Select shock attenuating method and system for automatic transmission

According to a select shock attenuating method, line pressure, under which hydraulic fluid is supplied to hydraulically actuable and frictionally engageable coupling or couplings which are assigned to establish a gear ratio in an automatic transmission, is controlled. The automatic transmission has a plurality of selectable modes. In one mode, an appropriate one gear ratio state for a motor vehicle to start under normal road condition is established after placing a manual valve to a drive range position from a non-drive range position, while in another mode the automatic transmission is conditioned to another gear ratio. In the one mode, control signal on which the line pressure depends is subject to a pulse-like change with a first height corresponding to a first precharge level and a first duration corresponding to a first precharge period of time, then to a gradual increase at a first rate until a first coupling period of time is expired, and then to a drop to such a level as to keep the line pressure high enough to maintain torque transmission. In the another mode, the control signal is subject to a pulse-like change with a second height corresponding to a second precharge level and a second duration corresponding to a second precharge period of time, then to a gradual increase at a second rate until a second coupling period of time is expired, and then to a drop to such a level as to keep the line pressure high enough to maintain torque transmission.

BACKGROUND OF THE INVENTION 
The present invention relates to a select shock attenuating method and 
system for an automatic transmission. The term "select shock" is used 
hereinafter to mean shock occurring in an automatic transmission during 
transition from neutral to any one of torque transmitting drive states. 
U.S. Pat. No. 4,730,521 issued to Hayasaki et al. on Mar. 15, 1989 
discloses an automatic transmission comprising a plurality of 
hydraulically actuable and frictionally engageable couplings, a hydraulic 
control system including a pressure regulator valve and a manual valve, 
and a control unit. A pressure regulator valve effects pressure regulation 
by generating line pressure under the control of an actuator in the form 
of a solenoid actuable actuator, also referred to as a line pressure 
solenoid. The manner of controlling the magnitude of the line pressure is 
well understood from claims 1 to 7 of U.S. Pat. No. 4,807,496 issued to 
Hayasaki et al. on Feb. 28, 1989. The control unit is programmed to 
condition the hydraulic control system in one state wherein hydraulic 
fluid under the line pressure is supplied to one of the plurality of 
hydraulically actuable and frictionally engageable couplings to establish 
one gear ratio after a manual selector valve has been placed to a forward 
drive range position from a neutral range position. After placing the 
manual selector valve to a reverse drive range position from the neutral 
range position, the control unit conditions the hydraulic control system 
in another state wherein hydraulic fluid under the line pressure is 
supplied to two of the plurality of hydraulically actuable and 
frictionally engageable couplings to establish another gear ratio. 
Japanese Patent Application First (unexamined) Publication No. 3-28571 
proposes a system for attenuating select shock which occurs during a 
transition after a manual selector valve has been placed to a forward 
drive range position from a neutral range position. According to this 
known system, the line pressure is momentarily increased toward a 
precharge level and then subject to an increase at a gradual rate. The 
precharge level and the rate of increase are determined after considering 
a stroke volume of a servo piston and torque bearing capacity of a 
hydraulically actuable and frictionally engageable coupling to be 
actuated. 
An object of the present invention is to provide an improved control method 
and system which reduces the generation of shock tending to occur during a 
transition to any of available drive states from a neutral state in an 
automatic transmission. 
SUMMARY OF THE INVENTION 
In one embodiment, the invention is embodied in an automatic transmission 
comprising a hydraulic control system including a pressure regulator valve 
capable of effecting pressure regulation to generate line pressure under 
the control of an actuator, a control unit programmed to condition a 
hydraulic control system in one state wherein hydraulic fluid under the 
line pressure is supplied to one of a plurality of hydraulically actuable 
and frictionally engageable couplings to establish one gear ratio from a 
neutral or to condition the hydraulic control system in another state 
wherein hydraulic fluid under the line pressure is supplied to two of the 
plurality of hydraulically actuable and frictionally engageable couplings 
to establish another gear ratio from the neutral. The improvement is such 
that during a transition from the neutral to the one state, a control 
signal supplied to the actuator is subject to a pulse-like change with a 
first height corresponding to a first precharge level and a first duration 
of time corresponding to a first precharge period of time, then to a 
gradual increase at a first rate until a first coupling period of time is 
expired, and then to a drop to such a level as to keep the line pressure 
high enough to maintain torque transmission, while during a transition 
from the neutral state to the another state, the control signal is subject 
to a pulse-like change with a second height corresponding to a second 
precharge level and a second duration of time corresponding to a second 
precharge period of time, then to a gradual increase at a second rate 
until a second coupling period of time is expired, and then to a drop to 
such a level as to keep the line pressure high enough to maintain torque 
transmission. 
