1 switches over two different states at a critical vehicle speed of 7 km/h. If the vehicle is running forward at 7 km/h or higher, the main speed shift mechanism 2 assumes a free rotating state as in the N (neutral) range. If the vehicle speed is not higher than 7 km/h, that is, if the vehicle is substantially stopped, the direct clutch C2 and the third brake B3 engage, and the fifth brake B5 also engages. In this state, the rotation of the input shaft 3a is transmitted to the sun gear S by the direct clutch C2, and the carrier CR reversely rotates while the ring gear R1 of the simple planetary gear set idles also in the reverse direction since the ring gear R2 of the double-pinion planetary gear set is stopped by the third brake B3. The reverse rotation of the carrier CR is transmitted to the auxiliary speed shift mechanism 5 by the counter gears 8, 17. The auxiliary speed shift mechanism 5 is maintained in the first speed state, where the carrier CR4 of the second simple planetary gear set is stopped in both directions by the fifth brake B5. With the combination of reverse rotation of the main speed shift mechanism 2 and the first speed rotation of the auxiliary speed shift mechanism 5, the automatic transmission 1 outputs reverse speed-reduced rotation from the output shaft 16. The shift processes, a feature of the invention, will next be described. Since shift occurs in many combinations of speeds as shown in "Shift" in Fig. 4, the description will be made for typical shift processes, that is, the 2nd-to-3rd shift, the 3rd-to-2nd shift, the 5th-to-2.5th (3rd low) shift, and the 5th-to-3.5th (4th low) shift. Other shift processes will be omitted from the description since they are similar to the typical shift processes. Referring to Fig. 5, the 2nd-to-3rd shift control performs starting solenoid timing control (F3), regulated pressure (P PC 2-3) control (F4), and then fifth brake (P B5C 2-3) hydraulic control (F5) . When the shift control is completed (F7), ending solenoid timing control (F8) for maintaining the third speed state is performed before the end of the 2nd-to-3rd shift control. In the 2nd-to-3rd shift control, the main speed shift mechanism 2 remains in the same state but the auxiliary speed shift mechanism 5 releases the fifth brake B5 and engages the fourth brake B4, as described above. Since no one-way clutch is involved in this shift control, the 2nd-to-3rd shift is performed as a so-called clutch-to-clutch shift, which requires the pressure regulating control for the engagement changeover in order to prevent shift shocks. Since in this hydraulic control circuit, the shift pressure control valve 20 is used commonly for all the hydraulic servos of the main speed shift mechanism and all the hydraulic servos of the auxiliary speed shift mechanism, the determination of the start of the 2nd-to-3rd shift by the control unit is immediately followed by the switching-on of the fifth solenoid valve No. 5 (F10) as shown in Figs. 6 and 8. Then the normally closed valve No. 5 is released so that the pressure relay valve (fluid pressure changeover valve) 21 switches to the position indicated by the left-hand side half of its illustration shown in Fig. 3. In this position, the line pressure port 21b communicates with the main speed shift mechanism-side output port 21d and the regulated pressure port 21c communicates with the auxiliary speed shift mechanism-side output port 21f. When this state is established, the control process goes on, as shown in Fig. 6, to start supplying the fourth brake hydraulic servo B-4 with the regulated fluid pressure from the shift pressure control valve 20 based on the linear solenoid valve SLS (F11), and start controlling the fifth brake hydraulic servo B-5 by using another linear solenoid valve SLU (F12), and start measuring time following the switching-on of the solenoid valve No. 5 (F13). After a predetermined time has elapsed (F16), the solenoid valve No. 4 is switched on (F17), and then the starting solenoid timing control ends. The control fluid pressure (SLS pressure) from the linear solenoid valve SLS is initially maintained at a relatively high constant level, and the control fluid pressure (SLU pressure) from the other linear solenoid valve SLU is also initially at a relatively high constant level, as indicated in Fig. 8. Before the shift control starts, that is, when the solenoid valve No. 5 is off, the pressure relay valve 21 is in the position indicated by the right-hand half of its illustration. In this position, the line pressure port 21e communicates with the auxiliary speed shift mechanism-side port 21f, but the line pressure is blocked by the U1 shift valve 26. Furthermore, the regulated pressure port 21c communicates with the main speed shift mechanism-side output port 21d, so that the regulated pressure is supplied to the second brake hydraulic servo B-2 (B2 pressure). The U1 shift valve 26 supplies the line pressure to the fifth brake hydraulic servo B-5 (B5 pressure) . When the solenoid valve No. 5 is switched on, the pressure relay valve 21 switches to the position indicated by the left half of its illustration, so that the line pressure port 21b communicates with the main speed shift mechanism-side output port 21d to make the B2 pressure equal to the line pressure, and so that the regulated port 21c communicates with the auxiliary speed shift mechanism-side output port 21f, while the U1 shift valve is in the position indicated by the left half of its illustration. Thereby, although the control fluid pressure (SLS pressure) of the linear solenoid valve SLS is relatively high, the fluid pressure (B4 pressure) of