Abstract:
In an automatic transmission for a vehicle comprising at least a first friction engaging unit engaged for attaining a low forward speed and a second friction engaging unit engaged together with the first friction engaging unit for attaining a high forward speed, an apparatus for preventing creeping of the vehicle comprising an electronic control unit responsive to release of an accelerator pedal and the halted state of the vehicle and operative to control shift control means for engaging said friction engaging units and to control fluid pressure control means for reducing the pressure of hydraulic fluid supplied to the second friction engaging unit to a level lower than that of hydraulic fluid supplied to the first friction engaging unit, the second friction engaging unit including a brake band for gripping a drum, one end of the brake band being urged into engagement with the drum by a hydraulic device and located with respect to the fixed other end in a direction that the drum tends to rotate when the high forward speed is attained.

Description:
This application is a continuation-in-part of my earlier application bearing U.S. Ser. No. 451,385, filed Dec. 20, 1982, entitled &#34;CREEPING PREVENTING APPARATUS IN AUTOMATIC TRANSMISSION FOR VEHICLE&#34;. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to improvements in automatic transmissions for vehicles. 
     In a vehicle provided with an automatic transmission in its power transmission system, the so-called &#34;creeping&#34; tending to gradually move the vehicle occurs frequently when the vehicle is brought to a halt with its engine under rotation. An automatic transmission designed for obviating such a danger is commonly known and is so constructed that a speed stage of a relatively small gear ratio (for example, a 3rd speed at which the gear ratio is 1.00) is attained and maintained in the transmission when the vehicle is brought to a halt and the engine is idling. However, in the known transmission of the construction above described, hydraulic fluid at a pressure (for example, 6 kg/cm 2 ) which is as high as that supplied during driving is supplied to hydraulically-actuated friction engaging units engaged to attain the 3rd speed, in the halted state of the vehicle too. Therefore, when the hydraulic fluid is then discharged from the friction engaging units to attain the 1st speed of a larger gear ratio for starting the vehicle from its halted state, that is, when the hydraulic fluid is quickly discharged as shown by a broken curve A in FIG. 1 for attaining the 1st speed within a short period of time, transmission of torque at the 1st speed is started from a point B, that is, after a length of time t 1  after the fluid discharge is started. Thus, the torque transmission at the 3rd speed is completely changed over to that at the 1st speed in a very short length of time t 2 . The known automatic transmission is therefore defective in that a very large shock is imparted during the shift for starting the vehicle. Further, when the hydraulic fluid is gradually discharged from the friction engaging units as shown by another broken curve C in FIG. 1 in order to prevent impartation of the shock due to such an abrupt shift, transmission of torque at the 1st speed is started from a point D, that is, after a length of time t 3  after the fluid discharge is started. In such a case, the length of time of acceleration at the 3rd speed is extended, and the speed stage is shifted to the  1st speed under the condition of high engine torque. Therefore, the known automatic transmission is also defective in that a similarly large shock is imparted during the shift. Furthermore, when the automatic transmission of the construction above described is so designed as to attain the 1st speed by engagement of one of the hydraulically-actuated friction engaging units and a mechanical one-way clutch, reverse rotation of the engine is prevented by the action of the one-way clutch when the vehicle is standing still midway of a steel ascent, so that the vehicle can be maintained in the halted state. However, when the accelerator pedal is then lightly depressed to attain the 1st speed, the vehicle will move in the reverse direction or backward due to the shortage of transmitted torque. Therefore, the known automatic transmission is also defective in that the driver driving the vehicle will be unsettled, and a danger may be given rise to. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention which has for its object to obviate all of the prior art defects pointed out above, there is provided, in an automatic transmission for a vehicle comprising a plurality of friction engaging units hydraulically selectively actuated to establish a plurality of speed stages, said plural friction engaging units including at least a first friction engaging unit engaged for attaining a low forward speed stage of a relatively large gear ratio and a second friction engaging unit engaged together with said first friction engaging unit for attaining a high forward speed stage of a relatively small gear ratio, said second friction engaging unit only being released for carrying out a shift from said high forward speed stage to said low forward speed stage, an apparatus for preventing creeping of the vehicle comprising first sensor means for sensing whether or not the accelerator pedal is released or whether or not the throttle valve is full closed, second sensor means for sensing whether or not the vehicle is in its halted state, shift control means for engaging said first and second friction engaging units in response to the two output signals from said first and second sensor means thereby attaining said high forward speed stage, and fluid pressure control means for reducing the pressure of hydraulic fluid supplied to said second friction engaging unit to a level lower than that of hydraulic fluid supplied to said first friction engaging unit, in response to the two output signals from said first and second sensor means, said second friction engaging unit including a drum connected to one element of a transmission gear mechanism, a brake band adapted to grip an outer circumferential surface of said drum, a hydraulic device for urging one end of said brake band into engagement with said drum, said one end of the brake band being located with respect to the other end thereof in a direction that said drum tends to rotate when said high forward speed stage is attained, and a fixing member for fixedly holding the other end of said brake band. 
     According to the apparatus of the present invention having such a feature, the pressure of hydraulic fluid supplied to the second friction engaging unit for attainment of the high forward speed stage under the halted state of the vehicle is reduced to a level lower than that of hydraulic fluid supplied to the first friction engaging unit. Therefore, as shown by a solid curve E in FIG. 1, a shorter length of time t 5  is only required until transmission of torque at the 1st speed is started when the vehicle is then started from its halted state, and a sufficient length of time t 6  can be ensured until the torque transmission at the 1st speed is completed, whereby the aforementioned defects of the prior art automatic transmission can be obviated to alleviate the shock due to the speed shift. Furthermore, in the condition that the vehicle is in a midway of an ascent, if the vehicle tends to move backward when the 1st speed is attained, the vehicle will also move backward even when the 2nd or 3rd speed is attained during stoppage of the vehicle, thereby providing an alarm to the driver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the operating characteristic of an embodiment of the automatic transmission according to the present invention as compared to that of a prior art automatic transmission, 
     FIG. 2 is a power train diagram of the automatic transmission embodying the present invention, 
     FIG. 3 is an explanatory sectional view taken along the line III--III in FIG. 2, 
     FIG. 4 is a table showing the relation between the operating conditions of the friction engaging units and the speed stages in the automatic transmission shown in FIG. 2, 
     FIG. 5, is a circuit diagram 5A and 5B showing the control system in the automatic transmission of the present invention, and 
     FIG. 6 is a table showing the relation between the operating conditions of the solenoid valves and the speed stages in the automatic transmission of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described in detail with reference to the drawings. 