In another embodiment, the present invention is embodied in an automatic 
transmission wherein one gear ratio is established owing to supply of 
hydraulic fluid under line pressure generated by a pressure regulator 
valve to one of hydraulically actuable and frictionally engageable 
couplings after placing a manual valve to a drive range position from a 
non-drive range position in a first mode, while in a second mode after 
placing the manual valve to the drive range position from the non-drive 
range position, another gear ratio is established owing to supply of 
hydraulic fluid under line pressure generated by the pressure regulator 
valve to at least two of the hydraulically actuable and frictionally 
engageable couplings. The improvement is such that after placing the 
manual valve to the drive range position from the non-drive range position 
in the first mode, a control signal, on which the pressure regulator valve 
effects pressure regulation in generating the line pressure, is subject to 
a pulse-like change with a first height corresponding to a first precharge 
level and a first duration of time corresponding to a first precharge 
period of time, then to a gradual increase at a first rate until a first 
coupling period of time is expired, and then to a drop to such a level as 
to keep the line pressure high enough to maintain torque transmission, 
while after placing the manual valve to drive range position from the 
non-drive range position in the second mode, the control signal is subject 
to a pulse-like change with a second height corresponding to a second 
precharge level and a second duration of time corresponding to a second 
precharge period of time, then to a gradual increase at a second rate 
until a second coupling period of time is expired, and then to a drop to 
such a level as to keep the line pressure high enough to maintain torque 
transmission.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an automatic stepwise operable transmission is 
drivingly coupled with a torque converter 10 which is in turn drivingly 
coupled with an output shaft 12 of an engine of an automotive vehicle. The 
engine has a throttle valve which opens in degrees. The automatic 
transmission provides four forward speeds with an overdrive and a single 
reverse. The transmission includes an input shaft 13 connected to a 
turbine runner of the torque converter 10, and an output shaft 14 
connected to a final drive gear assembly, not illustrated. It also 
includes a first planetary gear set 15, a second planetary gear set 16, a 
reverse clutch 18, a high clutch 20, a forward clutch 22, an overrunning 
clutch 24, a low & reverse brake 26, a band brake 28, a low one-way clutch 
29, and a forward one-way clutch 30. The torque converter 10 includes 
therein a lock-up clutch 11. The first planetary gear set 15 comprises a 
sun gear S1, a ring gear R1, and a pinion carrier PC1 rotatably supporting 
pinion gears P1, each meshing both the sun and ring gears S1 and R1. The 
planetary gear set 16 comprises a sun gear S2, a ring gear R2, and a 
pinion carrier PC2 rotatably supporting pinion gears P2, each meshing both 
the sun and ring gears S2 and R2. The carrier PC1 is connectable to the 
input shaft 13 via the high clutch 20, while the sun gear S1 is 
connectable to the input shaft 13 via the reverse clutch 18. The carrier 
PC1 is connectable to the ring gear R2 via the forward clutch 22 and the 
forward one-way clutch 30 arranged in series with the forward clutch 22 or 
via the overrunning clutch 24 arranged in parallel to both the forward 
clutch 22 and forward one-way clutch 30. The sun gear S2 is connected to 
the input shaft 13, while the ring gear R1 and the carrier PC2 are 
constantly connected to the output shaft 14. The low & reverse brake 26 is 
arranged to hold the carrier PC1 stationary, while the band brake 28 is 
arranged to hold the sun gear S1 stationary. The low one-way clutch 29 is 
arranged to allow a rotation of the pinion carrier PC1 in a forward 
direction (the same direction as a direction which the engine shaft 12 
rotates in), but preventing a rotation in the opposite reverse direction. 
In this transmission, rotating states of various rotary elements (S1, S2, 
R1, R2, PC1, and PC2) of planetary gear sets 15 and 16 are varied by 
actuating the hydralically actuable and frictionally engageable couplings, 
namely, the clutches 18, 20, 22, 24, and brakes 26, 28, in different kinds 
of combination, thereby to vary a ratio, i.e., a gear ratio, of a 
revolution speed of the input shaft 13 to a revolution speed of the output 
shaft 14. Four forward speeds and a single reverse speed are provided by 
actuating the clutches 18, 20, 22, and 24, and the brakes 26 and 28 in 
various combinations as shown in FIG. 2. In FIG. 2, the sign .largecircle. 
