Abstract:
A transmission control device according to the present invention comprises a main transmission having a torque converting device provided with a direct clutch for effecting engagement and disengagement of the input member with output member of a torque converting portion; a sub transmission having a speed change mechanism; a drive-mode detecting means for detecting the drive mode of the speed change mechanism of the sub transmission; and a controller for controlling the lock-up characteristics of the direct clutch by means of the output of the drive mode detecting means. 
     The transmission control device is capable of setting the lock-up characteristics of the direct clutch in accordance with the speed mode of the sub transmission by effecting a change-over of the lock-up characteristics of the direct clutch of the torque converting device on the basis of the output of the drive-mode detecting means for detecting the speed mode of the sub transmission, so that it is possible to optimize the power performance, operating performance, and fuel consumption in accordance with the speed mode of the sub transmission.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a transmission control device for controlling the operation of a transmission system comprising a main transmission having a torque converting device with a direct clutch and a sub-transmission having a speed change mechanism 
     Conventionally, a transmission system is known which combines a main transmission having a torque converting device provided with a direct clutch for effecting engagement and disengagement of the input member with output member of a torque converting portion as well as a sub-transmission capable of effecting speed change by selecting the drive mode of said speed change mechanism. For instance, such transmission systems includes a four-wheel drive automatic transmission comprising an electronic controller using as its input such as the states of running of the vehicle, including the vehicle speed and throttle opening; a hydraulic controller which inputs the output of the electronic controller; a main transmission having a plurality of speed-change steps controlled by the hydraulic controller and having a direct clutch effecting the engagement and disengagement between the input member and output member of the torque converting portion by a change-over of the hydraulic controller and the like; and a sub-transmission allowing the drive mode to be changed over between a high-speed mode and a low-speed mode In the case of such a four-wheel drive automatic transmission, even if the sub-transmission is changed over from the high-speed mode to the low-speed mode or from the low-speed mode to the high-speed mode, the engagement or disengagement of the direct clutch of the torque converting device has heretofore been controlled by fixed lock-up (direct clutch engagement) characteristics given by the vehicle speed, throttle opening, and the electronic controller regardless of the drive mode of the sub transmission. 
     As described above, the direct clutch of a conventional torque converting device is controlled by specific shifting characteristics, such as the vehicle speed and throttle opening, regardless of the drive mode of the sub-transmission. In consequence, if a torque converter is used as the torque converting device and the drive mode of the sub-transmission is set to the high-speed mode which is suitable for normal running and high-speed running, and if the lock-up characteristics of the direct clutch are set in such a manner as to effect an earlier lock-up so as to reduce fuel consumption at the time of normal running and high-speed running, in the performance curve of the torque converter, a lock-up is completed before the torque is increased sufficiently by a stator within a convertible range (a range in which a torque conversion is effected by the converter). As a result, in a case where the driver wishes to obtain a large torque for drive wheels by setting the drive mode of the sub transmission to a low speed, the direct clutch is locked up at an earlier point of time, thereby making it impossible to produce an increased torque for drive wheels within a convertible range. On the other hand, if the lock-up point of the lock-up characteristics of the direct clutch is set to a coupling range (a working range in which the torque ratio is 1) when the drive mode of the sub-transmission is set to the low speed, so as to produce a sufficiently increased torque for the drive wheels within a convertible range, and if the drive mode of the sub-transmission is set to a high-speed mode, and normal running and high-speed running are carried out, fuel consumption increases disadvantageously since the lock-up is delayed. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the above-described problems, and an object of the present invention is to increase the power performance and driving and operating performance by setting the lock-up characteristics of the direct clutch of a torque converting device in response to the drive mode of a sub-transmission. 
     Another object of the present invention is to provide a transmission control device which is capable of reducing fuel consumption. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
     To attain the aforementioned objects, the transmission control device according to the present invention comprises a main transmission having a torque converting device provided with a direct clutch for effecting engagement and disengagement of the input member with output member of a torque converting portion; a sub-transmission having a speed change mechanism; a drive-mode detecting means for detecting the drive mode of the speed change mechanism of the sub-transmission; and a controller for controlling the lock-up characteristics of the direct clutch by means of the output of the drive mode detecting means. 
     The transmission control device of the present invention having the aforementioned arrangement is capable of setting the lock-up characteristics of the direct clutch in accordance with the speed mode of the sub-transmission by effecting a change-over of the lock-up characteristics of the direct clutch of the torque converting device on the basis of the output of the drive-mode detecting means for detecting the speed mode of the sub-transmission, so that it is possible to optimize the power performance, operating performance, and fuel consumption in accordance with the speed mode of the sub-transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a cross sectional view of a four-wheel drive transmission system; 
     FIG. 2 is a schematic drawing of the drive mechanism of a four-wheel drive vehicle; 
     FIG. 3 is a cross sectional view of a torque converter and a direct clutch; 
     FIG. 4 is the hydraulic circuit diagram of the hydraulic controller of a main transmission; 
     FIG. 5 is the hydraulic circuit diagram of the hydraulic controller of a sub-transmission; 
     FIGS. 6 and 7 are block diagrams of an electronic controller; 
     FIG. 8 is a flowchart illustrating the operation of speed change control according to the present invention; 
     FIG. 9 illustrates lock-up characteristics showing the lock-up point of the direct clutch at the time when the drive mode of the sub-transmission is in the directly coupled state; 
     FIG. 10 illustrates lock-up characteristics showing the lock-up point of the direct clutch at the time when the drive mode of the sub-transmission is in the reduced-speed state; 
     FIG. 11 is a flowchart on speed change control illustrating another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The transmission control device of the invention will be fully understood from the following description of a preferred embodiment when the same is read in conjunction with the accompanying drawings. 
     Referring first to FIG. 1, the transmission system includes a main transmission 10 which is a four-speed automatic transmission with an overdrive, and a four-wheel drive sub-transmission 50 which is coupled to the output side of a planetary gear system incorporated in the main transmission 10. The main transmission 10 i directly mounted on an engine E and has an output shaft 32 which constitutes an input shaft of the four-wheel drive sub transmission 50. The sub-transmission 50 has two output shafts: namely, a first output shaft 52 coupled to a rear-wheel drive propeller shaft C and a second output shaft 54 coupled to a front-wheel drive propeller shaft B. 
     The main transmission 10 is constituted by a fluid type torque converter T, an overdrive mechanism OD and an underdrive mechanism UD having three forward speed gears and one reverse gear. 
     The torque converter T has a pump impeller 11 coupled to the output shaft of the engine E, a turbine impeller 13 coupled to the output shaft 12 of the torque converter T, a stator 15 connected to a stationary part through a one-way clutch 14, and a direct clutch 16. The output shaft 12 of the torque converter T serves as the input shaft of the overdrive mechanism OD. 
     The overdrive mechanism OD includes frictional engagement means such as a multi-disc clutch C0, multi-disk brake B0 and a one-way clutch F0, and a planetary gear device P0. The planetary gear device P0 has various constituent parts which are brought into and out of engagement with other parts such as a stationary part, e.g., a main transmission case 20, input shaft, output shaft and other parts, in accordance with the respective states of selective engagement of the above-mentioned frictional engagement means. 
     More specifically, the planetary gear device P0 has a carrier 21 connected to the output shaft 12, a ring gear 23 connected to the output shaft 22 of the overdrive mechanism OD, a sun gear 24 rotatably carried by the input shaft 12 and fixed to the main transmission case 20 through the brake B0, the sun gear 24 being connected to the carrier 21 through the clutch C0 and also through the one-way clutch F0 parallel to the clutch C0 and planetary pinions 25 rotatably carried by the carrier 21 and held in meshing engagement with the sun gear 24 and the ring gear 23. 
     The output shaft 22 of the overdrive mechanism 0D serves as the input shaft of the underdrive mechanism UD which has three forward speed gears and one reverse gear. 
     The underdrive mechanism UD has frictional engagement mans such as multi-disc clutches C1 and C2, a belt brake B1, multi-disc brakes B2 and B3, one-way clutches F1 and F2, a primary planetary gear device Pl and a secondary planetary gear device P2. 
     The primary planetary gear device P1 includes a ring gear 31 connected to the output shaft 22 through the clutch C1, a carrier 33 connected to the output shaft 32 of the underdrive mechanism UD, a sun gear 34 connected to the output shaft 22 through the clutch C2 and fixed to the main transmission case 20 through the belt brake B1 and also through a series connection of the one-way clutch F1 and the brake B2 which is parallel to the brake B1 and planetary pinions 35 rotatably carried by the carrier 33 and held in meshing engagement with the sun gear 34 and the ring gear 31. 
     The secondary planetary gear device P2 includes a carrier 36 which is fixed to the main transmission case 20 through the brake B3 and also through the one-way clutch F2 parallel to the brake B3, a sun gear 37 formed integrally with the sun gear 34 of the primary planetary gear device P1, a ring gear 38 connected to the output shaft 32, and planetary pinions 39 rotatably carried by the carrier 36 and held in meshing engagement with the sun gear 37 and the ring gear 38. 
     A main hydraulic controller 100 is accommodated by a oil pan 40 connected to the underside of the main transmission case 20 and is provided at the underside thereof with an oil strainer 101. The main hydraulic controller 100 operates in accordance with various factors representing the state of running of the vehicle, e.g., throttle opening of the engine E, vehicle running speed and so forth, so as to selectively engage and disengage the clutches and brakes, thereby effecting gear change over four forward speed gears including the overdrive O/D and the reverse gear which is selected manually. 