     Referring first to FIG. 2, a crankshaft 4 of an engine 2 which is a power source for driving a vehicle is coupled directly to a pump 8 of a torque convertor 6. Besides the pump 8, the torque convertor 6 includes a turbine 10, a stator 12 and a one-way clutch 14. The stator 12 is connected through the one-way clutch 14 to a casing 16, so that the one-way clutch 14 permits rotation of the stator 12 in the same direction as the direction of rotation of the crankshaft 4 but does not permit rotation of the stator 12 in the opposite direction. 
     Torque transmitted to the turbine 10 is transmitted by an input shaft 20 to a transmission gear unit 22 disposed on the rear side of the input shaft 20 to attain speed stages of four-forward and one-reverse speeds. The transmission gear unit 22 includes three clutches 24, 26, 28, two brakes 30, 32, one one-way clutch 34 and a planetary gearing 36 of Ravigneaux type. This planetary gearing 36 includes a ring gear 38, a long pinion gear 40, a short pinion gear 42, a front sun gear 44, a rear sun gear 46, and a carrier 48. The carrier 48 is arranged to rotatably support the two pinions 40, 42 and to be rotatable around its own axis. The ring gear 38 is connected to an output shaft 50. The front sun gear 44 is connected to the input shaft 20 through a kick-down drum 52 and the front clutch 24. The rear sun gear 46 is connected to the input shaft 20 through the rear clutch 26. The carrier 48 is connected to the casing 16 through the low-reverse brake 32 and the one-way clutch 34 arranged to be functionally parallel to each other and is also connected to the input shaft 20 through the 4th-spaced clutch 28 disposed at the rear end of the transmission gear unit 22. The kick-down drum 52 is arranged so that it can be fixedly connected to the casing 16 by the kick-down brake 30. Torque transmitted through the planetary gearing 36 is transmitted from an output gear 60 fixedly mounted on the output shaft 50 to a driven gear 64 through an idle gear 62, and thence, transmitted through a transfer shaft 66 fixed to the driven gear 64 and through a helical gear 68 to a differential gearing 72 to which drive shafts 70 are connected. 
     Each of the clutches and brakes above described is provided with an engaging mechanism such as a piston-cylinder assembly or a servo mechanism, so that it can be engaged or disengaged by hydraulic fluid under pressure supplied from an oil pump 74 (FIG. 5) driven by the engine 2 due to connection of the same to the pump 8 of the torque convertor 6. The hydraulic fluid is selectively supplied to the clutches and brakes under control of a fluid pressure control system described later depending on the driving condition of the vehicle sensed by various driving-condition sensors, and the selected combinations of the actuated clutches and brakes provide the speed stages of four-forward and one-reverse speeds. The table shown in FIG. 4 illustrates the relation between the operating conditions of the clutches and brakes and the speed stages. In FIG. 4, the symbol &#34; &#34; indicates that the corresponding clutch or brake is engaged, while the symbol &#34; &#34; indicates that the carrier 48 is locked against rotation by the function of the one-way clutch 34 immediately before the low-reverse brake 32 is engaged for the speed shift. 
     FIG. 3 shows a sectional view taken along the line III--III in FIG. 2, and the kick-down brake 30 will next be explained. 
     A brake band 502 is disposed around an outer circumferential surface of the kick-down drum 52. The direction of rotation of the input shaft 20 is shown by an arrow A. A bracket 506 fixed to the left-hand end 504 of the brake band 502 is held against a lock pin 508 fixed to the casing 16. A bracket 512 fixed to the right-hand end 510 of the brake band is adapted to be urged by a rod 352, described later, of a hydraulic actuator 514 to engage the brake band 502 with the kick-down drum 52. In FIG. 3, an arrow B shows the direction of free rotation of the kick-down drum 52 when the 1st forward speed is selected. An arrow C shows the direction of free rotation of the kick-down drum 52 when the 3rd forward speed is selected. However, when the 2nd forward speed is selected, the kick-down drum 52 is prevented from its rotation since a reaction of the drum 52 in the direction of the arrow B is supported by the brake band 502. 
     Description will next be directed to the fluid pressure control and electronic control applied to the transmission gear unit 22 shown in FIG. 2 for the attainment of the speed stages shown in FIG. 4. 
     A fluid pressure control system as shown in FIG. 5 is provided in the automatic transmission according to the present invention. Referring to FIG. 5, fluid under pressure pumped out by the pump 74 from an oil reservoir 76 through an oil filter 78 and a fluid passage 80 is supplied to the torque convertor 6 and is also selectively supplied to the fluid chambers of the piston-cylinder assemblies and servo mechanisms of the clutches 24, 26, 28 and brakes 30, 32 of the transmission gear unit 22 depending on the driving condition of the vehicle. For this purpose, the fluid pressure control system includes, as its principal components, a pressure regulator valve 82, a torque convertor control valve 84, a pressure reduction valve 86, a manual valve 88, a shift control valve 90, a rear clutch control valve 92, an N-R control valve 94, a fluid pressure control valve 96 controlling the fluid pressure during a shift, an N-D control valve 98, a 1st-2nd shift valve 100, a 2nd-3rd and 4th-3rd shift valve 102, a 4th-speed clutch control valve 104, and three solenoid valves 106, 108, 110. These components are interconnected by fluid passages. 
     The solenoid valves 106, 108 and 110 are of the same structure and are of the type closed when deenergized, so that individual orifices 114, 116 and 118 are on-off controlled by an electrical signal applied from an electronic control unit 112. These solenoid valves 106, 108, 110 include solenoids 120, 122, 124, valve members 126, 128, 130 disposed in the individual solenoids to open and close the associated orifices 114, 116, 118, and springs 132, 134, 136 normally biasing the associated valve members 126, 128, 130 in the closing direction, respectively. 