(circle) denotes that a particular coupling which it is assigned to is 
actuated or engaged, the signs .alpha.1 (alpha one) and .alpha.2 (alpha 
two) designate a ratio of number of teeth of the ring gear R1 to that of 
the sun gear S1 and a ratio of number of teeth of the ring gear R2 to that 
of the sun gear S2. 
FIG. 3 shows a hydraulic control system of the transmission. This hydraulic 
control system comprises a pressure regulator valve 40, a pressure 
modifier valve 42, a line pressure solenoid 44, a modifier pressure 
accumulator 46, a pilot valve 48, a torque converter relief valve 50, a 
lock-up control valve 52, a first shuttle valve 54, a lock-up solenoid 56, 
a manual selector valve 58, a first shift valve 60, a second shift valve 
62, a first shift solenoid 64, a second shift solenoid 66, a servo charger 
valve 68, a 3-2 timing valve 70, a 4-2 relay valve 72, a 4-2 sequence 
valve 74, a first reducing valve 76, a second shuttle valve 78, an 
overrunning clutch control valve 80, an overrunning clutch solenoid 82, an 
overrunning clutch reducing valve 84, a 1-2 accumulator 86, a 2-3 
accumulator 88, a 3-4 accumulator 90, a N-D accumulator 92, an accumulator 
control valve 94, and a filter 96. These components are interconnected as 
illustrated. As illustrated, they are connected also to the 
before-mentioned torque converter (the torque converter 10 includes an 
apply chamber 11a and a release chamber 11b for the lock-up clutch 11), 
the forward clutch 22, the high clutch 20, the band brake 28 (the band 
brake 28 including a second speed apply chamber 28a, a third speed release 
chamber 28b, and a fourth speed apply chamber 28c), the reverse clutch 18, 
the low & reverse brake 26, and the overrunning clutch 24. They are 
connected also to the variable capacity vane type oil pump 34, the oil 
cooler 36, the forward lubrication circuit 37, and the rear lubrication 
circuit 38 as illustrated. The detailed description of these valves is 
hereby omitted. The automatic transmission thus far briefly described is 
substantially the same as an automatic transmission of RE4R01A type which 
is manufactured by Nissan Motor Company Limited in Japan. The automatic 
transmission of the RE4R01A type is described in a service manual 
(publication No. A261C07) entitled "NISSAN FULL RANGE ELECTRONICALLY 
CONTROLLED AUTOMATIC TRANSMISSION RE4R01A TYPE" published by Nissan Motor 
Company Limited in March, 1987. U.S. Pat. No. 4,730,521 issued to Hayasaki 
et al. on Mar. 15, 1989 discloses the automatic transmission of the 
RE4R01A type. Thus, reference is made to the above-mentioned service 
manual and the U.S. Pat. No. 4,730,521 for full understanding of the 
automatic transmission of this type. 
In this automatic transmission, the magnitude of a line or system pressure 
is controllable by the line pressure solenoid 44. The manner of 
controlling the magnitude of the line pressure is described on pages I-22 
to I-24 of the above-mentioned service manual. Reference is made to claims 
1 to 7 of U.S. Pat. No. 4,807,496 issued to Hayasaki et al on Feb. 28, 
1989 for full understanding of features of the line pressure control. 
Briefly, the pressure regulator valve 40 effects pressure regulation to 
generate line pressure under the control of the line pressure solenoid 44 
which serves as an actuator for the pressure regulator valve 40. The line 
pressure solenoid 44 is of the duty cycle type which can vary ON duration 
per each cycle from 0 percent to 100 percent. A pilot pressure, i.e., a 
constant pressure, generated by the pilot valve 48 is fed to the pressure 
modifier valve 42. When it is in OFF state, the line pressure solenoid 44 
causes a needle valve to close a drain circuit for the pilot pressure, 
while when it is in ON state, the drain circuit is opened. Increasing the 
percentage of OFF duration per each cycle causes a decrease in flow rate 
of hydraulic fluid drained via the drain circuit, resulting in an increase 
in throttle pressure supplied to the pressure modifier valve 42. The 
throttle pressure is decreased by decreasing the percentage of OFF 
duration per each cycle since the flow rate of hydraulic fluid discharged 
via the drain circuit is increased. The term "throttle pressure" is used 
herein to mean a pressure regulated by ON-OFF operation of the line 
pressure solenoid 44. The pressure modifier valve 42 uses the pilot 
pressure as a source of pressure and effects regulation of pressure in 
response to the throttle pressure to generate a pressure modifier pressure 
variable with the throttle pressure. This pressure modifier pressure is 
supplied to the pressure regulator valve 40. The pressure regulator valve 
40 uses a pump discharge pressure displaced by the pump 34 as a source of 
pressure and effects pressure regulation in response to the pressure 
modifier pressure to generate the line pressure. The hydraulic fluid under 
the line pressure is supplied to the manual valve 58. 