the fourth brake hydraulic servo B-4 is supplied with no additional fluid pressure since the regulated pressure is blocked by the U1 shift valve 26. The control pressure (SLU pressure) from the other linear solenoid valve SLU is relatively high and supplied to the solenoid relay valve 22, the ports 21g, 21h of the pressure relay valve 21, and the control fluid chamber 35a of the B-5 control valve 35. By suitable control, the output port 35b of the B-5 control valve 35 outputs a suitably regulated pressure. This regulated pressure is blocked by the U1 shift valve 26, which is in the position indicated by the left half of its illustration. When the solenoid valve No. 4 is switched on after a predetermined time has elapsed, the U1 shift valve 26 is switched to the position indicated by the right half of its illustration, thus establishing the third speed state. At the same time, the regulated pressure control of the linear solenoid valves SLU, SLS is performed. Then the regulated pressure from the output port 21f of the pressure relay valve 21 acts on the fourth brake hydraulic servo B-4 (B4 pressure), so that the B4 pressure is gradually increased by the control fluid pressure (SLS pressure) of the linear solenoid valve SLS. The release regulated pressure from the B-5 control valve 35 based on the linear solenoid pressure (SLU pressure) acts on the fifth brake hydraulic servo B-5 (B5 pressure), so that the B5 pressure gradually decreases. Thereby, the fourth brake B4 smoothly engages and the fifth brake B5 is released, thus establishing the third speed state. The ending solenoid timing control is then performed as illustrated in Fig. 7. This control operation controls the discontinuation of the regulated pressure supply from the shift pressure control valve 20 to the fourth brake hydraulic servo B-4 (F19), and controls the discontinuation of the fluid pressure supply to the fifth brake hydraulic servo B-5 (F20) . After a predetermined time elapses on the basis of a timer (F22, F23), the solenoid valve No. 5 is switched off (F24), and the 2nd-to-3rd shift is completed. In this state, the B5 pressure based on the control fluid pressure SLU is low, for example, zero, but the B4 pressure based on the control fluid pressure SLS is sufficiently high so that the fourth brake B4 can be completely engaged. When the solenoid valve No. 5 is switched off, the pressure relay valve 21 switches to the position indicated by the right half of its illustration, so that the line pressure from the port 21e is conducted to the auxiliary speed shift mechanism-side output port 21f and the regulated pressure from the port 21c is conducted to the main speed shift mechanism-side output port 21g. Thereby, the B2 is supplied with the regulated pressure based on the SLS pressure, and the B4 pressure becomes equal to the line pressure, so that the fourth brake hydraulic servo B-4 receives the line pressure that is achieved by a high pressure corresponding to the brake B4 of the auxiliary speed shift mechanism 5, which is disposed downstream in transmission flow from the main speed shift mechanism 2 currently assuming the second speed state, and receives higher torques. The fourth brake hydraulic servo B-4 thus reliably maintains the brake B4 in engagement. The 3rd-to-2nd shift control will be described with reference to Figs. 9 to 12, where Fig. 9 shows the main flowchart, Fig. 10 shows the flowchart of the starting solenoid timing control, Fig. 11 shows the flowchart of the ending solenoid timing, and Fig. 12 shows the timing chart. The control unit determines whether to start the 3rd-to-2nd shift control on the basis of the throttle opening and the vehicle speed. In this situation, the automatic transmission is in the third speed state, where solenoid valve No. 5 is off and the solenoid valve No. 4 is on so that the line pressure from the pressure relay valve 21 is supplied to the fourth brake hydraulic servo B-4 (B4 pressure) and the regulated fluid pressure based on the control fluid pressure SLS is supplied to the second brake hydraulic servo B-2 (B2 pressure) . In addition, the zero pressure based on the SLU pressure is blocked by the solenoid relay valve 22, and the fifth brake hydraulic servo B-5 (B5 pressure) is drained through the solenoid relay valve 22. When the solenoid valve No. 5 is switched on, the pressure relay valve 21 switches to the position indicated by the left half of its illustration, so that the line pressure from the port 21b is supplied as the B2 pressure through the output port 21d and the regulated pressure from the port 21c is supplied as the B4 pressure by way of the port 21f, the U1 shift valve 26, the U2 shift valve 27 and the B-4 check valve 40. In addition, the regulated pressure (SLS pressure) is supplied to the upper control fluid chamber 22a of the solenoid relay valve 22 so that the solenoid valve 22 switches to the position indicated by the right half of its illustration. In this state, by suitable control of the linear solenoid valves SLU, SLS based on signals from the control unit, the SLS pressure gradually decreases so that the B4 pressure gradually decreases. On the other hand, the SLU pressure gradually increases after remaining at a fixed level, so that the B5 pressure gradually increases. Thereby, the fourth brake B4 is gradually released and the engaging force of the fifth brake B5 gradually increases, to smoothly perform the 3rd-to-2nd shift by the engagement changeover. After it is determined that the 3rd-to-2nd shift is completed, the solenoid valve No. 4 is switched off so that the U1 shift valve switches to the position indicated by the