     The electronic control unit 112 includes means such as means for sensing starting of a speed shift. The solenoid valve 106 whose duty factor or on-duty is controlled on the basis of the sensed driving condition of the vehicle is energized and deenergized under control of the electronic control unit 112. The electronic control unit 112 controls the fluid pressure by varying the open duration of the solenoid valve 106 by controlling the pulse width of a pulse current of 50 Hz supplied to the valve 106, and also controls the opening and closure or on-off of the combination of the solenoid valves 108 and 110. Elements applying inputs to the electronic control unit 112 include a load sensor 138 sensing the opening of the throttle valve (not shown) or the vacuum in the intake manifold (not shown) of the engine 2, a rotation speed sensor 140 sensing the rotation speed (rpm) of the engine 2, another rotation speed sensor 142 sensing the rotation speed (rpm) of the kick-down drum 52 shown in FIG. 2, another rotation speed sensor 144 sensing the rotation speed (rpm) of the driven gear 64 thereby sensing the rotation speed (rpm) of the output shaft 50 and the speed of the vehicle, an accelerator pedal switch 146 provided for sensing the position of the accelerator pedal (not shown) and turned on when the accelerator pedal is released, a selected position sensor 148 sensing the selected position of said manual valve 88, and a fluid temperature sensor 150 sensing the temperature of the hydraulic fluid. 
     When the rotation speed of the driven gear 64 is sensed to be 0 rpm or about 0 rpm by said rotation speed sensor 144, this sensor 144 acts also as a sensor which senses the halted state of the vehicle since the vehicle speed is 0 km/h or about 0 km/h in such a case. Said accelerator pedal switch 146 is designed to be turned off when the accelerator pedal is merely depressed within the range of play. 
     The hydraulic fluid delivered from the oil pump 74 is supplied by way of a fluid passage 152 to the pressure regulator valve 82, manual valve 88 and pressure reduction valve 86. 
     The manual valve 88 includes a spool 154 which is actuated by the driver through an actuating lever (not shown) to occupy any one of four positions P, R, N and D as desired. In the D position, the fluid passage 152 communicates with fluid passages 156 and 158, and the transmission gear unit 22 is placed in the forward drive mode for forward drive at the 1st speed to the 4th speed depending on the on-off combination of the solenoid valves 108 and 110 as described later. In the N position, the fluid passage 152 communicates only with the fluid passage 158, and the fluid passage 156 communicates with a fluid exhaust port 160 to establish the neutral mode in the transmission gear unit 22. In the R position, the fluid passage 152 communicates with fluid passages 162 and 164 to establish the reverse drive mode of the speed stages in the transmission gear unit 22. In the P position, all of the fluid passages communicating with the manual valve 88 communicate with the fluid exhaust port 160 or a fluid exhaust passage 166, so that the transmission gear unit 22 is substantially placed in its neutral mode. It is so arranged that, when the P position of the manual valve 88 is selected, a parking mechanism (not shown) mechanically interlocked with the actuating lever is actuated to lock the output shaft 50 against rotation. Various shifts in the forward drive mode, such as an automatic shift to the 4th speed, an automatic shift between the 1st, 2nd and 3rd speeds, an automatic shift between the 1st and 2nd speeds only, and speed fixation at the 1st speed can be achieved by setting the spool 154 of the manual valve 88 in the D position and applying signals corresponding to the individual shifts to the electronic control unit 112 from auxiliary switches (not shown). 
     The pressure regulator valve 82 includes a spool 172 having pressure receiving surfaces 168 and 170, and a spring 174. When the hydraulic fluid supplied from the fluid passage 152 by way of the fluid passage 158 in the D position acts upon the pressure receiving surface 168 of the spool 172, the pressure of the hydraulic fluid in the fluid passage 152 is regulated to be a constant pressure of 6 kg/cm 2  (which will be referred to hereinafter as a line pressure), while when the pressure of the hydraulic fluid supplied from the fluid passage 152 by way of the fluid passage 162 in the R position of the manual valve 88 acts upon the pressure receiving surface 170 of the spool 172, the pressure of the hydraulic fluid in the fluid passage 152 is regulated to be 14.6 kg/cm 2 . 
     The torque convertor control valve 84 includes a spool 176 and a spring 178. The hydraulic fluid supplied from the pressure regulator valve 82 by way of a fluid passage 180 passes through a passage 182 formed in the spool 176 to impart pressure to the pressure receiving surface at the right-hand end of the spool 176. By the balance between the force created by the fluid pressure acting upon this pressure receiving surface of the spool 176 and the biasing force of the spring 178, the pressure of the hydraulic fluid supplied from the fluid passage 180 is regulated to be 2.5 kg/cm 2 , and the hydraulic fluid at such a regulated pressure is delivered to the torque convertor 6 by way of a fluid passage 184. The hydraulic fluid discharged from the torque convertor 6 is supplied through an oil cooler 186 to the individual lubricated parts of the transmission. 
     The pressure reduction valve 86 includes a spool 188 formed with opposite pressure receiving surfaces 192, 194, and a spring 190. The force created by the differential fluid pressure due to the difference between the areas of the opposite pressure receiving surfaces 192 and 194 formed on the spool 188 is balanced by the biasing force of the spring 190, so that the pressure of the hydraulic fluid supplied from the fluid passage 152 is reduced to be regulated at 2.4 kg/cm 2 , and the hydraulic fluid at such a reduced and regulated pressure is delivered to a fluid passage 196. The hydraulic fluid whose pressure is so regulated or reduced by the valve 86 and which appears in the fluid passage 196 is supplied through an orifice 198 to the N-R control valve 94, to the fluid pressure control valve 96 and to the orifice 114 of the solenoid valve 106. 