     The sub-transmission 50 has a sub-transmission case 51 which is coupled to the rear end of the main transmission case 20 by means of a plurality of bolts, such that the output shaft 32 of the main transmission 10 constitutes the input shaft of the sub-transmission 50. More specifically, the sub-transmission 50 has frictional engagement means such as a clutch C3, a brake B4 and a clutch C4 which are driven by the input shaft constituted by the common output shaft of the planetary gear devices P1 and P2. The sub transmission 50 has a first output shaft 52 connected in series to the output shaft 32, a planetary gear device P3 interposed between the output shaft 32 and the first output shaft 52, a four-wheel drive sleeve 53 rotatably fitted around the first output shaft 52, a second output shaft 54 disposed in parallel with the output shaft 32 and projecting in the direction opposite to the first output shaft 52, and a transfer mechanism 55 through which the sleeve 53 is connected to the second output shaft 54. The planetary gear device P3 includes a sun gear 56 splined to the end of the output shaft 32, planetary pinions 57 meshing with the sun gear 56, a ring gear 58 meshing with the planetary pinions 57, and a carrier 59 rotatably carrying the planetary pinions 57 and connected to the free end of the first output shaft 52 of the sub-transmission 50. A parking gear 61 is formed on the outer peripheral surface of a cylinder 60 which is connected to the carrier 59. The arrangement is such that, when a shift lever (not shown) of the main transmission 10 is operated to select a parking position, a stopper 62 engages with the parking gear 61 thereby fixing the first output shaft 52. 
     The frictional engagement means, i.e., clutches and brakes, of the sub-transmission 50 are selectively operated to be engaged and disengaged by a sub hydraulic controller 400 which is disposed in an oil pan 63 under the sub transmission case 51. 
     FIG. 3 shows a cross sectional view of the torque converting device with a direct clutch. 
     The torque converter T has the direct clutch 16 and is constituted by a power transmission case 70, i.e., a case for the torque converter T, and a torque converting portion 71 for transmitting power by means of a fluid (working oil) interposed in the power transmission case 70. The direct clutch 16 is interposed between the power transmission case 70 and the torque converting portion 71 and is driven by a working oil supplying means 72. 
     The torque converter T is disposed inside the torque converter case 20 between the engine E and the overdrive mechanism OD, and the torque converter T and the overdrive mechanism OD are partitioned by an oil pump housing 102a. 
     The power transmission case 70 is coupled with the crankshaft Ea of the engine E via a stator wheel Eb and includes a front cover 70a containing the direct clutch 16, an annular, planar rear cover 70b welded onto the inner periphery of the front cover 70a and containing the torque converting portion 71, and a pump drive sleeve 70c provided along the inner peripheral wall of the rear cover 70b. The rear end of the pump drive sleeve 70c is provided rotatably via a metal bearing 72a and an oil seal 72b to the inner surface of a cylindrical portion 102c formed projecting forwardly of an oil pump cover 102b fastened between the torque converter case 20a and the main transmission case 20. Furthermore, the rear end of the pump drive sleeve 70c is splined to the inner periphery of the external gear 102e of an internal gear oil pump 102 having the external gear 102e and an internal gear 102f both being provided inside the oil pump housing 102a constituted by the oil pump cover 102b and a rear cover 102d so as to drive the external gear 102e. 
     The torque converting portion 71 is comprised of the pump impeller formed integrally inside the rear cover 70b and causing the working oil to flow from the inner peripheral side to the outer peripheral side by means of a centrifugal force; the turbine impeller 13 provided in correspondence with the pump impeller 11 and adapted to receive the working oil made to flow to the outer peripheral side by means of the pump impeller 11, and cause the working oil to flow to the inner peripheral side again, thereby transmitting the rotation of the pump impeller 11; and the stator 15 which changes the direction of flow of the working oil between the pump impeller 11 and the turbine impeller 13 so as to increase the torque. The inner periphery of the stator 15 is coupled with the outer race 71b of a one-way clutch 71a which is rotatable only in one direction, while the inner race 71c of the one-way clutch 71a is splined to the outer periphery of the front end of a fixed sleeve 71d which is coupled to the rear cover 102d of the oil pump housing 102a fastened to the main transmission case 20, so that the turbine impeller 13 will rotate when the flow rate of the working oil passing through the stator 15 exceeds a predetermined level. In addition, a turbine flange 13a  for supporting the turbine impeller 13, together with the inner peripheral portion of the first guide plate 16 of the direct clutch 16, is fixed and connected, by means of a rivet 12c, to the outer peripheral flange 12b of an output shaft coupling hub 12a, to which the central side thereof is splined to the output shaft 12 disposed via metal bearings 72c, 72d provided at the front and rear ends of the inner periphery of the fixed sleeve 71d. 
     The direct clutch 16 is interposed between the front cover 70a of the power transmission case 70 and the turbine impeller 13 of the torque transmitting portion 71 and is constituted by a disk-shaped piston 16e is and a damper mechanism 16f for absorbing impact at the time of lock-up engagement. The piston 16e includes an inner peripheral-side cylindrical portion 16b, an outer peripheral-side cylindrical portion 16c, and an annular, planar portion 16d and is detachably engaged with the front cover 70a of the power transmission case 70, which serves as the input member of the torque converter T. The inner peripheral-side cylindrical portion 16b of the piston 16e externally fitted via a seal ring 12e to the annular recess 12d of an output shaft coupling hub such as to be axially slidable. The annular, planar portion 16d is formed as a flat, annular lock-up engageable surface 16g corresponding to a frictional member 70d provided inside the front cover 70a and adapted to increase a frictional force at the time of lock-up engagement, while a spline 16h having a plurality of notches opening rearwardly is formed in the outer peripheral-side cylindrical portion 16c. 
     The damper mechanism 16f is comprised of a driven plate 16k and a drive plate 16n. The driven plate 16k includes a group of dampers 16i each constituted by a compression coil spring, a first guide plate 16a and a second guide plate 16j which contain the dampers 16i and carry the same such as to be slidable in the peripheral direction. The driven plate 16n has a spline 161 whose outer periphery is splined to the spline 16h of the outer peripheral-side cylindrical portion 16c as well as a damper-urging window 16m which is slidably carried in the peripheral direction between the first guide plate 16a and the second guide plate 16j and in which the dampers 16i are provided and which urges the dampers 16i by virtue of the sliding of the driven plate 16k. The first guide plate 16a and the second guide plate 16j are secured by means of a rivet 16p, while the inner periphery of the first guide plate 16a of the driven plate 16k is coupled to the output shaft coupling hub 12a, as described above. 
     The working oil supplying means 72 is comprised of a first oil passage 72k and a second oil passage 72n. The first oil passage 72k includes an oil passage 1R which communicates with a below-mentioned lock-up relay valve 120 formed inside the rear cover 102d of the oil pump housing 102a; an oil passage 72f formed in the fixed sleeve 71d in such a manner as to correspond to the oil passage 1R, an oil passage 72g formed between the metal bearings 72c and 72d and also between the output shaft 12 and the fixed sleeve 71d, an oil passage 72i communicating with the oil passage 72g via an oil passage 72h and formed in the axial center of the output shaft 12; and an oil passage 72j communicating with the oil passage 72i and formed between the front cover 70a and the piston 16e. Meanwhile, the second oil passage 72n includes an oil passage 1S which communicates with the below-mentioned lock-up relay valve 120 allowing communication between the pump drive sleeve 70c and the fixed sleeve 71d, an oil passage 721 formed between the pump drive sleeve 70c and the fixed sleeve 71d, and an oil passage 72m communicating with the oil passage 72l, passing between the pump drive sleeve 70c and the one-way clutch 71a, and allowing communication between the stator 15 and the pump impeller 11. A change-over of the supplying direction of the working oil between the first oil passage 72k and the second oil passage 72n is effected by means of the main hydraulic controller 100 to be described below. 
     The operation of the direct clutch 16 of the torque converter T having the above-described construction will be described hereinunder. 
     When Below-Mentioned Main Hydraulic Controller 100 Is Not Locked Up 
     The working oil supplying means 72 is designed to form a circulatory passage which fills the power transmission case 70 with working oil via the first oil passage 72k and discharges the working oil through the second oil passage 72n. 
     Since the supply of the working oil to the power transmission case 70 is effected via the portions between the front cover 70a and the piston 16e, the frictional member 70d secured to the front cover 70a and the lock-up engageable surface 16g of the piston 16e are separated from each other by a difference in hydraulic pressure, thereby releasing their frictional engagement surfaces. In consequence, the hydraulic oil flows between the frictional member 70d and the lock-up engageable surface 16g, fills the power transmission case 70, circulates through the torque converting portion 71, and is discharged through the second oil passage 72n. At this time, the output which is inputted by the pump impeller 11 via the power transmission case 70 is transmitted to the turbine impeller 13 by means of the transmission of the working oil circulating through the torque converting portion 71. Accordingly, a rotational torque based on the action of the torque converting portion 71 of the torque converter T alone is outputted to the output shaft 12, with the result that the torque is not transmitted to the direct clutch 16. 
     When Above-Mentioned Main Hydraulic Controller 100 Is Locked Up 
     The hydraulic oil supplying means 72 is designed to form a circulatory passage which fills the power transmission case 70 with working oil through the second oil passage 72n and discharges the working oil through the first oil passage 72k. 
     Since the supply of the working oil to the power transmission case 70 is effected from the side of the torque converting portion 71, the internal pressure of power transmission case 70 is increased due to the filling pressure of the working oil, while the working oil between the front cover 70a and the piston 16e is discharged through the first oil passage 72k. In consequence, the frictional member 70d provided on the front cover 70a is engaged with the lock-up engageable surface 16g of the piston 16e by means of the filling pressure of the working oil inside the power transmission case 70. As a result, the output transmitted from the crankshaft Ea to the power transmission case 70 via the stator wheel Eb is transmitted to the output shaft 12 via the damper mechanism 16f and the output shaft coupling hub 12a, thereby allowing the output of rotation of the engine E to be directly coupled with the output shaft 12. 