left half of its illustration. Thereby the regulated fluid pressure from the B-5 control valve 35 based on the control fluid pressure SLU is discontinued, so that the SLU pressure increases and the B5 pressure is supplied with the line pressure. In addition, the regulated fluid pressure from the shift pressure control valve 20 based on the control fluid pressure SLS is discontinued, so that the SLS pressure increases and the B4 pressure is drained. After the measurement of a predetermined time length by a timer ends, the solenoid valve No. 5 is switched off so that the pressure relay valve 21 switches to the position indicated by the right half of its illustration. In this state, the line pressure conducted by way of the input port 21e and the output port 21f is blocked by the U1 shift valve 26, and the regulated fluid pressure based on the control fluid pressure SLS is added to the B2 pressure through the port 21c and the output port 21d, and the B4 pressure is maintained in the drained state. In cases, as in the 3rd-to-2nd shift, where the release pressure control of the friction engagement element (B4) of the auxiliary speed shift mechanism 5 is to be performed on the basis of the control fluid pressure SLS, the pressure relay valve 21 is controlled so that the regulated pressure is raised for a predetermined time on the basis of the SLS pressure, while the auxiliary speed shift mechanism-side shift valve 26 or 27 is switched in position so that the regulated pressure is discontinued. Subsequently the pressure relay valve 21 is switched in position. Thereby, the regulated pressure based on the SLS pressure becomes a high pressure, so that the pressure relay valve 21 switches in position. Thus a high pressure is applied to a predetermined friction engagement element of the main speed shift mechanism that has been receiving the line pressure, so as to prevent low pressure on that friction engagement element. In the control of shifts between the 5th and 4th speeds, which are frequently performed in automatic transmissions, the shift control is performed by engaging or releasing the direct clutch C2 of the main speed shift mechanism 2 while maintaining the auxiliary speed shift mechanism 5 in the same state. In the control, the solenoid valve No. 5 remains off, and the pressure relay valve 21 is maintained in the position indicated by the left half of its illustration. The pressure relay valve 21 thus remains in the aforementioned state, whether it is during the 4th-to-5th shift or the 5th-to-4th shift, or whether the 4th or 5th speed is maintained. The time lag is thus reduced. During the shift operation from the 5th speed to the 4th speed, the release control of the direct clutch C2 of the main speed shift mechanism 2 is performed, and the third clutch C3 of the auxiliary speed shift mechanism 5 is maintained in engagement and the second brake B2 of the main speed shift mechanism 2 is also maintained in engagement. In the 5th speed state of this operation, the M2 shift valve 25 is in the position indicated by the left half of its illustration, where the input port 25a communicates with the output port 25b so that the fluid pressure from the shift pressure control valve 20 is supplied to the hydraulic servo C-2 by way of the ports 21c, 21d of the relay valve 21 and the M1 shift valve 23. In the 4th speed state, the M2 shift valve 25 is switched to the position indicated by the right half of its illustration, where the output port 25b communicates with the drain port. As stated above, it is necessary to continue supplying the line pressure to the second brake hydraulic servo B-2 in this state. However, the positional switching of the M2 shift valve 25 disconnects the input port 25c and the output port 25d from each other, so that the line pressure supply from the fluid passage e to the hydraulic servo B-2 discontinues. In addition, the switching of the M2 shift valve 25 establishes communication between the regulated pressure input port 25a and the output port 25d, so that the fluid pressure from the shift pressure control valve 20 is supplied to the hydraulic servo B-2. As a result, the hydraulic servo B-2 will undergo a low pressure. To prevent this problem, the B-2 check valve 39 constituted by a two-way check valve is provided. One input port 39a of the check valve 39 is connected to a fluid passage extending from the output port 25d, and the other input port 39b is connected to a fluid passage extending from the output port 26b of the U1 shift valve 26. The output port 39c of the check valve 39 is connected to the hydraulic servo B-2. Thereby, although the line pressure supply to the input port 39a of the B-2 check valve 39 discontinues, the output port 26b of the U1 shift valve 26 is supplied with the line pressure from the output port 21f of the pressure relay valve 21. This line pressure is conducted to the third clutch hydraulic servo C-3 through the ports 27a, 27b of the U2 shift valve 27, and is also conducted to the other input port 39b of the B-2 check valve 39. Therefore, even if the M2 shift valve 25 is switched in position, the line pressure, serving as the C-3 engaging pressure, is supplied to the hydraulic servo B-3 through the other input port 39b of the B-2 check valve 39, so that the second brake B2 is maintained in engagement. The 5th-to-3rd low (2.5th) shift control will be described with reference to Figs. 13-16, where Figs. 13-15 are flowcharts and Fig. 16 is a timing chart. In the 5th speed state, the main speed shift mechanism 2 is locked up by the forward clutch C1 and the direct clutch C2 being engaged, and the auxiliary speed shift mechanism 