     The N-R control valve 94 includes a spool 206 formed with pressure receiving surfaces 200, 202, 204, and a spring 208. The pressure of the hydraulic fluid supplied from the fluid passage 164 is controlled to be regulated at a predetermined value by the balance between the force created by the fluid pressure acting upon the pressure receiving surface 200 and the sum of the force created by the differential fluid pressure due to the difference between the areas of the pressure receiving surfaces 202, 204 and the biasing force of the spring 208, so that the hydraulic fluid at such a regulated pressure is delivered to a fluid passage 210. This hydraulic fluid appearing in the fluid passage 210 is used for the control of the low-reverse brake 32 during the shift to the reverse speed. 
     The fluid pressure control valve 96 includes a spool 218 formed with pressure receiving surfaces 212, 214, 216, and a spring 220. The pressure of the hydraulic fluid supplied from the fluid passage 156 is controlled to be regulated at a predetermined value by the balance between the force created by the fluid pressure acting upon the pressure receiving surface 212 and the sum of the force created by the differential fluid pressure due to the difference between the areas of the pressure receiving surfaces 214, 216 and the biasing force of the spring 220, so that the hydraulic fluid at such a regulated pressure is delivered to a fluid passage 222. The hydraulic fluid appearing in the fluid passage 222 is used for the control of the front clutch 24, rear clutch 26, kick-down brake 30 and low-reverse brake 32 when the vehicle is driven in the forward direction or is in the halted state. 
     The duty factor or on-duty of the solenoid valve 106 is controlled by the electronic control unit 112 which supplies a constant-frequency pulse current of 50 Hz whose pulse width is varied depending on the driving condition of the vehicle. By increasing or decreasing the pulse width of the pulse current, the ratio between the open and closed durations of the orifice 114 is varied so as to control the pressure of the hydraulic fluid in the fluid passage 196 downstream of the orifice 198, hence, to control the fluid pressure P 1  acting upon the pressure receiving surface 200 of the N-R control valve 94 and upon the pressure receiving surface 212 of the fluid pressure control valve 96. Suppose, for example, that the diameter of the orifice 198 is 0.8 mm, and the diameter of the orifice 114 is 1.4 mm. Then, the fluid pressure P 1  is regulated within the range of about 0.3 kg/cm 2  and 2.1 kg/cm 2 , and, therefore, the pressure of the hydraulic fluid of regulated pressure appearing in the fluid passages 210 and 222 increases or decreases between about 0 kg/cm 2  and the pressure value of the hydraulic fluid supplied by way of the fluid passage 164 or 156, in proportional relation to an increase or a decrease in the fluid pressure P 1 . 
     The operation starting timing and operating duration of the solenoid valve 106 are determined on the basis of electrical signals applied from means including the shift sensor built in the electronic control unit 112 for sensing the shift starting timing and the engaging timing sensor composed of the two rotation speed sensors 142 and 144, besides those from the load sensor 138 and various rotation speed sensors 140, 142 and 144. 
     The shift control valve 90 is controlled by the combination of the on-off states of the solenoid valves 108 and 110. The shift control valve 90 includes three spools 224, 226, 228 and two stoppers 230, 232. The spool 224 is formed with lands 234, 236, an annular groove 238, and a fluid passage 242 providing a communication passage between the groove 238 and a fluid chamber 240 disposed on the left-hand side of the land 234. The spool 226 is formed with lands 244, 246 of different diameters, an annular groove 248, and abutments 250, 252 engageable with the respective spools 224, 228. The spool 228 is formed with lands 254, 256, an annular groove 258, and a fluid passage 262 providing a communication passage between the groove 258 and a fluid chamber 260 disposed on the right-hand side of the land 256. The stopper 230 is fixed to the valve casing in the position intermediate between the spools 224 and 226, and the stopper 232 is also fixed to the valve casing in the position intermediate between the spools 226 and 228. 
     The fluid passage 156 communicates always with a fluid passage 264 through the annular groove 248 of the spool 226, and the fluid passage 264 communicates through an orifice 266 with the orifice 116, left-hand fluid chamber 240 and right-hand fluid chamber 260 and communicates also through another orifice 268 with the orifice 118 and a fluid chamber 270 defined between the spools 224 and 226. FIG. 6 shows the relation between the combination of the on-off states of the solenoid valves 108, 110 and the speed stages. 
     The rear clutch control valve 92 includes a spool 278 formed with a land 272, another land 274 having a diameter smaller than that of the land 272, and an annular groove 276, another spool 290 formed with three lands 280, 282, 284 having the same diameter as that of the land 274 and annular grooves 286, 288, and a spring 292. When the force created by the pressure of the hydraulic fluid supplied to a left-hand fluid chamber 294 to act upon the pressure receiving surface of the land 272 of the spool 278 becomes higher than the sum of the force created by the pressure of the hydraulic fluid supplied to a right-hand fluid chamber 296 to act upon the pressure receiving surface of the land 284 of the spool 290 and the biasing force of the spring 292, the two spools 278 and 290 are urged toward and maintained in their rightmost positions in FIG. 5. In the rightmost positions of these spools 278 and 290, the fluid pressure acts between the lands 274 and 280. Therefore, the spool 278 only is urged toward its leftmost position when the hydraulic fluid is discharged from the fluid chamber 294. Then, when the force created by the pressure of the hydraulic fluid acting upon the left-hand pressure receiving surface of the land 280 of the spool 290 becomes lower than the sum of the force created by the fluid pressure in the fluid chamber 296 acting upon the right-hand pressure receiving surface of the land 284 and the biasing force of the spring 292, the spool 290 is urged toward its leftmost position. 
     The N-D control valve 98 includes a spool 304 formed with lands 298, 300 and an annular groove 302, and a spring 306. Depending on the direction of the sum of the force created by the fluid pressure acting upon the pressure receiving surfaces 308, 310 and 312 of the spool 304 and the biasing force of the spring 306, the spool 304 is selectively urged toward its leftmost position illustrated in FIG. 5 and its rightmost position (not shown). 