     FIG. 4 shows the detail of the main hydraulic controller 100 for controlling the main transmission 10. 
     As will be seen from this Figure, the main hydraulic controller 100 has the following parts: an oil strainer 101; an oil pump 102; a cooler by-pass valve 115 for regulating the oil pressure in an oil cooler O/C; a pressure relief valve 116; a release clutch control valve 117; a release brake control valve 118; a lock-up relay valve 120; a pressure regulator valve 130 for regulating the pressure of a working fluid or oil which is supplied by the oil pump 102 to an oil passage 1; a second pressure regulator valve 150 through which the oil is supplied both to a lubricating oil supply passage L1 leading to the main transmission 10 and a working oil supply passage L2 for the sub-transmission 50; a cut-back valve 160; a lock-up control valve 170; a first accumulator control valve 180; a second accumulator control valve 190; a throttle valve 200; a manual valve 210 which distributes the line pressure in the oil passage 1 selectively to oil passages 2 to 6; a 1-2 shift valve 220; a 2-3 shift valve 230; a 3-4 shift valve 240; an intermediate coast modulator valve 245 which regulates the pressure of the oil supplied to the brake B1; a low coast modulator valve 250 for regulating the pressure of the oil supplied to the hydraulic servomotor B-3; an accumulator 260 for smoothing the engagement of the clutch C2; an accumulator 270 for smoothing the engagement of the brake B0; an accumulator 280 for smoothing the engagement of the clutch C2; an accumulator 290 for smoothing the engagement of the brake B2; hydraulic servomotors C-0, C-1 and C-2 for the clutches CO, C1 and C2; hydraulic servomotors B-0, B-1, B-2 and B-3 for the brakes B0, B1, B2 and B3; flow-rate controller valves for controlling the flow rates of oil flowing therethrough, the flow-rate control valves being respectively denoted by 301, 303, 304, 305, 306, 307, 308 and 309 and provided with respective check valves; a shuttle valve 302; oil strainers ST1, ST2, ST3 and ST4; a first solenoid valve S1 adapted for controlling the 2-3 shift valve 230 under the control of a later-mentioned electronic controller or computer 600; a second solenoid valve S2 adapted for controlling both the 1-2 shift valve 220 and the 3-4 shift valve 240 under the control of the computer 600; a third solenoid valve S3 adapted to control both the lock-up relay valve 120 and the lock-up control valve 170 under the control of the computer 600; and oil passages providing communication between the valves and the hydraulic cylinders of respective clutches and brakes. 
     The working oil is sucked by the hydraulic pump 102 from the oil pan 40 through an oil strainer 101, and is supplied to the oil passage 1 at a predetermined line pressure regulated by the pressure regulator valve 130. 
     The pressure regulator valve 130 has a spool 132 which is urged by a spring 131 disposed on the upper side thereof as viewed in FIG. 4, and a plunger 138 disposed in series with and in contact with the spool 132. In operation, a throttle pressure introduced through an oil passage 1W and acting on the uppermost land 136 produces a force which, together with the force produced by the spring 131, acts to cause a downward displacement of the spool 132. When reversing, downward force is produced also by the line pressure introduced through the oil passage 5 and acting on the lowermost land 137 of the plunger 138, in addition to the downward force produced by the throttle pressure and the spring 131. On the other hand, the line pressure is fed back to the lowermost land 133 of the spool 132 so as to produce a force which acts to displace the spool 132 upwardly. In consequence, the spool 132 is displaced to a position where the upward force and the downward force balance each other, so as to change the area of communication between the oil passage 1 and the oil passage 1Q, as well as the area of communication between the oil passage 1 and the drain port 135, thereby maintaining the line pressure at a level corresponding to the state of running of the vehicle. 
     The throttle valve 200 has a throttle plunger 201 which is adapted to be displaced by a cam 205 which in turn is rotated in accordance with the amount by which the accelerator pedal is depressed. The throttle valve 200 also has a spool 202 disposed on the lower side of the throttle plunger 201 as viewed in FIG. 4 with a spring 203 acting as an intermediary therebetween. The stroking of the throttle plunger 201 causes an axial movement of the spool 202 through the action of the spring 203 so that the spool 202 changes the line pressure supplied from the oil passage 1 to the throttle pressure corresponding to the throttle opening and delivers the throttle pressure to the oil passage 1W. 
     The second pressure regulator valve 150 has a spool 152 which is urged by a spring 151 acting on the upper side thereof as viewed in FIG. 4. The spool 152 is axially displaced to a position where a balance is obtained between the downward force exerted by the spring 151 and an upward force which is generated by an oil pressure fed back through the oil passage 1Q and acting on the lower side thereof, thus varying the area of communication between the oil passage 1Q and the lubricating oil supply passage 1V, as well as the area of communication between the oil passage 1Q and the drain port 155, thereby regulating the pressure in the oil passage 1Q to a predetermined secondary line pressure (torque converter pressure), while delivering excessive oil to the oil passage 1V. The oil supplied to the oil passage 1V is distributed both to the lubricating oil passage L1 of the main transmission 10 and the working oil supply passage L2 leading to the sub-transmission 50. 
     The manual valve 210 is directly connected to the shift lever arranged in the vicinity of the driver&#39;s seat, and is adapted to take one of the positions P (parking), R (reverse), N (neutral), D (drive), S (second) and L (low) as the shift lever is manually operated such as to select one of the ranges P, R, N, D, S and L. 
     The first solenoid valve S1, when not energized, blocks the oil passage 2E which is communicated with the oil passage 2 through an orifice 322. In this state, a solenoid pressure of high level equal to that of the line pressure is maintained in the oil passage 2E. However, when energized, the first solenoid valve S1 allows the oil passage 2E to communicate with the drain, so that the solenoid pressure in the oil passage 2E is reduced to a low level. 
     Similarly, the second solenoid valve S2 operates, when not energized, to establish a solenoid pressure of a high level in the oil passage 1H which communicates with the oil passage 1 through an orifice 332 and, when energized, to drain the oil in the oil passage 1H thereby reducing the solenoid pressure to a low level. 
     The third solenoid valve S3 controls the pressure in an oil chamber 121 which is formed on the upper side of the lock-up relay valve 120 as viewed in FIG. 4 and connected to the oil passage 2D which in turn is communicated with the oil passage 2A through an orifice 342. The third solenoid valve S3 also controls the pressure in an oil chamber 171 which is formed on the upper side of the lock-up control valve 170 as viewed in FIG. 4. More specifically, the third solenoid valve S3 operates, when energized, to establish the high solenoid pressure in the oil chambers 121 and 171 thereby displacing the spools 122 and 172 downwardly as viewed in FIG. 4, whereas, when de-energized, it allows the oil chambers 121 and 171 to be drained so as to reduce the solenoid pressure to a low level, thereby allowing the spools 122 and 172 to return to the upper positions by virtue of the force produced by the line pressure derived from the oil passage 1 and the forces of the springs 123 and 173. 
     The 1-2 shift valve 220 has a spool 222 which is urged by a spring 221 acting on the lower side thereof as viewed in FIG. 4. When the solenoid pressure of the high level is maintained in the oil passage 1H by the second solenoid valve S2 which is not energized, this high solenoid pressure is introduced into an oil chamber 224 which is defined on the upper side of the 1-2 shift valve 220, so that the spool 222 is moved downwardly to a first speed position. Conversely, when the second solenoid valve S2 is energized to drain the oil passage 1H and thereby to reduce the solenoid pressure to the low level, the spool 222 is returned to the upper position, away from the first speed position. When a third speed or a fourth speed is selected, the line pressure is introduced into an oil chamber 223 which is formed on the lower side of the spool 222 through the oil passage 1, the 2-3 shift valve 230 and then through the oil passage 1C which is communicated with the oil passage 1B, so that the spool 222 is held at the upper position regardless of the level of the solenoid pressure. 
     The 2-3 shift valve 230 has a spool 232 urged by a spring 231 acting on the lower side thereof, as viewed in FIG. 4. When the first solenoid valve S1 is energized, the solenoid pressure in the oil passage 2E takes the low level, so that the spool 232 is held by the force of the spring 231 at an upper position, as viewed in FIG. 4, where it serves for first and second speeds, as well as for reversing. Conversely, when the first solenoid valve S1 is not energized, the high solenoid pressure established in the oil passage 2E is applied to an oil chamber 234 so as to produce a force which acts to displace the spool 232 downwardly, as viewed in FIG. 4, to a lower position where it serves for the third speed and fourth speed operations. However, when the line pressure is supplied to an oil chamber 233 which is formed on the lower side of the spool 232, the spool 232 is fixed at the upper position regardless of the level of the solenoid pressure. 
     The 3-4 shift valve 240 has a spool 242 which is urged by a spring 241 acting on the lower side thereof. When the second solenoid valve S2 is not energized, the solenoid pressure of the high level is introduced through the oil passage 1H into an oil chamber 243 on the upper side of the spool 242, so that the spool 242 is held at a lower position as viewed in FIG. 4 where it serves for fourth speed (overdrive) operation. In contrast, when the second solenoid valve S2 is energized, the oil passage 1H is drained so that the spool 242 is set at an upper position where it serves for operations other than that for the fourth speed. When the line pressure is fed to an oil chamber 244 defined on the lower side of the spool 242 from the oil passage 1 or the oil passage 3 via the 2-3 shift valve 230 and the oil passage 1A, the spool 242 is fixed at the upper position by the force generated by the line pressure and the force of the spring 241. 