5 is also locked up by the UD direct clutch C3 being engaged. For the 3rd low speed, the main speed shift mechanism 2 is maintained in the locked-up state, but the auxiliary speed shift mechanism 5 becomes the first speed state, where the fifth brake B5 is engaged. Thus, the 5th-to-3rd low shift control maintains the main speed shift mechanism 2 in the current state, and performs the engagement changeover control where the auxiliary speed shift mechanism 5 releases the UD clutch C3 and engages the fifth brake B5. If a driver depresses the accelerator pedal for a quick acceleration to, for example, pass another vehicle, during steady traveling at the 5th speed, the control unit determines that the 5th-to-3rd low shift control is to be started. In the 5th speed state at this moment of determination, the solenoid valve No. 5 is off, so that the regulated fluid pressure from the shift pressure control valve 20 based on the control fluid pressure SLS is supplied to the direct clutch hydraulic servo C-2 (C2 pressure) through the port 21c and the output port 21d, and so that the line pressure is supplied to the UD direct clutch hydraulic servo C-3 (C3 pressure) through the port 21e, the output port 21f, the U1 shift valve 26 set in the position indicated by the right half of its illustration, and the U2 shift valve 27 set in the position indicated by the right half of its illustration, and so that the fifth brake hydraulic servo B-5 (B5 pressure) is drained through the solenoid relay valve 22 set in the position indicated by the left half of its illustration. When the solenoid valve No. 5 is switched on in this state, the pressure relay valve 21 switches to the position indicated by the left half of its illustration, so that the regulated pressure based on the control pressure SLS is applied through the U1 shift valve to the upper control fluid chamber 22a of the solenoid relay valve 22 to switch the valve to the position indicated by the right half of its illustration. As a result, the control fluid pressure SLU from the linear solenoid valve SLU is applied to the control fluid chamber 35a of the B-5 control valve 35 for suitable pressure regulation on the valve, and the regulated fluid pressure is then supplied as the B5 pressure through the U1 shift valve 26 set in the position indicated by the right half of its illustration, and the solenoid relay valve 27 set in the position indicated by the right half of its illustration. In addition, since the line pressure port 21b is now in communication with the output port 21d, the line pressure acts as the C2 pressure. Furthermore, since the regulated pressure port 21c is in communication with the output port 21f, the regulated pressure from the shift pressure control valve 20 based on the control fluid pressure SLS acts as the C3 pressure, conducted through the U1 shift valve 26 set in the position indicated by the right half of its illustration, and the U2 shift valve 27 set in the position indicated by the right half of its illustration. Then, the release pressure regulation of the control fluid pressure SLS gradually decreases the C3 pressure. On the other hand, the control fluid pressure SLU gradually increases after remaining at a fixed low level, so that the B5 pressure gradually increases after remaining at a fixed low level. Thereby, the UD direct clutch C3 is gradually released, and the fifth brake B5 receives gradually increasing engagement pressures after the clutch plate has been maintained in contact for a predetermined time. The 5th-to-3rd low shift is thus performed smoothly. After the completion of this shift is determined, the solenoid valve No. 3 is immediately switched on. Thereby, the U2 shift valve 27 switches from the position indicated by the right half to the position indicated by the left half of its illustration, to drain the C3 pressure. In addition, since the line pressure supply to the upper control fluid chamber 26a of the U1 shift valve 26 is blocked, the valve 26 switches to the position indicated by the left half of its illustration so that the line pressure acts on the B5 pressure. Thereby the fifth brake B5 and the direct clutch C2 are reliably engaged and the UD direct clutch C3 is reliably released, achieving the 3rd low speed state without a failure. After a predetermined time elapses on the basis of a timer, the solenoid valve No. 5 is switched off, so that the pressure relay valve 21 thereby switches to the position indicated by the right half of its illustration. Thereby the regulated pressure based on the control fluid pressure SLS is applied as the C2 pressure through the port 21c and the output port 21d. Although the line pressure supply through the port 21e and the output port 21f is blocked by the U1 shift valve 26, the line pressure is applied as the B5 pressure as described above. Thus the 3rd low speed state is reliably maintained. The 5th-to-4th low shift control will be described. The flow of this control is substantially the same as the above-described control flow, except that the P B4C 5-4low hydraulic control step (F25) and the P B4C =P ST B4 step (F26) control the B4 pressure based on the linear solenoid valve SLT. The following description will be made with reference to the timing chart of Fig. 20. During the 5th-to-4th low shift, the main speed change mechanism 2 remains locked up, and the auxiliary speed change mechanism 5 shifts to the second speed state by releasing the UD direct clutch C3 and engaging the fourth brake B4. At the time of the determination of the start of the 5th-to-4th low shift, the regulated fluid pressure based on the control fluid pressure