     The 1st-2nd shift valve 100 includes a spool 314 and a spring 316. The spool 314 is selectively urged toward its leftmost position illustrated in FIG. 5 and its rightmost position (not shown) respectively when the hydraulic fluid at the line pressure acting upon the left-hand pressure receiving surface of the spool 314 is discharged and supplied respectively. Thus, when the hydraulic fluid at the line pressure is supplied to impart the line pressure to the pressure receiving surface 318, the spool 314 is urged toward its rightmost position by the force created by the line pressure, while, when the hydraulic fluid at the line pressure is discharged, the spool 314 is urged toward its leftmost position by the biasing force of the spring 316. 
     The 2nd-3rd and 4th-3rd shift valve 102 and the 4th-speed clutch control valve 104 include similarly spools 320, 322 and springs 324, 326 respectively. Fluid chambers 328, and 330 to which the hydraulic fluid at the line pressure is supplied are provided on the left-hand side of the respective spools 320, 322, and fluid chambers 332, 334 are provided on the right-hand side of the respective spools 320, 322. These spools 320 and 322 are similarly selectively urged toward the illustrated leftmost position and the rightmost position (not shown). 
     The shift control by selective engagement of the friction engaging units will now be described. 
     When the manual valve 88 is changed over from the illustrated N position to its D position, the hydraulic fluid at the line pressure of 6 kg/cm 2  in the fluid passage 152 is supplied to the fluid passage 156 to be led to the shift control valve 90. When both of the solenoid valves 108 and 110 are in their energized state at that time, both of the orifices 116 and 118 are opened, and the fluid pressure in the fluid chambers 240, 260 and 270 is not increased. Consequently, the spool 226 is urged toward its illustrated leftmost position by the force created by the differential fluid pressure due to the difference between the areas of the pressure receiving surfaces of the lands 244 and 246, so that the hydraulic fluid at the line pressure is not supplied to the other fluid passages leading to the shift control valve 90. The hydraulic fluid at the line pressure in the fluid passage 156 is also supplied to the fluid chamber of the rear clutch 26 through the fluid pressure control valve 96, fluid passage 222, N-D control valve 98, fluid passage 336, rear clutch control valve 92 and fluid passage 338. The hydraulic fluid at the line pressure in the fluid passage 156 is also supplied to the fluid chamber of the low-reverse brake 32 through the fluid passage 222, 1st-2nd shift valve 100 and fluid passage 340. Thus, the rear clutch 26 and the low-reverse brake 32 are engaged to establish the 1st speed stage. 
     At this time, the fluid pressure control valve 96 functions to prevent abrupt build-up of fluid pressure in the fluid chamber of the rear clutch 26 thereby alleviating the shock due to the shift. 
     The function of the N-D control valve 98 at this time will be described now. When the hydraulic fluid is supplied from the fluid passage 222 to the annular groove 302 of the spool 304, the spool 304 is urged rightward due to the area difference of the pressure receiving surfaces 310 and 312 against the biasing force of the spring 306. As a result, communication between the fluid passages 222 and 336 is now interrupted, and the fluid passage 156 communicates with the fluid passage 336 through a fluid passage 342 and communicates also with a fluid passage 344. Therefore, as far as the manual valve 88 is maintained in its D position, the fluid passage 156 communicates with the fluid passage 336 without being led through the fluid pressure control valve 96, so that the pressure of the hydraulic fluid supplied to the fluid chamber of the rear clutch 26 is not subjected to the fluid pressure reduction control by the fluid pressure control valve 96 carrying out such a control during a shift. Thus, the troubles such as the shift shock and revving-up or racing of the engine due to the slip of the rear clutch 26 during the shift to the 1st speed can be avoided. 
     Then, when the accelerator pedal is depressed after the attainment of the 1st speed to increase the speed of the vehicle, the electronic control unit 112 generates a command signal commanding attainment of the 2nd speed in response to the output signals from the load sensor 138 and rotation speed (vehicle speed) sensor 144, so that the solenoid valve 108 is deenergized while the solenoid valve 110 is maintained in its energized state. Consequently, the hydraulic fluid at the line pressure in the fluid passage 264 is supplied through the orifice 266 to the annular groove 238, fluid passage 242, fluid chamber 240 and also to the fluid chamber 260, fluid passage 262 and annular groove 258 of the shift control valve 90, so that the spool 224 is urged rightward together with the spool 226 until the rightward movement of the spool 224 is stopped by engagement with the stopper 230. In such positions of the spools 224 and 226, the hydraulic fluid at the line pressure in the fluid passage 156 is supplied through the annular groove 248 to a fluid passage 346 and is then supplied from the fluid passage 346 to the 1st-2nd shift valve 100 to act upon the pressure receiving surface 318 and also to the fluid chamber 330 of the 4th-speed clutch control valve 104, so that the spools 314 and 322 of the respective valves 100 and 104 are urged toward their rightmost positions. Consequently, the hydraulic fluid at the line pressure in the fluid passage 222 is supplied by way of a fluid passage 348 to an engaging-side fluid chamber 350 of the kick-down brake 30 to urge the rod 352 leftward against the biasing force of a spring 354 thereby causing engagement of the brake band 502 with the kick-down drum 52. On the other hand, the hydraulic fluid in the fluid passage 340 is discharged by way of the fluid passage 210 thereby releasing the engagement of the low-reverse brake 32. Thus, the 2nd speed is attained. At this time, the fluid pressure control valve 96 responds to the fluid pressure control by the solenoid valve 106, so that the pressure of the hydraulic fluid in the fluid passage 222, hence, the pressure of the hydraulic fluid supplied to the fluid chamber 350 of the kick-down brake 30 is reduced by the function of the valve 96 during only the period of shift thereby preventing impartation of a shift shock. 