     The cut-back valve 160 has a spool 162 which is displaced by a downward force exerted by a spring 161 and an upward force produced by the line pressure introduced through the oil passage 2A. More specifically, when the line pressure is supplied through the oil passage 2A, the spool 162 is set at the upper position as viewed in FIG. 4 so as to bring the oil passage 1X into communication with the oil passage 1W which maintains the throttle pressure mentioned before, thereby outputting the throttle pressure as the cut-back pressure which is applied to the lower land 207 of the spool 202 of the throttle valve 200, thereby reducing the level of the throttle pressure in the oil passage 1W. The reduction in the throttle pressure causes the spool 132 of the pressure regulator valve 130 to move upwardly, thereby allowing the line pressure in the oil passage 1 to be relieved through a drain port 135, thus effecting a so-called &#34;cut-back&#34; of the line pressure. 
     The first accumulator control valve 180 has a spool 181 disposed at a lower portion thereof as viewed in FIG. 4, and a plunger 183 connected in series to the spool 181 and downwardly urged by a spring 182. In operation, the spool 181 is axially displaced so as to attain a balance between an upward force produced by the line pressure introduced into a lower oil chamber 184 through the oil passage 1 and a downward force which is the sum of the force of the spring 182 and the force produced by the output pressure fed back from the oil passage 1M to an upper oil chamber 185 through an orifice 333, thereby producing the output pressure through regulating the line pressure which is transmitted from the oil passage 1. The thus obtained output pressure is delivered to the second accumulator control valve 190 through the oil passage 1M. 
     The second accumulator control valve 190 has a spool 192 which is urged by a spring 191 acting on the lower side thereof. The spool 192 has an upper land 193 in which is formed an orifice 196 which provides communication between an upper oil chamber 194 formed on the upper side of the spool 192 and an intermediate oil chamber 195 formed under the land 193. The spool 192 is axially displaced upon balance being attained between an upward force which is the sum of the force exerted by the spring 191 and the force produced by a throttle modulator pressure which acts on a lower land 197 of the spool 192 through the oil passage 1W, and a downward force which is produced by a feedback pressure fed back from the oil passage 1M to act in the upper oil chamber 194 through the orifice 196, thereby producing an output pressure at the oil passage 1M. The output pressure from the oil passage 1M is supplied through the oil passage 1K to back-pressure chambers 272, 282 and 292 of the accumulators 270, 280 and 290 through respective back-pressure ports 271, 281 and 291, thereby controlling the back pressure in these accumulators. The back pressure in turn is applied through the oil passage 1K to the upper land 193 of the spool 192, so that the latter is displaced downwardly as viewed in FIG. 4, thereby allowing the oil passage 1K to be communicated with a drain port 199 through an intermediate oil chamber 195, whereby the back pressure in the oil passage 1K is relieved. 
     As stated before, the manual valve 210 is operated manually by means of the shift lever (not shown) of the main transmission 10 disposed in the vicinity of the driver&#39;s seat. The shift lever is adapted to be operated manually by the driver so as to take one of the ranges of P (parking), R (reverse), N (neutral), D (drive), S (second) and L (low) of the main shift position Mp. Table 1 shows the gear stages, i.e., the fourth (4), third (3), second (2) and first (1) gears obtainable in the respective ranges of the main shift position Mp, as well as the states of clutches and brakes corresponding to the respective gear stages. 
     
                                           TABLE 1__________________________________________________________________________Mp    S1 S2      S3 C1           C2 C0                B1 B2                     B3                       B0                         F1 F2 F3__________________________________________________________________________P     0  X X  X X  E X  X X X f  f  fR     0  X X  X E  E X  X E X f  f  fN     0  X X  X X  E X  X X X f  f  fD  1  0  X X  E X  E X  X X X f  L  (L)   2  0  0 ⊚         E X  E X  E X X L  f  (L)   3  X  0 ⊚         E E  E X  E X X f  f  (L)   4  X  X ⊚         E E  X X  E X E f  f  fS  1  0  X X  E X  E X  X X X f  L  (L)   2  0  0 ⊚         E X  E E  E X X (L)                            f  (L)   3  X  0 ⊚         E E  E X  E X X f  f  (L)   (3) X  X X  E E  E X  E X X f  f  (L)L  1  0  X X  E X  E X  X E X f  (L)                               (L)   2  0  0 X  E X  E E  E X X (L)                            f  (L)   (1) X  X X  E X  E X  X E X f  (L)                               (L)__________________________________________________________________________ 
    
     In Table 1, marks 0 and X represent the fact that the solenoid valves S1 and S2 are energized and de-energized, respectively, whereas the mark ⊚  shows that the solenoid valve S3 is energized to lock-up the transmission. The mark X appearing in the column of the solenoid valve S3 indicates that the solenoid valve S3 is de-energized to release the lock-up state. A symbol E shows that the clutches and the brakes are engaged, while the symbol X in the columns of clutches and brakes indicate that the clutches and brakes are released. A symbol L indicates that the one-way clutch is in the engaged state when the power is transmitted from the engine even though the power transmission is ensured also by a clutch or a brake connected in parallel with this one-way clutch, i.e., it indicates that the one-way clutch is locked. A symbol (L) shows that the one-way clutch is engaged only in the engine driving condition but is disengaged in the engine braking condition. Finally, a symbol f indicates that the one-way clutch is in the free state. 
     Table 2 shows the states of communication between the oil passage 1 and the oil passages 2 to 6, as obtained in the respective positions of the shift lever of the main transmission 10. 
     
                       TABLE 2______________________________________     P   R        N     D     S   L______________________________________Oil passage 2       X     X        X   0     0   0Oil passage 3       X     X        X   X     0   0Oil passage 4       X     X        X   X     X   0Oil passage 5       X     0        X   X     X   XOil passage 6       X     0        0   0     0   0______________________________________ 
    
     In Table 2, a mark 0 shows establishment of the communication that allows delivery of the line pressure to the respective oil passages, whereas a mark X indicates that the respective oil passages are drained. 
     The operation of the main hydraulic controller 100 in response to the manual shifting operation of the manual valve 210 will be explained hereinunder. 
     When Manual Valve 210 Selects N or P Range 
     In this state, the oil passage 1 communicates with none of the oil passages 2 to 5, as will be seen from Table 2, and the first and second solenoid valves S1 and S2 are energized and de-energized, respectively, as shown in Table 1. The spool 222 of the 1-2 shift valve 220 is therefore displaced to the lower position by the force produced by the high solenoid pressure, while the spool 232 of the 2-3 shift valve 230 takes the upper position under the force of the spring 231, thus allowing the line pressure to be supplied from the oil passage 1A to the lower oil chamber 244 of the 3-4 shift valve 240 and thereby upwardly displacing the spool 242 of the 3-4 shift valve 240. In this state, only the servomotor C-0 of the clutch C0 is communicated with the oil passage 1 through the 3-4 shift valve 240, oil passage 1F, flow-rate control valve 301 with check valve, and the oil passage 1E, while detouring the manual valve 210, so that only the clutch C0 is engaged. 
     When Manual Valve 210 Selects D Range 
     In this state, the line pressure is supplied to the oil passage 2, so that the clutch C1 is engaged. 
     The vehicle then starts with the first speed gear. Namely, when the D range is selected, the first solenoid valve S1 and the second solenoid valve S2 are energized and de-energized, respectively, as shown in Table 2, so that the spool 222 of the 1-2 shift valve 220 takes the lower position. In this state, the oil passages 3B and 2A leading to the brakes B1 and B2 are drained, and the oil passage 5C leading to the brake B3 is not supplied with oil, so that the brakes B1, B2 and B3 are released. At the same time, since the spool 232 of the 2-3 shift valve 230 is set at the upper position, the oil passage 1B is drained and the clutch C0 thereby disengaged. Meanwhile, the line pressure is supplied from the oil passage 1A to the lower oil chamber 244 of the 3-4 shift valve 240, so that the spool 242 is held at the upper position such as to maintain the clutch C0 in the engaged state by the oil pressure which is supplied from the oil passage 1 through the 3-4 shift valve 240 and the oil passage 1F. On the other hand, the oil passage 1B is drained so that the clutch C2 is disengaged and the brake B0 is released because the oil passage 1D is drained as a result of the supply of the line pressure to the oil passage 1F. In consequence, the power train is completed with the first speed gear, thereby starting the vehicle with the first speed gear. 
     Up-shift to the second speed gear is conducted in accordance with the state of running of the vehicle, in a manner which will be explained hereinunder. 
     When the vehicle running speed has reached a predetermined level, the computer 600 produces an output which allows the second solenoid valve S2 to be energized, so that the solenoid pressure acting in the upper oil chamber 224 of the 1-2 shift valve 220 is reduced to the low level, thereby causing the spool 222 of the 1-2 shift valve 220 to be moved upward, whereby the oil is supplied to the servomotor B-2 of the brake B2 from the oil passage 2, 1-2 shift valve 220, oil passage 2A, flow-rate control valve 308 with check valve, and the oil passage 2B, thus engaging the brake B2 and accomplishing the up-shift to the second speed gear. 
     As the vehicle speed is further increased, an up-shift to the third speed gear is effected in the following way. Namely, the computer 600 produces an output which serves to de-energize the first solenoid valve S1, so that the spool 232 of the 2-3 shift valve 230 is moved downwardly, thereby engaging the clutch C2 by the oil pressure which is supplied from the oil passage 1 through the 2-3 shift valve 230, oil passage 1B, shuttle valve 302, flow-rate control valve 303 with check valve and the oil passage 1P. Meanwhile, the line pressure is supplied from the oil passage 1C to the lower oil chamber 223 so as to fix the spool 222 of the 1-2 shift valve 220 at the upper position where it serves for a speed other than the first speed. 