SLS acts as the C2 pressure, and the line pressure acts as the C3 pressure, as described above. In addition, the pressure (B4 pressure) of the fourth brake hydraulic servo B-4 is drained through the 3-4 timing valve 36. When the solenoid valve No. 4 is switched on, the control fluid pressure is supplied to the lower control fluid chamber 22b of the solenoid relay valve 22 through the U2 shift valve 27 set in the position indicated by the right half of its illustration, to switch the solenoid relay valve 22 to the position indicated by the left half of its illustration. After a predetermined time elapses on the basis of a timer, the solenoid valve No. 5 is switched on to switch the pressure relay valve 21 to the position indicated by the left half of its illustration. In this state, the regulated pressure based on the control pressure SLS acts as the C3 pressure, conducted through the U1 shift valve 26 set in the position indicated by the right half of its illustration, and the U2 shift valve 27 set in the position indicated by the right half of its illustration. The line pressure acts as the C2 pressure, conducted through the ports 21b, 21d. Since the regulated pressure, lower than the line pressure, is applied to the C3 pressure and acts on the lower control fluid chamber 36a of the 3-4 timing valve 36, the valve 36 is switched to the position indicated by the right half of its illustration, by the line pressure acting on the upper control fluid chamber. The throttle linear solenoid valve SLT is suitably controlled by a signal from the control unit, so that the control fluid pressure (SLT pressure) from its output port acts on the control fluid chamber 33a of the B-4 control valve 33 for the pressure regulation on the valve 33. The regulated fluid pressure from the valve 33 is supplied to the B4 pressure through the U1 shift valve 26 set in the position indicated by the right half of its illustration, the B-4 check valve 40, and the 3-4 timing valve 36 set in the position indicated by the right half of its illustration. The control fluid pressure SLS is gradually decreased so that the C3 pressure gradually decreases, and the control fluid pressure SLT is gradually increased after being maintained at a fixed low pressure so that the B-4 pressure gradually increases after remaining at a fixed low pressure. Thereby, the UD direct clutch C3 is gradually released and the engaging force of the fourth brake B4 is smoothly increased after the brake B4 has remained in the contact state. When the control unit determines that the 5th-to-4th low shift is completed, the control unit switches on the solenoid valve No. 3. The U2 shift valve 27 thereby switches to the position indicated by the left half of its illustration, so that the control fluid pressure from the solenoid valve No. 4 changes its acting object, from the lower control fluid chamber 22b of the solenoid relay valve 22 to the upper control fluid chamber 26a of the U1 shift valve 26. Thus, the solenoid relay valve 22 is switched to the position indicated by the right half of its illustration, and the U1 shift valve 26 is maintained in the position indicated by the right half of its illustration. In this state, the regulated pressure from the output port 21f of the pressure relay valve 21 based on the control fluid pressure SLS is conducted through the U1 shift valve 26 set in the position indicated by the right half of its illustration, and the U2 shift valve 27 set in the position indicated by the left half of its illustration, and the regulated fluid pressure is blocked by the B-4 check valve 40. The regulated pressure based on the control fluid pressure SLT is applied as the B4 pressure, through the B-4 check valve 40 and the 3-4 timing valve 36. The B4 pressure increases by an increase of the control fluid pressure SLT. The C3 pressure is drained through the U2 shift valve 27. After a predetermined time elapses on the basis of a timer, the solenoid valve No. 5 is switched off. The pressure relay valve 21 thereby switches to the position indicated by the right half of its illustration, so that the line pressure from the port 21e is outputted from the output port 21f and applied as the B4 pressure, and so that the fluid pressure based on the control fluid pressure SLS is applied as the C2 pressure through the ports 21c, 21d. The aforementioned hydraulic control circuit U comprises a first fluid passage a for supplying fluid pressure from the pressure control valve 20 to the pressure relay valve 21, second fluid passages b, c for supplying fluid pressure from the pressure relay valve 21 to the first speed shift mechanism-side shift valves 23, 25 and to the second speed shift mechanism-side shift valves 26, 27, and third fluid passages d 1 , d 3 extending from the first fluid passage a to the second fluid passages b, c, not via the pressure relay valve 21. The third fluid passages d1, d3 are provided with one-way check valves 45, 46 for connecting first fluid passage a to the second fluid passages b, c, respectively, when the fluid pressure in the second fluid passages b, c becomes lower than the fluid pressure in the first fluid passage a. It is normally necessary to provide sufficiently large torque capacity for predetermined friction engagement elements by supplying thereto line pressure after the engagement control of the fluid pressure supply to the friction engagement elements using pressure control valves. The above-described hydraulic control circuit U is able to constantly provide sufficiently large torque capacity by preventing closure of either of the second fluid passages b, c during the switchover