     Then, when both of the solenoid valves 108 and 110 are deenergized under command of the electronic control unit 112 for attainment of the 3rd speed, the hydraulic fluid at the line pressure is supplied through the orifice 268 to the fluid chamber 270 of the shift control valve 90. The spool 226 of the shift control valve 90 is urged rightward by the line pressure acting upon the pressure receiving surface of the land 244 until it is stopped at the position at which it is abutted by the spool 228, and the fluid passage 156 communicates now with a fluid passage 356. The hydraulic fluid at the line pressure let to the fluid passage 356 is supplied to the fluid chamber 328 of the 2nd-3rd and 4th-3rd shift valve 102, and the spool 320 of this valve 102 is urged toward its rightmost position by the line pressure, with the result that the fluid passage 348 communicates through an orifice 358 with a fluid passage 360. The hydraulic fluid at the line pressure led to the fluid passage 360 is supplied to the fluid chamber 334 of the 4th-speed clutch control valve 104 through a selector valve 362 and a fluid passage 364, to a release-side fluid chamber 366 of the kick-down brake 30, and to the front clutch 24 through a selector valve 368. Because of the arrangement in which the fluid passage 360 communicates with the release-side fluid chamber 366 of the kick-down brake 30 and, at the same time, communicates with the front clutch 24, the brake 30 is released and the clutch 24 is engaged in overlapping relation. 
     During this shift from the 2nd speed to the 3rd speed too, the fluid pressure control valve 96 operates to reduce the pressure of the hydraulic fluid supplied to the fluid passage 222 and maintain such a reduced pressure for a short period of time, in entirely the same manner as the fluid pressure control during the shift from the 1st speed to the 2nd speed described above. As the orifice 358 is disposed in the fluid passage 348 communicating with the fluid passage 222 through the 1st-2nd shift valve 100, by the function of this orifice 358, the pressure of the hydraulic fluid in the release-side fluid chamber 366 of the kick-down brake 30 is maintained at the same low level as that in the fluid chamber of the front clutch 24, so that the front clutch 24 is engaged in parallel relation to the release of the kick-down brake 30. When, thereafter, the fluid pressure control valve 96 ceases its control operation, and the fluid pressure rises to the line pressure level of 6 kg/cm 2  again, the engagement of the front clutch 24 is completed to establish the 3rd speed. In this case, the rotation speed of the input shaft 20 and kick-down drum 52 approaches the rotation speed of the output shaft 50 until finally the former becomes equal to the latter. The state of attainment of the same rotation speed or the state immediately before the attainment of the same rotation speed is sensed by the associated rotation speed sensors 142 and 144 as completion of the shift from the 2nd speed to the 3rd speed. As soon as the completion of the above shift is detected, the operation of the fluid pressure control valve 96, hence, the operation of the solenoid valve 106 is ceased, and the pressure of the hydraulic fluid supplied to the front clutch 24 is raised to the line pressure level of 6 kg/cm 2 . When the pressure of the hydraulic fluid supplied to the front clutch 24 is thus raised, the pressure of the hydraulic fluid supplied to the fluid chamber 334 of the 4th-speed clutch control valve 104 is also raised, with the result that the spool 322 of this valve 104 is urged toward its leftmost position. The hydraulic fluid at the line pressure is supplied to the 4th-speed clutch 28 by way of a fluid passage 370 to engage the 4th-speed clutch 28. The fluid passage 370 communicates with the fluid chamber 334 of the 4th-speed clutch control valve 104 through the selector valve 362 and fluid passage 364. Therefore, once the hydraulic fluid at the line pressure is supplied to the fluid passage 370, the spool 322 of the 4th-speed clutch control valve 104 is maintained in its leftmost portion until the hydraulic fluid at the line pressure is discharged from the fluid passage 370. Thus, a trouble such that the 4th-speed clutch 28 is released or slips during a shift from the 3rd speed to the 4th speed, hence, the desired shift is impossible, or the transmission gear unit 22 takes its neutral position, can be reliably avoided. 
     When the solenoid valve 108 is energized again, while the solenoid valve 110 is kept deenergized under command of the electronic control unit 112 for attainment of the 4th speed, the fluid pressure in each of the fluid chambers 240 and 260 of the shift control valve 90 is reduced, and the spool 228 is urged rightward together with the spool 226 until finally it takes its rightmost position. As a result, the hydraulic fluid at the line pressure in the fluid passage 156 is supplied by way of a fluid passage 372 to the fluid chamber 294 of the rear clutch control valve 92 and is also supplied through the fluid passage 372 and a check valve 374 to the fluid passage 370. The spools 278 and 290 of the rear clutch control valve 92 are urged toward their rightmost positions by the hydraulic fluid at the line pressure supplied to the fluid chamber 294, with the result that the fluid passage 372 communicates with a fluid passage 376, and the hydraulic fluid at the line pressure is supplied to the fluid chamber 332 of the 2nd-3rd and 4th-3rd shift valve 102 to urge the spool 320 of this valve 102 toward its illustrated leftmost position. In this case, the hydraulic fluid in the fluid chamber of the rear clutch 26 is discharged through a fluid exhaust port 378 of the rear clutch control valve 92 to immediately release the rear clutch 26. Also, the hydraulic fluid in the fluid chamber of the front clutch 24 and the hydraulic fluid in the fluid chamber 366 of the kick-down brake 30 are discharged through a fluid exhaust port 380 of the 2nd-3rd and 4th-3rd shift valve 102 and an orifice 382 to release the front clutch 24 and to engage the kick-down brake 30. As in the case of the shift from the 1st speed to the 2nd speed or the shift from the 2nd speed to the 3rd speed, the fluid pressure control valve 96 operates to reduce the pressure of the hydraulic fluid in the fluid passage 222 for a short period of time during the shift, so that the engaging fluid pressure in the fluid chamber 350 of the kick-down brake 30 is reduced to ensure smooth engagement. Then, when the pressure of the hydraulic fluid is raised to the line pressure level of 6 kg/cm 2 , the brake 30 is completely engaged to establish the 4th speed. 
     Down shifts will next be described. In the case of down shifts, the route of actuating fluid pressure is changed over in a relation reverse to that of the up shifts described hereinbefore. 