     For up-shifting to the fourth speed gear, the computer 600 produces an output which serves to de-energize the second solenoid valve S2 so that the solenoid pressure which has been supplied from the oil passage 1H to the upper oil chamber 243 of the 3-4 shift valve 240 is returned to the high level, thus allowing the spool 242 of the 3-4 shift valve 240 to be moved downwardly. As a result, the oil passage 1F is drained, while the oil passage 1D is supplied with the pressurized oil, so that the flow-rate control valve 305 with check valve is supplied with the pressurized oil through the oil passage 1G, whereby the clutch C0 is disengaged and the brake B0 engaged, thus accomplishing the up-shift to the fourth speed gear. 
     When Manual Valve 210 Selects S Range 
     In this case, the line pressure is supplied not only to the oil passage 2 but also to the oil passage 3, as will be seen from Table 2, and the up-shift from the first speed gear to the second and third speed gears is conducted in the same manner as that in the case of the D range. However, up-shift to the fourth speed gear is prevented because the spool 242 of the 3-4 shift valve 240 is held at the upper position by the line pressure which is supplied to the lower oil chamber 244 of the 3-4 shift valve 240 from the oil passage 1 or the oil passage 3 through the 2-3 shift valve 230 and the oil passage 1A. When the second speed gear is selected, the line pressure is supplied to the servomotors of the clutches C0 and C1 and to the servomotor of the brake B2 as in the case of the D range. In addition, the line pressure is supplied also to the intermediate coast modulator valve 245 from the oil passage 3 through the 2-3 shift valve 230, oil passage 3A, 1-2 shift valve 220 and the oil passage 3D, so that pressurized oil of a pressure regulated by the intermediate coast modulator valve 245 is supplied to the oil passage 3B, thereby engaging the brake B1. Thus, the second speed in the S range permits the engine braking effect and affords a greater torque transmission capacity by virtue of the fact that both the brakes B2 and B1 are always held in the engaged state. 
     It is to be understood also that, when the manual valve 210 is operated from the D range to the S range during running at fourth speed gear in the D range, the line pressure is introduced into the lower oil chamber 244 of the 3-4 shift valve 240, thereby effecting down-shift to the third speed gear without delay. 
     When Manual Valve 210 Selects L range 
     In this case, the line pressure is supplied to the oil passages 2, 3 and 4, as will be seen from Table 2. The up-shift from the first speed gear to the second speed gear is conducted in the same manner as that in the D range but the up-shift to the third speed gear is prevented because the spool 232 of the 2-3 shift valve 230 is fixed at the upper position by the line pressure introduced into the lower oil chamber 233 of the 2-3 shift valve 230 from the oil passage 4. When the first speed gear is selected, the oil pressure is transmitted to the servomotor B-3 from the oil passage 4 through the 2-3 shift valve 230, oil passage 4A, low coast modulator valve 250, oil passage 4B, 1-2 shift valve 220 and the oil passage 5C, so that the brake B3 is held in the engaged state thereby allowing engine braking effect. When the second speed gear is selected, the operation is identical to that followed when the manual shift valve 210 selects the S range. It is to be understood also that, when the manual valve 230 is shifted to the L range during running with the third speed gear in the S range, down-shift to the second speed gear is effected without delay by the introduction of the line pressure into the lower oil chamber 233 of the 2-3 shift valve 230. Then, when the vehicle decelerates to a predetermined speed, the computer 600 produces an output for energizing the second solenoid valve S2, thereby effecting a down-shift from the second speed gear to the first speed gear. 
     When Manual Valve 210 Selects R Range 
     In this case, the pressure exists in the oil passage 5, while the oil passages 2, 3 and 4 are drained, as will be understood from Table 2. Since no pressure exists in the oil passages 2 and 3 leading to the servomotors of the clutch C1 and the brakes B1, B2, the clutch C1 and the brakes B1, B2 are released. The oil pressure supplied to the oil passage 5 is transmitted to the servomotor C-2 through the shuttle valve 302, flow-rate control valve 303 with check valve and the oil passage 1P, so that the clutch C2 is engaged. At the same time, the spool 222 of the 1-2 shift valve 220 is held in the upper position because the line pressure is supplied to the lower oil chamber 223 of this valve through the oil passage 1C, so that the line pressure is supplied to the oil passage 5C thereby engaging the brake B3. Meanwhile, the spool 232 of the 2-3 shift valve 230 is held in the upper position because the solenoid pressure acting in the upper oil chamber 243 of this valve reaches the low level as a result of energization of the first solenoid valve S1. Therefore, the line pressure is introduced from the oil passage 1 to the lower oil chamber 244 of the 3-4 shift valve 240 through the 2-3 shift valve 230 and the oil passage 1A, so that the spool 242 of the 3-4 shift valve is held in the upper position so as to allow the line pressure to be transmitted to the servomotor C-0 from the oil passage 1 through the 3-4 shift valve 240 and the oil passage 1F, thus engaging the clutch C0. At the same time, the oil passage 1D leading to the servomotor B-0 is drained so that the brake B0 is released, thus allowing reversing of the vehicle. 
     When the manual valve 210 is either in the D or S range, the line pressure is transmitted to the oil passage 2. If the spool 222 of the 1-2 shift valve 220 is in the upper position for speed ranges other than first speed, the line pressure is transmitted further to the oil passage 2A and is then introduced into the upper oil chamber 121 of the lock-up relay valve 120 through the oil passage 2D. When the line pressure exists in the upper oil chamber 121, if the third solenoid valve S3 is energized in response to the output from the computer 600 so as to maintain the pressure in the upper oil chamber 121 at the high level, the spool 122 of the lock-up relay valve 120 is moved downwardly, thereby bringing the oil passage 1Q into communication with the oil passage 6B. Consequently, the direct clutch 16 in the torque converter T is engaged, thereby switching the torque converter T into the direct-coupling state. 
     In contrast, if the line pressure does not exist in the oil passage 2A or if the solenoid pressure of the low level exists in the upper oil chamber 121 because of de-energization of the third solenoid valve S3 by the output of the computer 600, the spool 122 is held in the upper position by the force produced by the line pressure introduced into the lower oil chamber 124 from the oil passage 1. In this state, the oil passage 1Q is held in communication with the oil passage 1R, so that the direct clutch 16 in the torque converter T is held in the disengaged state. 
     When the spool 122 is held in the upper position, i.e., when the torque converter T is not locked up, the secondary line pressure (torque converter pressure) supplied from the torque converter T to the oil passage 1S is supplied to the oil cooler 0/C through the lock-up relay valve 120 and the oil passage 1T. In contrast, when the spool 122 is held in the lower position, i.e., when the torque converter is locked up, the pressurized oil is supplied to the oil cooler 0/C from the oil passage 1Q through the orifice 127 in the sleeve 125 and the oil passage 1T and also from the oil passage 1 through the orifice 126 in the sleeve 125 of the lock-up relay valve 120 and the oil passages 1U and 1T. 
     FIG. 5 shows the detail of the sub hydraulic controller 400 for controlling the four-wheel drive sub transmission 50. 
     The sub hydraulic controller 400 is constituted by the following major parts: a transfer manual valve 410 operated manually by means of a shift lever provided in the vicinity of the driver&#39;s seat and adapted to distribute the pressurized oil supplied through the oil passage 6 of the main hydraulic controller 100 for the main transmission 100 both to the oil passages 7 and 8; a relay valve 420; an inhibitor valve 440 for switching the states of engagement of the clutch C3 and the brake B4; a third accumulator control valve 460; a shift timing valve 470; an orifice control valve 480 for smoothing the engagement of the brake B4; an accumulator 490 for smoothing the engagement of the clutch C3; an accumulator 500 for smoothing the engagement of the brake B4, hydraulic servomotors C-3, C-4 and B-4 of the clutches C3, C4 and the brake B4; flow-rate control valves 511, 512 and 513 provided with check valves and adapted to control the flow rates of the pressurized oil flowing therethrough; oil strainers ST5 and ST6; a fourth solenoid valve S4 adapted to be selectively operated by an output from the computer 600; and oil passages providing communication between respective valves and servo cylinders of respective clutches and brakes. 
     The transfer manual valve 410 has a spool 411 which is directly connected to a shift lever (not shown) provided in the vicinity of the driver&#39;s seat. The transfer manual valve 410 also has an in-port 412 communicating with the oil passage 6 of the main hydraulic controller 100 for the main transmission 10, an out-port 413 communicating with the oil passage 7, an out-port 414 communicating with the oil passage 8, and drain ports 415 and 416. When the spool 411 of the transfer manual valve 410 is held in a two-wheel drive direct coupling (H2) position, the oil passage 6 is communicated with the oil passage 7, while the oil passage 8 is communicated with the drain port 416. When the spool 411 is held in a four-wheel drive direct coupling (H4) position, the oil passage 6 communicates with both the oil passage 7 and the oil passage 8. Finally, when the spool 411 is held in a reduced speed four-wheel drive (L4) position, the oil passage 6 is communicated with the oil passage 8, while the oil passage 7 is drained through the drain port 415. 
     The relay valve 420 has a spool 421 and a plunger 422 arranged in series with the spool 421. The spool 421 has lands 424 and 425 of an equal diameter and arranged at the upper and lower ends thereof, respectively, as viewed in FIG. 5, the upper land 424 being urged by a spring 423. The plunger 422 has an upper land 426 of the same diameter as the lands 424, 425 of the spool 421 and a lower land 427 of a diameter greater than the diameter of the upper land 426. The spool 421 and the plunger 422 in cooperation define an upper oil chamber 431 on the upper side of the upper land 424, a first intermediate oil chamber 432 between the upper land 424 and the lower land 425, a second intermediate oil chamber 433 between the spool 421 and the plunger 422, a third intermediate oil chamber 434 between the upper land 426 and the lower land 427, and a lower oil chamber 435 on the lower side of the lower land 427. 