operation of the fluid pressure changeover valve, which would otherwise result in low pressure during the pressure changeover. The hydraulic control circuit U is further provided with the M1 shift valve 23 for achieving a low speed (for example, the first or second speed) of the main speed shift mechanism 2, and the second speed shift mechanism-side shift valves 26, 27 for achieving a predetermined speed (for example, the second and third speeds, or the third speed) of the auxiliary speed shift mechanism 5, and the solenoid valves No. 3, No. 4 for the changeover control of the second speed shift mechanism-side shift valves. The solenoid valves No. 3, No. 4 are disposed in the fluid passages e, f connecting the fluid pressure source to the second speed shift mechanism-side shift valves. The M1 shift valve 23 is disposed in the fluid passages e, f, and closes the fluid passages for the low speed of the main speed shift mechanism. Normally, during the low speed of the main speed shift mechanism 2, the friction engagement elements engaged for a higher speed of the auxiliary speed shift mechanism receive increased loads because the main speed shift mechanism 2 inputs increased torques to the auxiliary speed shift mechanism. However, the above-described hydraulic control circuit U prevents the auxiliary speed shift mechanism 5 from shifting to a higher speed during a low speed of the main speed shift mechanism, by discontinuing the source pressure on the solenoid valves No. 3, No. 4, which change the position of the shift valves 26, 27 for controlling the shift of the auxiliary speed shift mechanism (for example, to the second and third speed, or to the third speed). The hydraulic control circuit U thus increases the service life of the high-speed friction engagement elements of the auxiliary speed shift mechanism. The M1 shift valve 23 in the fluid passages e, f is disposed between the fluid pressure source and the solenoid valves No. 3, No. 4. In a construction in which the first speed shift mechanism-side shift valves are disposed in fluid passages between the solenoid valves No. 3, No. 4 and the second speed shift mechanism-side shift valves, the flow loss would become large if the connection between the fluid pressure source and the drain fluid passage should be established by the solenoid valve. However, this hydraulic control circuit U disconnects the fluid pressure source from the drain fluid passage by the first speed shift mechanism-side shift valves to reduce flow loss when either of the solenoid valves is in the position for the connection between the fluid pressure source and the drain fluid passage. In the hydraulic control circuit U, a fluid passage i connecting to the line pressure port 21e of the pressure relay valve 21 is connected to the ports 31a, 31b of the manual valve 31 by the DR check valve 37. Thereby, if the manual valve 31 is set in the P (parking) range, the ports 31a, 31b are drained, so that the line pressure fluid passage i is opened. For the P range, it is necessary to put the various hydraulic servos in the release state. However, in some cases, the shift pressure control valve 20 is maintained in a low pressure state due to the relation between its spool and spring, and the low pressure is supplied to the hydraulic servos through the ports 21c, 21d of the pressure relay valve 21 set in the position indicated by the right half of its illustration, the fluid passage b, and the M1 and M2 shift valves 23, 25. In this construction, however, since the regulated pressure fluid passage a connecting through the ports 21d, 21c of the pressure relay valve 21 to the (fifth) fluid passage b connecting to the hydraulic servos has a larger fluid pressure than the (sixth) fluid passage c, the check valve 45 becomes open to connect the regulated pressure fluid passage a to the fluid passage c via the (seventh) fluid passage d 2 . The fluid passage c is connected to the line pressure (fourth) fluid passage i, which is open, through the ports 21f, 21e of the pressure relay valve 21. Thereby, if the fluid pressure in the fifth fluid passage b connecting to the regulated fluid passage a is higher than the fluid pressure in the sixth fluid passage c connecting to the open fourth fluid passage i, the check valve 45 becomes open so that the fluid passage b connecting to the various hydraulic servos is drained through the fluid passage i. This construction thus ensures the drainage of the hydraulic servos even if a low pressure occurs in the shift pressure control valve 20. According to the embodiment, although in the above description, the pressure relay valve 21 is set in the position indicated by the right half of its illustration by the solenoid valve No. 5 being off in the P range, the drainage of the hydraulic servo will also be ensured if the pressure relay valve 21 is in the position indicated by the left half of its illustration. More specifically, if a low pressure from the regulated pressure fluid passage caused by the pressure control valve 20 is supplied to the hydraulic servos through the ports 21c, 21f of the relay valve 21 and the U1 and U2 shift valves 26, 27, the check valve 46 becomes open since the regulated pressure fluid passage a has a higher pressure than the (sixth) fluid passage b. Therefore, the regulated pressure fluid passage a connecting to the (fifth) fluid passage c through the ports 21f, 21c becomes connected to the (sixth) fluid passage b through the (seventh) fluid passage d 2 provided with the check valve 46. The fluid passage b is connected to the line pressure (fourth) fluid passage i, which is open, through the ports 21d, 21b.