     When, for a shift from the 4th speed to the 3rd speed, both of the solenoid valves 108 and 110 are deenergized under command of the electronic control unit 112, the hydraulic fluid at the line pressure is discharged from the fluid passage 372. At this time, the line pressure of the hydraulic fluid from the fluid passage 336 is acting upon the left-hand pressure receiving surface of the land 280 of the rear clutch control valve 92, and the fluid pressure in the fluid passage 222 is reduced by the function of the fluid pressure control valve 96, as in the case of the up shift. The spool 278 of the rear clutch control valve 92 only is urged toward its leftmost position to discharge the hydraulic fluid in the fluid passage 376, and the spool 320 of the 2nd-3rd and 4th-3rd shift valve 102 is urged toward its rightmost position. Therefore, the hydraulic fluid from the fluid passage 348 is gradually supplied to the fluid chamber 366 of the kick-down brake 30 and to the front clutch 24 through the orifice 358 and fluid passage 360. Upon substantial completion of release of the kick-down brake 30 and engagement of the front clutch 24, the fluid pressure control valve 96 ceases to operate, and the pressure of the hydraulic fluid in the fluid passage 222 is raised. Consequently, the spool 290 of the rear clutch control valve 92 is urged toward its leftmost position, and the hydraulic fluid from the fluid passage 336 is supplied by way of the fluid passage 338 to the rear clutch 26 to engage the rear clutch 26 thereby attaining the 3rd speed. The engagement of the rear clutch 26 is delayed relative to the engagement of the front clutch 24 for the purpose of alleviating the shift shock occurring as a result of earlier engagement of the rear clutch 26 having a larger torque capacity. 
     In the case of a down shift from the 3rd speed to the 2nd speed, the solenoid valve 110 is energized, while the solenoid valve 108 is kept deenergized, under command of the electronic control unit 112, to discharge the hydraulic fluid from the fluid passage 356. As a result, the spool 322 of the 4th-speed clutch control valve 104 is urged toward its rightmost position, and the hydraulic fluid in the fluid passage 370 is discharged by way of the fluid passage 372. At the same time, the spool 320 of the 2nd-3rd and 4th-3rd shift valve 102 is urged toward its leftmost position, and the hydraulic fluid in the fluid passage 360 is also discharged. Thus, both of the 4th-speed clutch 28 and the front clutch 24 are released, and the kick-down brake 30 is engaged to attain the 2nd speed. In the case of this down shift too, the fluid pressure control valve 96 operates in a manner as described above to ensure smooth engagement of the kick-down brake 30, thereby preventing impartation of the shift shock during the shift. 
     In the case of a down shift from the 2nd speed to the 1st speed, both of the solenoid valves 108 and 110 are energized under command of the electronic control unit 112. The spools 224 and 226 of the shift control valve 90 are urged toward their leftmost positions to discharge the hydraulic fluid from the fluid passage 346, and the spool 314 of the 1st-2nd shift valve 100 and the spool 322 of the 4th-speed clutch control valve 104 are urged toward their leftmost positions. Thus, the hydraulic fluid in the fluid chamber 350 of the kick-down brake 30 is discharged to release the brake 30, and the low-reverse brake 32 is engaged to attain the 1st speed. 
     As described already, in the embodiment of the present invention, various shift modes including an automatic shift between the 1st speed, 2nd speed and 3rd speed, an automatic shift between only the 1st speed and the 2nd speed, and fixation at the 1st speed are achieved by manipulation of auxiliary switches (not shown). When a signal corresponding to any one of the various shift modes is applied to the electronic control unit 112 from the associated auxiliary switch, the electronic control unit 112 controls the solenoid valves 108 and 110 in response to the signal, so that the shift to the 4th speed, 3rd speed or 2nd speed, except the shift indicated by the signal, may not be done. 
     When the manual valve 88 is changed over to the R position, the fluid passage 152 communicates with the fluid passages 162 and 164. The fluid passage 164 communicates with the low-reverse brake 32 through the N-R control valve 94, fluid passage 210, 1st-2nd shift valve 100 and fluid passage 340, and the fluid passage 162 communicates with the front clutch 24 through the selector valve 368. Therefore, both of the front clutch 24 and the low-reverse brake 32 are engaged to establish the reverse speed stage. In this case too, the solenoid valve 106 operates for a short period of time during the shift as in the case of the forward shift, and the pressure of the hydraulic fluid supplied to the low-reverse brake 32 is reduced under control of the N-R control valve 94 to prevent impartation of the shift shock. 
     Suppose that, when the manual valve 88 in the system shown in FIG. 5 is in its D position, the accelerator pedal switch 146 is turned on to indicate that the accelerator pedal is released, and the rotation speed sensor 144 associated with the driven gear 64 senses the rotation speed of 0 rpm (corresponding to the vehicle speed of 0 km/h), that is, senses that the vehicle is in its halted state or is nearly halted in that the vehicle speed is lower than about 5 km/h. (According to the sensitivity of such a sensor, it is generally difficult to sense a vehicle speed lower than 5 km/h, and the vehicle speed lower than 5 km/h is sensed to be 0 km/h.) Then, in response to the output signals from the accelerator pedal switch 146 and rotation speed sensor 144, the electronic control unit 112 controls energization of the solenoid valves 108 and 110 to attain the 2nd speed, and the spool 244 of the shift control valve 90 is brought to the position at which the fluid passage 156 communicates with the fluid passage 346. In that position of the spool 244 of the shift control valve 90, both of the rear clutch 26 and the kick-down brake 30 are engaged to establish the 2nd speed stage, as when the 2nd speed is attained during the forward drive. At the same time, the solenoid valve 106 is energized under command of the electronic control unit 112 and continues to be maintained in the energized state during the period of time in which the 2nd speed stage is maintained in the halted state of the vehicle. The pressure of the hydraulic fluid supplied from the fluid pressure control valve 96 to the kick-down brake 30 through the fluid passage 222, 1st-2nd shift valve 100 and fluid passage 348 is maintained at a value of about 1.5 kg/cm 2  which is lower than the line pressure (6 kg/cm 2 ) of the hydraulic fluid supplied to the rear clutch 26. 