     The operation of the relay valve 420 is as follows. When the pressurized oil is supplied to the lower oil chamber 435 from the oil passage 6A, the spool 421 and the plunger 422 are displaced upwardly, so that the oil passage 7 is brought into communication with the line pressure supply passage 7A through the first intermediate oil chamber 432, so that the supply and discharge of the line pressure to and from the lower oil chamber 456 of the inhibitor valve 440 is possible through the switching of the transfer manual valve 410. When the line pressure is supplied to the lower oil chamber 456 of the inhibitor valve 440 through the transfer manual valve 410 while the oil passage 7 is communicated with the line pressure supply passage 7A, the feedback pressure is supplied to the second intermediate oil chamber 433, so that the spool 421 is fixed at the upper position. In this state, if the solenoid valve S4 is energized in response to the output from the computer 600 so as to allow the lower oil chamber 435 to be drained, the plunger 422 alone is moved to the lower position, while the spool 421 remains in the upper position, so as to maintain the supply of the line pressure to the lower oil chamber 456 of the inhibitor valve 440. If the line pressure in the oil passage 7 is relieved through the transfer manual valve 410 in this state, or if the lower oil chamber 435 of the relay valve 420 is drained as a result of energization of the fourth solenoid valve S4 by the output from the computer 600 while the line pressure in the lower oil chamber 456 of the inhibitor valve 440 being drained through the transfer manual valve 410 which provides communication between the oil passage 7 and the line pressure supply passage 7A, the spool 421 and the plunger 422 are displaced downwardly by the force produced by the spring 423 so as to bring the line pressure supply passage 7A into communication with the drain port 436 through the first intermediate oil chamber 432. When the spool 421 is held in the lower position, the supply and discharge of the line pressure to and from the lower oil chamber 456 of the inhibitor valve 440 through the transfer manual valve 410 does not proceed, so that the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the lower positions as viewed in FIG. 5. 
     The inhibitor valve 440 has a spool 441 which is adapted to be moved between a first position which is on the lower side as viewed in FIG. 5 and a second position which is on the upper side as viewed in FIG. 5. The spool 441 serves as a change-over valve which conducts supply and discharge of the line pressure to and from the servomotors C-3 and B-4 of the clutch C3 and the brake B4 as it is moved between the lower first position and the upper second position. The inhibitor valve 440 also has a plunger 442 disposed in series with the spool 441 and adapted to be moved between a lower first position and an upper second position. The plunger 442, when in the upper second position, holds the spool 441 in the second position. The spool 441 has an upper sleeve-like land 445, a lower land 447 and an intermediate land 446 all having an equal diameter, and a spring 450 acting on the upper land 445 thus serving as means for setting the spool 441 in the first position. On the other hand, the plunger 442 has an upper land 448 of the same diameter as the lands of the spool 441, and a lower land 449 having a greater diameter than the upper land 448. Thus, the spool 441 and the plunger 442 define an upper oil chamber 451 on the upper side of the sleeve-like land 445, a first intermediate oil chamber 452 between the sleeve-like land 445 and the intermediate land 446, a second intermediate oil chamber 453 between the intermediate land 446 and the lower land 447, an oil chamber 454 between the spool 441 and the plunger 442, and a lower oil chamber 456. 
     The operation of this inhibitor valve 440 is as follows. When the spool 441 is held in the lower position, i.e., the first position, the upper oil chamber 451 is communicated with the oil passage 6A through an oil port 443 in the sleeve-like land 445, while the first intermediate oil chamber 452 and the second intermediate oil chamber 453 provide, respectively, communication between the line pressure passage 6 and a speed-reduction oil passage 6B and between a direct-coupling oil passage 6C and a drain port 457. 
     Conversely, when the spool 441 is held in the upper position, i.e., the second position, the upper oil chamber 451 is communicated with a drain port 458 through the oil port 443 formed in the sleeve-like land 445, while the first intermediate oil chamber 452 and the second intermediate oil chamber 453 provide, respectively, communication between the speed-reduction oil passage 6B and a drain port 459 and between the oil passage 6 and the direct-coupling oil passage 6C. On the other hand, the oil chamber 454 is communicated with an oil passage 6A which delivers an oil pressure corresponding in degree to the signal supplied to the fourth solenoid valve S4. This oil pressure acting in the oil chamber 454 serves to normally bias the spool 441 to the second position, i.e., the upper position, while urging the plunger 442 to the first position, i.e., the lower position. An oil pressure maintained in the lower oil chamber 456 serves to normally maintain the plunger 442 in the second position, i.e., in the upper position. 
     The orifice control valve 480 is a control valve which regulates the pressure of the working oil supplied to the oil passage 6B and delivers the regulated pressure to the hydraulic servomotor B-4. The orifice control valve 480 has a spool 485 which is biased upwardly by a spring 481. The spool 485 has an upper land 482, an intermediate land 483 and a lower land 484. The orifice control valve 480 has an upper oil chamber 486 on the upper side of the upper land 482, an upper intermediate oil chamber 487 between the upper land 482 and the intermediate land 483, a lower intermediate oil chamber 488 between the intermediate land 483 and the lower land 484, and a lower oil chamber 489 accommodating the spring 481. The spool 485 has an orifice 483A through which the upper intermediate oil chamber 487 is communicated with the lower intermediate oil chamber 488. 
     The upper oil chamber 486 is communicated with the oil passage 1W in the main hydraulic controller 100. The position of the spool 485 is changed in accordance with the balance between the force exerted by the spring 481 and the force produced by an oil pressure corresponding to the throttle opening and applied to the upper oil chamber 486, so as to deliver the line pressure in the oil passage 6B to the hydraulic servomotor B-4 while selectively making orifices 6a and 6b in the oil passage 6B effective. 
     The fourth solenoid valve S4 is controlled by the computer 600 such that it is switchable from the de-energized state to the energized state on condition that the shift lever (not shown) is in the L4 range (reduced speed four-wheel drive) and that the state of running of the vehicle meets a predetermined condition, and from the energized state to the de-energized state on condition that the shift lever is in the H2 (two-wheel drive direct coupling) or H4 (four-wheel drive direct coupling) range and that the state of running of the vehicle meets a predetermined condition. When de-energized, the fourth solenoid valve S4 acts to establish a solenoid pressure of high level equal to that of the line pressure in the oil passage 6A which communicates with the oil passage 2 through the orifice 514, whereas, when energized, it acts to drain the oil passage 6A, thereby generating a solenoid pressure of low level. 
     As stated before, the transfer manual valve 410 is operated by means of the shift lever of the sub transmission 50 provided in the vicinity of the driver&#39;s seat. This shift lever has sub shift positions Sp corresponding to the respective ranges of H2 (two-wheel drive direct coupling), H4 (four-wheel drive direct coupling) and L4 (reduced-speed four-wheel drive). The states of the brake B4 and the clutches C3 and C4, as well as the states of running of the vehicle, are shown in Table 3 in relation to respective ranges of the sub shift positions Sp. 
     
                       TABLE 3______________________________________Sp    S4       C3    B4      C4  Running state______________________________________H2    0        X     E       X   L2 α  E     X       X   H2H4    0        X     E       E   L4 α  E     X       E   H4L4    X        E     X       E   H4 β   X     E       E   L4______________________________________ 
    
     In Table 3, a symbol α represents the fact that, once the solenoid valve S4 is de-energized, the direct coupling condition is maintained even if the solenoid valve S4 is energized thereafter, whereas a symbol β represents the fact that, once the solenoid valve S4 is energized, the reduced-speed coupling condition is maintained even if the solenoid valve S4 is de-energized thereafter. A symbol E represents the fact that the clutch or the brake is held in the engaged state, while a symbol X represents the fact that the clutch or the brake is in the disengaged or released state. 
     Table 4 shows the respective states of communication between the oil passage 6 and the oil passages 7 and 8, in respective shift ranges of the sub-transmission. 
     
                       TABLE 4______________________________________      H2         H4    L4______________________________________Oil passage 7        0            0     XOil passage 8        X            0     0______________________________________ 
    
     In Table 4, the mark 0 represents the fact that the oil passage is supplied with the line pressure, while the mark X represents the fact that the oil passage is drained. 
     The operations of the sub-transmission 50 in respective ranges are as follows. 
     (A) When Transfer Manual Valve 410 Selects H2 Range 
     In this case, the oil passage 7 is supplied with the line pressure, while the oil passage 8 is drained. In consequence, the hydraulic servomotor C-4 is drained so as to disengage the clutch C4. In this state, the power of the engine is not transmitted to the sleeve 53, so that the vehicle runs in the two-wheel drive mode. 
     When the fourth solenoid valve S4 is de-energized in accordance with the output from the computer 600, the lower oil chamber 435 of the relay valve 420 is supplied with the solenoid pressure of high level, so that the spool 421 and the plunger 422 are held in the upper positions as viewed in the drawing, whereby the oil passage 7 and the line pressure supply passage 7A are communicated with each other through the relay valve 420, thereby allowing the line pressure signal to be applied to the lower oil chamber 456 of the inhibitor valve 440. In consequence, the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the second positions, i.e., upper positions, so that the oil passage 6B and, hence, the hydraulic servomotor B-4 are drained to release the brake B4. Since the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the upper positions, the oil passage 6C is communicated with the oil passage 6 and, hence, with the oil passage 6D through the third accumulator control valve 460, thereby supplying the line pressure to the hydraulic servomotor C-3 and thus engaging the clutch C3. The sub-transmission 50 therefore operates in the H2 (two-wheel drive direct coupling) range. 