Claims: A hydraulic control apparatus for an automatic transmission, comprising: a main transmission mechanism (2) and an auxiliary transmission mechanism (5) each of which has a plurality of friction engagement elements; and a hydraulic control circuit for achieving a plurality of speeds by supplying fluid pressure to and discharging fluid pressure from a hydraulic servo unit that operates said plurality of friction engagement elements, the hydraulic control circuit comprising: a main transmission mechanism-side hydraulic servo for controlling the friction engagement elements provided in the main transmission mechanism (2); a main transmission mechanism-side shift valve for switching over the supply and discharge of fluid pressure with respect to the main transmission mechanism-side hydraulic servo in accordance with various speed signals; an auxiliary transmission mechanism-side hydraulic servo for controlling the friction engagement elements provided in the auxiliary transmission mechanism (5); an auxiliary transmission mechanism-side shift valve for switching over the supply and discharge of fluid pressure with respect to the auxiliary transmission mechanism-side hydraulic servo in accordance with various speed signals; a fluid pressure source for generating a line pressure; a pressure regulating valve (20) for regulating the line pressure from the fluid pressure source; characterized by a fluid pressure changeover valve (21) for selectively supplying the line pressure and the fluid pressure from the pressure regulating valve (20) to the main transmission mechanism-side shift valve and the auxiliary transmission mechanism-side shift valve. A hydraulic control apparatus for an automatic transmission according to claim 1, wherein the fluid pressure changeover valve (21) comprises: a regulated pressure input port (21c) for inputting the fluid pressure from the pressure regulating valve (20), a line pressure input port (21e) for inputting the line pressure from the fluid pressure source, a first output port (21d) for outputting to the main transmission mechanism-side shift valve, and a second output port (21f) for outputting to the auxiliary transmission mechanism-side shift valve; and wherein the fluid pressure changeover valve (21) changes over a position where the regulated pressure input port (21c) and the first output port (21d) communicate with each other and the line pressure input port (21e) and the second output port (21f) communicate with each other, and a position where the regulated pressure input port (21c) and the second output port (21f) communicate with each other and the line pressure input port (21e) and the first output port (21d) communicate with each other. A hydraulic control apparatus for an automatic transmission according to claim 1 or 2, wherein the main transmission mechanism (2) receives, as input, drive force from an engine and achieves a plurality of reduced speeds, and the auxiliary transmission mechanism (5) receives, as input, output from the main transmission mechanism; and wherein the auxiliary transmission mechanism-side shift valve supplies the fluid pressure from the fluid pressure source to the hydraulic servo that operates a friction engagement element that is engaged when the auxiliary transmission mechanism (5) is at a predetermined speed; and wherein at least when the auxiliary transmission mechanism (5) completes shifting to a predetermined speed, the fluid pressure changeover valve (21) switches from the position where the fluid pressure from the pressure regulating valve (20) is supplied to the auxiliary transmission mechanism-side shift valve, to the position where the line pressure from the fluid pressure source is supplied to the auxiliary transmission mechanism-side shift valve. A hydraulic control apparatus for an automatic transmission according to claim 1 or 2, wherein the main transmission mechanism (2) achieves the highest speed and the speed next to the highest by engaging or releasing a predetermined friction engagement element of the main transmission mechanism (2); and wherein at least after the shift to the highest speed is completed, the fluid pressure changeover valve (21) is held in the same position as assumed during the shift to the highest speed. An apparatus according to any one of claims 1 to 4, wherein when speed is to be shifted from a first speed to a second speed by releasing at least a predetermined friction engagement element of the auxiliary transmission mechanism (5) from engagement while a predetermined friction engagement element of the main transmission mechanism is maintained in engagement: the main transmission mechanism-side shift valve maintains the predetermined friction engagement element of the main transmission mechanism (2) in engagement by supplying fluid pressure to the hydraulic servo that operates the predetermined friction engagement element, during the operation of the shift from said first speed to said second speed, and the auxiliary transmission mechanism-side shift valve changes over the supply of the fluid pressure from the pressure regulating valve (20) to the hydraulic servo that operates the predetermined friction engagement element of the auxiliary transmission mechanism (5) and the discontinuation thereof, during the operation of the shift from said first speed to said second speed, and the fluid pressure changeover valve (21) supplies the main transmission mechanism-side shift valve with the fluid pressure from the pressure regulating valve (20) raised within a predetermined time, after a predetermined time following the discontinuation of supply of the fluid pressure from the pressure regulating valve (20) by the auxiliary transmission mechanism-side shift valve, at the time of completion of the shift from said first speed to said second speed. An apparatus according to any one of claims 1 to 5, further comprising: a first fluid passage for supplying the fluid pressure from the pressure regulating valve (20) to the fluid pressure changeover valve (21); a second fluid passage for supplying the fluid pressure from the fluid pressure changeover valve (21) to at least one of the main transmission mechanism-side shift valve and the auxiliary transmission mechanism-side shift valve; and