     Then, when the driver depresses the accelerator pedal for starting driving of the vehicle, the accelerator pedal switch 146 is turned off, and, in response to the signal indicative of the turn-off of the switch 146, the pulse width (the duty factor) of the pulse current supplied to the solenoid valve 106 is gradually increased under control of the electronic control unit 112. As a result, the pressure of the hydraulic fluid being supplied to the kick-down brake 30 (that is, the pressure of the hydraulic fluid delivered to the fluid passage 222 from the fluid pressure control valve 96) is gradually reduced as shown by the solid curve E in FIG. 1, and, finally, the continuous current for continuously opening the solenoid valve 106 is supplied to the valve 106, so that the pressure of the hydraulic fluid supplied to the kick-down brake 30 is reduced to about 0 kg/cm 2 . During this period of time, the shift control valve 90 is maintained in the position attaining the 2nd speed, and the pressure of the hydraulic fluid supplied to each of the clutches and brakes, except the rear clutch 26, is reduced by the functions of the fluid pressure control valve 96 and N-D control valve 98. Therefore, torque transmission at the 1st speed is started from the point F in FIG. 1 by the combination of the rear clutch 26 and the one-way clutch 34, and the shift to the 1st speed is completed as soon as the pressure of the hydraulic fluid supplied to the kick-down brake 30 is reduced to about 0 kg/cm 2 . Thereafter, the electronic control unit 112 generates the signal turning on both of the solenoid valves 108 and 110 at substantially the same time as it supplies the continuous current to the solenoid valve 106, thereby urging the spool 226 of the shift control valve 90 toward the position at which the 1st speed is attained. Therefore, the hydraulic fluid at high fluid pressure or the line pressure of 6 kg/cm 2  is now supplied to the low-reverse brake 32 to establish the  1st speed in the normal driving condition of the vehicle. 
     As described above, in the halted state of the vehicle, the 2nd speed is established, and the pressure of the hydraulic fluid having a low value of about 1.5 kg/cm 2  is supplied to the kick-down brake 30. At this time, an actuating torque due to a reaction of the kick-down drum 52 acts in the direction of the arrow B in FIG. 3. This direction of the arrow B corresponds to a self-locking direction of the brake band 502, so that the brake band 502 engages with the kick-down drum 52 with a large engaging force although the low pressure of the hydraulic fluid is supplied to the hydraulic actuator 514 of the kick-down brake 30. Consequently, the kick-down drum 52 is prevented from its rotation without occurrence of a slip of the kick-down drum 52 in the direction of the arrow B. That is, the 2nd speed is maintained without substantially causing a shift from the 2nd speed to the 1st speed attained by the combination of the rear clutch 26 and the one-way clutch 34. However, when the vehicle is stopped midway of an ascent and tends to move backward, an input from the ring gear 38 in the reverse direction creates through the effect of the one-way clutch 34 mounted on the carrier 48 an actuating torque which acts on the kick-down drum 52 in the direction of the arrow C in FIG. 3. This direction of the arrow C corresponds to a counter self-locking direction of the brake band 502, so that the engaging force of the brake band 502 with the brake drum 52 is decreased to occur the slip of the brake drum in the direction of the arrow C. Consequently, the vehicle moves backward due to the slip of the brake drum 52, maintaining the attainment of the 2nd speed. Thus, by setting appropriately the pressure of the hydraulic fluid supplied to the kick-down brake 30, the vehicle can move backward even during attainment of the 2nd speed in a midway of an ascent the inclination of which is such that the vehicle moves backward when the 1st speed is attained. 
     It will be apparent from the foregoing detailed description of the embodiment of the present invention that, when the vehicle is brought to a halt with the accelerator pedal being released, the 2nd speed of the small gear ratio is established in the transmission gear unit 22 to prevent occurrence of creeping in the halted state of the vehicle, and the pressure of the hydraulic fluid supplied to the kick-down brake 30 for attainment of the 2nd speed is reduced to about 1.5 kg/cm 2  which is lower than the line pressure (6 kg/cm 2 ) of the hydraulic fluid supplied to the rear clutch 26 which is also engaged at the time of attainment of the 1st speed. Therefore, torque transmission at the 1st speed attained by the combination of the rear clutch 26 and the one-way clutch 34 for starting the vehicle starts in a short period of time of t 5  after the discharge of the hydraulic fluid from the kick-down brake 30 is started, and the complete shift to the 1st speed attained by the combination of the rear clutch 26 and low-reverse brake 32 is also carried out smoothly in a short period of time of t 6 , so that the undesirable or uncomfortable shift shock can be alleviated. Furthermore, the kick-down drum 52 is prevented from its rotation by the brake band 502 in the self-locking direction thereof, so that the attainment of the 2nd speed is ensured and the occurrence of creeping is effectively prevented without causing a shift to the 1st speed. 
     According to the present invention, the pressure of the hydraulic fluid supplied to the kick-down brake 30 in the halted state of the vehicle is controlled to be lower than the line pressure. Therefore, when the vehicle is temporarily stopped midway of a steep ascent, the vehicle tends to gradually move backward due to the slip of the kick-down drum 52 relative to the brake band 502 in the counter self-locking direction thereof, and such a situation provides an alarm to the driver, so that the driver must actuate the foot brake to prevent the backward movement of the vehicle. Thus, the driver will carefully or intentionally sufficiently depress the accelerator pedal when he starts then the vehicle, and, consequently, the danger that the vehicle moves backward due to light depression of the accelerator pedal for re-starting the vehicle can be reliably avoided. 
     When, in the aforementioned embodiment, it is desired to drive the vehicle utilizing the creeping, the driver may merely depress the accelerator pedal within the range of its play. When the accelerator pedal is so depressed, the 1st speed is attained without increasing the rotation speed of the engine 2, and the desired drive utilizing the creeping can be achieved. 
     The aforementioned embodiment employs the accelerator pedal switch 146 for sensing whether or not the accelerator pedal is released. This switch 146 may be replaced by an idle switch sensing whether or not the engine throttle valve is full closed. When such an idle switch is employed, discharge of the hydraulic fluid from the kick-down brake 30 starts after the throttle valve is opened, and the 1st speed is attained after the rotation speed of the engine 2 is increased up to a certain level. The functional effects exhibited by such a modification are the same as those of the aforementioned embodiment, although the effect of shift shock alleviation is somewhat reduced. In the modification using the idle switch, a cancel switch cancelling the output signal from the idle switch may be connected in series with the idle switch as a means for intentionally producing creeping, and the cancel switch may be disposed at a position permitting manipulation by the driver.