     In this state, the spool 421 is fixed in the upper position by virtue of the line pressure fed back from the line pressure supply passage 7A to the second intermediate oil chamber 433 of the relay valve 420. Therefore, even if the fourth solenoid valve S4 is energized to supply the solenoid pressure of the low level to the lower oil chamber 435, the plunger 422 alone is moved to the lower position while the spool 421 remains in the upper position, thus maintaining the supply of the line pressure signal to the lower oil chamber 456 of the inhibitor valve 440. Thus, the sub-transmission 50 is maintained in the H2 (two-wheel drive direct coupling) range, even though the fourth solenoid vale S4 is energized. 
     (B) When Transfer Manual Valve 410 Selects H4 Range 
     In this case, the line pressure is supplied both to the oil passages 7 and 8. When the fourth solenoid valve S4 is de-energized in response to the output of the computer 600, the solenoid pressure of high level is supplied to the lower oil chamber 435 of the relay valve 420, so that the spool 421 and the plunger 422 are held in the upper positions so as to provide communication between the oil passage 7 and the line pressure supply passage 7A, thereby allowing the line pressure to be supplied to the lower oil chamber 456 of the inhibitor valve 440. Consequently, the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the second positions, i.e., upper positions, so that the oil passage 6B and, hence, the hydraulic servomotor B-4 are drained through the drain port 459, thereby releasing the brake B4. 
     Meanwhile, the oil passage 6C is held in communication with the oil passage 6 because the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the second or upper positions. The oil passage 6C is communicated also with the oil passage 6D through the third accumulator control valve 460. Therefore, the line pressure is supplied to the hydraulic servomotor C-3, thereby engaging the clutch C3. On the other hand, the line pressure supplied to the oil passage 8 is introduced into the hydraulic servomotor C-4, thereby engaging the clutch C4, and thus allowing the sub-transmission 50 to operate in the H4 (four-wheel drive direct coupling) range. 
     In this state, the line pressure is fed back to the second intermediate oil chamber 433 of the relay valve 420 from the line pressure supply passage 7A, so that the spool 421 is fixed in the upper position. When the fourth solenoid valve S4 is energized in this state, he plunger 422 alone is moved downwardly, while the spool 421 remains in the upper position, thereby maintaining the supply of the line pressure to the lower oil chamber 456 of the inhibitor valve 440 and, hence, the H4 (four-wheel drive direct coupling) range of the sub-transmission 50. 
     (C) When Transfer Manual Valve 410 Selects L4 Range 
     In this case, the oil passage 7 is drained, while the oil passage 8 is supplied with the line pressure, as will be seen from Table 4. Thus, the oil passages 7 and 7A are drained, while the oil passage 8 is supplied with the line pressure, regardless of the position of the relay valve 420, so that the clutch C4 is engaged to maintain the four-wheel drive mode of the sub-transmission 50. 
     When the transfer manual valve 410 is shifted from the high speed range H2 or H4 down to the low speed range L4, if the fourth solenoid valve S4 is de-energized in response to the output from the computer 600, the solenoid pressure of high level is supplied from the oil passage 6A to the oil chamber 454 of the inhibitor valve 440, while the line pressure which has been supplied to the lower oil chamber 456 of the same valve is drained through the oil passage 7A, relay valve 420, oil passage 7 and the transfer manual valve 410. In consequence, the plunger 442 of the inhibitor valve 440 is moved to the first or lower position, but the spool 441 of the same valve remains in the second or upper position so as to maintain the H4 (four-wheel drive direct coupling) range of the sub transmission 50. 
     If the fourth solenoid valve S4 has been energized in response to the output of the computer 600 when the transfer manual valve 410 is shifted to the L4 range, or if the fourth solenoid valve S4 is switched from the de-energized state to the energized state while the transfer manual valve 410 has been set in the L4 range, the solenoid pressure supplied to the oil passage 6A is changed to the low level, so that the spool 441 of the inhibitor valve 440 is set in the first or lower position by the force of the spring 450. As a result, the oil passage 6B is brought into communication with the oil passage 6, so that the hydraulic servomotor B-4 is supplied with the line pressure through the orifice 6a, orifice 6b or the orifice control valve 480, thereby engaging the brake B4. On the other hand, the oil passage 6C is communicated with the drain port 457 through the first intermediate oil chamber 452, so that the hydraulic servomotor C-3 is drained so as to disengage the clutch C3, thereby shifting the sub transmission 50 to L4 (reduced-speed four-wheel drive) range. Once the L4 range is attained, the spool 441 of the inhibitor valve 440 is not moved even if the fourth solenoid valve S4 is de-energized, because the solenoid pressure of the high level established in the oil passage 6A is supplied not only to the oil chamber 454 of the inhibitor valve 440 but also to the upper oil chamber 451 of the same valve through the oil port 443 in the sleeve-like land 445 of the spool 441, so that the sub transmission 50 is maintained in the L4 (reduced-speed four-wheel drive) range. 
     (D) When Transfer Manual Valve 410 is Shifted to H2 or H4 Range From L4 Range While Sub Transmission 50 is in Reduced-Speed Four-Wheel Drive Mode 
     In this case, if the fourth solenoid valve S4 has been energized by the output from the computer 600, the lower oil chamber 435 of the relay valve 420 is drained so that the spool 421 and the plunger 422 are held as a unit in the lower positions by the force of the spring 423. Therefore, the communication between the oil passage 7 and the oil passage 7A is interrupted by the upper land 424 and the lower oil chamber 456 of the inhibitor valve 440 is drained. In consequence, the spool 441 and the plunger 442 of the inhibitor valve 440 are held in the lower position by the force of the spring 450. As a result, the clutch C3 is released while the brake B4 is engaged, so that the sub transmission 50 is held in L2 (reduced-speed two-wheel drive) or L4 (reduced-speed four-wheel drive) range. However, when the fourth solenoid valve S4 is de-energized by the output of the computer 600, the solenoid pressure of the high level is introduced into the lower oil chamber 435 of the relay valve 420 as stated before, so that the spool 421 and the plunger 422 of the relay valve 420 are moved to the upper positions so as to provide communication between the oil passage 7 and the line pressure supply passage 7A. This in turn permits the line pressure to be supplied to the lower oil chamber 456 of the inhibitor valve 440, so that the spool 441 and the plunger 442 are moved to and held in the second or upper positions. As a result, the oil passage 6B is brought into communication with the drain port 459 and, hence, drained, so as to release the brake B4. Meanwhile, the oil passage 6C is communicated with the oil passage 6 because the spool 441 and the plunger 442 are held in the upper positions. In consequence, the oil passage 6 is brought into communication with the oil passage 6D through the third accumulator control valve 460, so that the clutch C3 is engaged to change-over the sub-transmission 50 to the H2 (two-wheel drive direct-coupling) range or H4 (four-wheel drive direct-coupling) range. 
     The computer for effecting the speed-change control of a four-wheel drive automatic transmission will be described hereinunder with reference to the block diagrams shown in FIGS. 6 and 7. 
     The computer 600 which controls the main hydraulic controller 100 and the sub hydraulic controller 400 by energizing and de-energizing the solenoid valves Si to S4 is composed of: a throttle opening sensor 601 for detecting the amount by which the accelerator pedal is depressed; a vehicle speed sensor 602 for converting a signal which is detected from the rotational speed of the output shaft of the sub-transmission 50 to the vehicle speed; a main transmission output shaft speed sensor 603 for detecting the rotational speed of the output shaft 32 of the main transmission 10 which constitutes the input shaft of the sub-transmission 50; a power select switch 604 for effecting a change-over between the shift patterns, e.g., economy, normal and power, at the option of the driver; a brake lamp switch 605; a main transmission shift lever position sensor 606 for detecting the position (Mp) of the set range of the main transmission 10; a sub transmission shift lever position sensor 607 for detecting the position (Sp) of the set range of the sub-transmission 50; an I/O port 608 which is an input port from the various factors representing the state of running of the vehicle and also serves as an output port to the solenoid valves S1 to S4; a central processing unit CPU; a random access memory RAM for shift point processing; and a read only memory ROM for storing data of shift patterns at the shift point and the lock-up point. 
     The transmission control will now be explained with reference to the flow chart shown in FIG. 8. 