a third fluid passage that connects the first fluid passage to the second fluid passage without going through the fluid pressure changeover valve (21), the third fluid passage being provided with connecting means for connecting the first fluid passage and the second fluid passage to each other when the fluid pressure in the second fluid passage becomes less than the fluid pressure in the first fluid passage. An apparatus according to any one of claims 1 to 6, further comprising: a predetermined main transmission mechanism-side shift valve for achieving a low speed of the main transmission mechanism (2); a predetermined auxiliary transmission mechanism-side shift valve for achieving a predetermined speed of the auxiliary transmission mechanism (5); and a solenoid valve for controlling the changeover of the auxiliary transmission mechanism-side shift valve, the solenoid valve being disposed in a fluid passage that connects the fluid pressure source and the auxiliary transmission mechanism-side shift valve in communication, the predetermined main transmission mechanism-side shift valve being disposed in said fluid passage, the predetermined main transmission mechanism-side shift valve closing said fluid passage when a low speed is maintained in the main transmission mechanism (2). An apparatus according to any one of claims 1 to 6, further comprising: a predetermined main transmission mechanism-side shift valve for achieving a low speed of the main transmission mechanism (2); a predetermined auxiliary transmission mechanism-side shift valve for achieving a predetermined speed of the auxiliary transmission mechanism (5); and a solenoid valve for controlling the changeover of the auxiliary transmission mechanism-side shift valve, the solenoid valve being disposed in a fluid passage that connects the fluid pressure source and the auxiliary transmission mechanism-side shift valve in communication, the predetermined main transmission mechanism-side shift valve being disposed in said fluid passage, the predetermined main transmission mechanism-side shift valve closing said fluid passage when a low speed is maintained in the main transmission mechanism (2), the predetermined main transmission mechanism-side shift valve in the fluid passage being provided between the fluid pressure source and the solenoid valve. An apparatus according to any one of claims 1 to 8, further comprising: changeover means for changing the connection of a fourth fluid passage that is provided between the fluid pressure source and the pressure regulating valve (20) and that connects to an input port of the pressure regulating valve (20), over to a line pressure supply passage extending from the fluid pressure source and to a drain fluid passage; a fifth fluid passage for, when the fluid pressure changeover valve is in a predetermined position, connecting to a regulated pressure passage extending from the pressure regulating valve (20) to supply the regulated pressure from the pressure regulating valve (20) to one of the main transmission mechanism-side shift valve and the auxiliary transmission mechanism-side shift valve via the fluid pressure changeover valve (21); a sixth fluid passage for, when the fluid pressure changeover valve (21) is in the predetermined position, connecting to the fourth fluid passage to supply the line pressure from the fluid pressure source to the other one of the main transmission mechanism-side shift valve and the auxiliary transmission mechanism-side shift valve via the fluid pressure changeover valve (21); and a seventh fluid passage provided with connecting means for connecting the regulated pressure passage and the sixth fluid passage to each other when the fluid pressure in the regulated pressure passage is higher than the fluid pressure in the sixth fluid passage, the seventh fluid passage being capable of connecting the regulated pressure passage to the sixth fluid passage by the connecting means. A hydraulic control apparatus for an automatic transmission according to any one of claims 1 to 9, wherein when a first friction engagement element of the main transmission mechanism (2) and a second friction engagement element of the auxiliary transmission mechanism (5) are maintained in engagement, the hydraulic control apparatus shifts from a first speed to a second speed by releasing a third friction engagement element of the main transmission mechanism (2) from engagement; wherein during the operation of the shift from said first speed to said second speed, the auxiliary transmission mechanism-side shift valve maintains the second friction engagement element of the auxiliary transmission mechanism in engagement by supplying fluid pressure to a second hydraulic servo that operates the second friction engagement element; wherein during the operation of the shift from said first speed to said second speed, the main transmission mechanism-side shift valve changes over the supply of the fluid pressure from the pressure regulating valve (20) to a third hydraulic servo that operates the third friction engagement element of the main transmission mechanism (2), and the discontinuation thereof, and also changes over the supply of the fluid pressure from the pressure regulating valve (20) to a first servo that operates the first friction engagement element of the main transmission mechanism (2), and the supply of the line pressure to the first hydraulic servo; and wherein the hydraulic control apparatus comprises a changeover valve for selectively supplying the first hydraulic servo with the fluid pressure from the main transmission mechanism-side shift valve and the fluid pressure supply to the second hydraulic servo, the changeover valve supplying the first hydraulic servo with the fluid pressure supply to the second hydraulic servo when the fluid pressure from the main transmission mechanism-side shift valve becomes less than the fluid pressure supply to the second hydraulic servo.

Inventors: HAYABUCHI MASAHIRO, NISHIDA MASAAKI, SUZUKI AKITOMO, TSUKAMOTO KAZUMASA, YAMAMOTO YOSHIHISA, HAYABUCHI, MASAHIRO, NISHIDA, MASAAKI, SUZUKI, AKITOMO, TSUKAMOTO, KAZUMASA, YAMAMOTO, YOSHIHISA
Applicants: AISIN AW CO, AISIN AW CO., LTD.
Application year: 1996
Main classification: F16H61
IPC codes: F16H61