     The ignition switch (not shown) of a vehicle is turned &#34;ON&#34; to start the engine E of the vehicle. Various factors representing the running state of the vehicle, such as a throttle opening sensor 601, a vehicle speed sensor 602, a main transmission output shaft speed sensor 603, a power select switch 604, a brake lamp switch 605, a main transmission shift lever position sensor 606, and a sub transmission shift lever position sensor 607 are read (701). Judgment is made (702) as to whether or not the present shift position (Mp) of the main transmission 10 is set at the P (parking) range. If the answer is YES (when the shift position Mp is set in the P range), an output which de-energizes (indicated by a numeral 0) the solenoid valves S1, S2 and S3 is produced (703), and the process then proceeds to the step (704). If the answer is NO (when the shift position Mp is not set in the P range), judgment is made (705) as to whether or not the shift position Mp is set in the R (reverse) range. If YES (when the shift position Mp is set in the R range), the process proceeds to the step (703), and if the answer is NO (when the shift position Mp is not set in the R range), decision is made (706) as to whether the shift position Mp is set in the N (neutral) range or not. If YES (when the shift position Mp is set in the N range), the process proceeds to the step (703), whereas if the answer is NO (when the shift position Mp is not set in the N range), judgment is made (707) as to whether the power select position PS is set at the power P or not. If YES (when the power select position PS is set at the power P), the power running is carried out (708) in which speed change is controlled by a shift pattern in which the shift point is set at a comparatively high point, and the process then proceeds to the step (704). If the answer is NO (when the power select position is not set the power P), whether the power select position PS is set at the economy E or not is decided (709). If YES (when the power select position PS is set at the economy E), the economy running is carried out (710) in which speed change is controlled by a shift pattern in which the shift point is set at a comparatively low point, and thereafter the step (704) is executed. In the case of NO (when the power select position PS is not set the economy E but at the normal N), judgment is made (711) as to whether the brake lamp switch 605 is ON or not. If YES (when the brake lamp switch 605 is ON), normal brake lamp energized running is carried out (712) in which speed change is controlled by a shift pattern in which the shift point is set at a comparatively high point, and the process then proceeds to the step (704). In NO is the answer, namely, when the brake lamp switch 605 is OFF, whether the fourth solenoid valve S4 for detecting the speed-change range of the sub transmission 50 is in the energized state or not is judged (713) in this embodiment so as to judge the state of running of the vehicle. (If the solenoid valve S4 is in the energized state, the vehicle is judged to be in the speed-reduced state of running, while if the solenoid valve S4 is in the de-energized state, the vehicle is judged to be in the directly coupled state of running.) If the answer is NO (when the fourth solenoid valve S4 is in the de-energized state), judgment is made (714) as to whether the state of running of the vehicle is in the first-speed region in the shift pattern in which the speed-change range of the sub-transmission 50 is the directly coupled range when the power select position PS is the normal. If YES is the answer, namely, if it is in the first-speed region, an output is generated (715) which energizes (indicated by the numeral 1) the solenoid valve S1 alone and de-energizes the solenoid valves S2 and S3, and thereafter the step (704) is executed. On the other hand, if NO (in a region other than the first-speed region), decision is given (716) as to whether or not vehicle is in the second speed region in the shift pattern in which the transmission gears of the sub transmission 50 are in the directly coupled state when the power select position PS is the normal. In the case of YES (in the second-speed region), decision is made (717) as to whether or not the vehicle is in the second-speed direct clutch engageable range (i.e., the region right to the solid line a in FIG. 9) at the lock-up point when the power select position PS shown in FIG. 9 is the normal and the transmission gears of the sub-transmission are in the directly coupled state. If the answer is YES (in the second-speed direct clutch engageable range), an output which energizes the solenoid valves S1, S2 and S3 is produced (718), whereas if NO is the answer in the step (717), namely, when the vehicle is in a region other than the second-speed direct clutch engageable range, an output which energizes the solenoid valves S1 and S2 and de-energizes the solenoid valve S3 is produced (719), and thereafter the step (704) is executed. If the answer is NO in the step (716), namely, when the vehicle is in a region other than the second-speed region, decision is made (720) as to whether or not the shift position Mp is set at the L (low) range. In the case of YES (when the shift position Mp is set the L range), whether the vehicle is in the region for shift-down to first speed or not is judged (721), and if the answer is YES (in the region for shift-down to first speed), the process proceeds to the step (715). If the answer is NO (in a region other than the region for shift-down to first speed) in the step (721), the process proceeds to the step (719). If the answer is NO (when the shift position Mp is not set in the L range) in the step (720), decision is given (722) as to whether the vehicle is in the third-speed region or not in the shift pattern in which the power select position PS is the normal and the transmission gears of the sub transmission 50 are in the directly coupled state. If the answer is YES (in the third-speed region), judgment is made (723) as to whether or not the vehicle is in the third-speed direct clutch engageable range (i.e., the region right to the solid line b in FIG. 9) at the lock-up point when the power select position PS is the normal and the transmission gears of the sub-transmission 50 are in the directly coupled state. If YES (in the third-speed direct clutch engageable region), an output which energizes the solenoid valves S2 and S3 and de-energizes the solenoid valve S1 is generated (724), whereas if the answer is NO in the step (723), namely, when the vehicle is in a region other than the third-speed direct clutch engageable region), an output which energizes the solenoid valve S2 and de-energizes the solenoid valves S1 and S3 is produced (725), and the operation proceeds to the step (704). If the answer is NO in the step (722), namely, when the vehicle is in a region other than the third-speed region, judgment is made (726) as to whether or not the shift position Mp is set in the S (second) range. If YES (when the shift position Mp is set in the S range), judgment is made (727) as to whether or not the vehicle is in the region for shift-down to second speed. If YES (in the region for shift-down to second speed), the step (721) is executed, whereas if NO is the answer in the step (727), namely, when the vehicle is in a region other than the region for shift-down to second speed, the process proceeds to the step (725). If the answer is NO in the step (726), namely, when the shift position Mp is not set in the S range), i.e., when the vehicle is in the fourth-speed region, judgment is made (728) as to whether the vehicle is in the fourth-speed direct clutch engageable region (i.e., the region right to the solid line c in FIG. 9) when the power select position PS shown in FIG. 9 is the normal and the transmission gears of the sub-transmission 50 are in the directly coupled state. If YES (in the fourth-speed direct clutch engageable region), an output which energizes the solenoid valve S3 and de-energizes the solenoid valves S1 and S2 is produced (729), whereas if NO is the answer, namely, when the vehicle is in a region other than the fourth-speed direct clutch engageable region), an output which de-energizes the solenoid valves S1, S2 and S3 is generated (730), and the process proceeds to the step (704). If the answer is YES in the step (713), namely, when the solenoid valve S4 is energized, judgment is made (731) as to whether the vehicle is in he first-speed region in the shift pattern in which the power select position PS is the normal and the transmission gears of the sub-transmission 50 are in the reduced-speed state. In the case of YES (in the first-speed region), the process proceeds to the step (715), whereas if NO is the answer, namely, when the vehicle is in a region other than the first-speed region, judgment is made (732) as to whether or not the vehicle is in the second-speed region in the shift pattern in which the power select position PS is the normal and the transmission gears of the sub-transmission 50 are in the reduced-speed state. If YES (in the second-speed region), judgment is made (733) as to whether or not the vehicle is in the second-speed direct clutch engageable region (i.e., the region right to the solid line f in FIG. 10) in the case of lock-up characteristics in which, in terms of the state of running of the vehicle, as shown in FIG. 10, the power select position is the normal, sub-transmission 50 are in the reduced-speed state. If YES (in the second-speed direct clutch engageable region, the process proceeds to the step (718), whereas if the answer is NO (in a region other than the second-speed direct clutch engageable region) in the step (733), the step (719) is executed. If the answer is NO in the step (732), namely, when the vehicle is in a region other than the second-speed region, judgment is made (734) as to whether or not the shift position Mp is set in the L range. If YES (when the shift position Mp is set in the L range), judgment is made (735) as to whether or not the vehicle is in the region for shift-down for first speed. In the case of YES (in the region for shift-down to first speed), the process proceeds to the step (715), whereas if the answer is NO in the step (735), namely, when the vehicle is in a region other than the region for shift-down to first speed), the step (719) is executed. If the answer is NO in the step (734), namely, when the shift position Mp is not set in the L range, decision is made (736) as to whether or not the shift position Mp is set in the S range. If YES (when the shift position Mp is set in the S range), judgment is made (737) as to whether the vehicle is in the region for shift-down to second speed. If YES (in the region for shift-down to second speed), the process proceeds to the step (735), whereas if the answer is NO in the step (737), namely, when the vehicle is in a region other than the region for shift-down to second speed), the process proceeds to the step (725). If the answer is NO in the step (736), namely, when the shift position Mp is not set in the S range), decision is made (738) as to whether or not the vehicle is in the third-speed direct clutch engageable region (i.e., the region right to the solid line g in FIG. 10) in the lock-up characteristics in which the power select position PS shown in FIG. 10 is the normal and the transmission gears of the sub-transmission 50 are in the reduced-speed state. If YES (in the third direct clutch engageable region), the process proceeds to the step (724), whereas if the answer is NO (in a region other than the third-speed direct clutch engageable region), the process proceeds to the step (725). In the step (704), a transfer shifting process 800 is carried out to effect control accompanying the shifting of the sub-transmission 50. 
     In the above-described embodiment, the energized state of the solenoid valve S4 is detected as a means of detecting the drive mode (the direct-coupling mode and the reduced-speed mode) of the sub-transmission, but, alternatively, other detecting means may be employed, such as by detecting the position of the spool 441 of the inhibitor valve 440 of the sub hydraulic controller 400, detecting the engagement of the clutch C3 or the brake B4, detecting the hydraulic pressure in the hydraulic servomotor C-3 or the hydraulic servomotor B-4, or detecting the driving state (such as the state of rotation of the ring gear 58 or the carrier 59) of the planetary gear device P3. 
     FIG. 11 illustrates another preferred embodiment of the transmission control device of the present invention. 
     In this embodiment, as shown in the flowchart in FIG. 11, the arrangement is made in such a manner that when if the answer in the step (732) is YES, the process proceeds to the step (719), while if the answer of the step (736) is NO, the process proceeds to the step (725), thereby preventing a lock-up at the time when the drive mode of the sub-transmission is set to the low-speed mode, and allowing a large torque to be generated for the drive wheels when the drive mode of the sub-transmission is set to the reduced-speed mode. 
     Although the invention has been described through its preferred forms, it is to be understood that the described embodiments are only illustrative and various changes and modifications may be imparted thereto. For instance, the planetary gear type speed change gears in the main transmission of the described embodiments may be substituted by other types of speed changing mechanism which employ a plurality of meshing gear trains, stepless speed change gear, and so forth, and the drive mode may be detected by detecting the shift position, the drive gears, and a ratio between the rotational speed input and rotational speed output of the sub-transmission. 
     It is to be understood also that the described four-wheel drive transfer employing the sub-transmission for transmitting power to front wheels and rear wheels is only illustrative and the invention can apply equally well to any type of transmission system which has a main transmission and a sub-transmission.