FIG. 4 shows an automatic transmission control unit 300 which controls the 
solenoids 44, 56, 64, 66 and 82. The control unit 300 comprises an input 
interface 311, a reference pulse generator 312, a CPU (a central processor 
unit) 313, a ROM (a read only memory) 314, a RAM (a random access memory) 
315, and an output interface 316. They are interconnected by an address 
bus 319, and a data bus 320. Fed to this control unit 300 are output 
signals of an engine revolution speed sensor 301, an output shaft 
revolution speed sensor (a vehicle speed sensor) 302, a throttle opening 
degree sensor 303, a select position switch 304, a kickdown switch 305, an 
idle switch 306, a full throttle switch 307, an oil temperature sensor 
308, an input shaft revolution speed sensor (a turbine revolution speed 
sensor) 309, an overdrive switch 310, and an automatic transmission (A/T) 
mode switch 340. The output shaft revolution speed sensor 302 detects a 
revolution speed of the output shaft 14. The input shaft revolution speed 
sensor 309 detects a revolution speed of the input shaft 13. The outputs 
of the control unit 300 are supplied to the shift solenoids 64 and 66, 
overrunning clutch solenoid 82, lock-up solenoid 56, and line pressure 
solenoid 44. 
As shown in FIG. 3, the manual selector valve 58 has P (park), R (reverse), 
N (neutral), D (drive), 2 and 1 range position. With manipulation of a 
selector (not shown), the manual selector valve 58 may be placed to any 
one of the range positions. The select position switch 304 is provided to 
detect which one among the range positions the manual selector valve 58 is 
placed at. The A/T mode switch 340 is manually operable and has a power 
mode position, an automatic mode position and a snow mode position. 
The shift valves 60 and 62 which are actuable by the corresponding shift 
solenoids 64 and 66. In order to condition the hydraulic control system in 
the first speed or gear ratio state, both shift solenoids 64 and 66 are 
energized and become ON states, respectively. In order to condition the 
hydraulic control system in the second speed or gear ratio state, the 
shift solenoid 64 is deenergized and becomes OFF state, while the shift 
solenoid 66 is energized and becomes ON state. In the third speed or gear 
ratio state, both of the shift solenoids 64 and 66 are not energized and 
assume OFF states, respectively. In the fourth speed or gear ratio state, 
the shift solenoid 64 is energized and assumes ON state, while the shift 
solenoid 66 is not energized and assumes OFF state. This relationship is 
tabulated in FIG. 5. 
The shift solenoids 64, 66 and the line pressure solenoid 44 are under the 
control of the automatic transmission control unit 300. 
Reference is made to pages I-22 to I-27 of the service manual (publication 
No. A261C07) and to the U.S. Pat. No., 4,730,521 for explanation of 
actuation of the solenoids 44, 64 and 66, and valves 42, 60 and 62. The 
necessary control functions are performed in the control unit 300. 
FIG. 6 illustrates a D range economy pattern which is used in the control 
unit 300 when the manual selector valve 58 is placed at the D range 
position and the automatic mode is selected by the A/T mode switch 340. In 
FIG. 6, the fully drawn lines show a set of 1-2 upshift points, a set of 
2-3 upshift points, and a set of 3-4 upshift points, respectively, while 
the broken lines show a set of 2-1 downshift points, a set of 3-2 
downshift points, and a set of 4-3 downshift points, respectively. 
FIG. 7 illustrates a D range snow pattern which is used in the control unit 
300 when the manual selector valve 58 is placed at the D range position 
and the snow mode is selected by the A/T mode switch 340. In FIG. 7, the 
fully drawn lines show a set of 2-3 upshift points and a set of 3-4 
upshift points, while the broken lines show a set of 3-2 downshift points 
and a set of 4-3 downshift points. As different from the D range economy 
pattern, with the D range snow pattern, the automatic transmission is 
conditioned to establish the second speed or gear ratio after placing the 
manual selector valve 58 to the D range position from the N range 
position, and the first speed or gear ratio is not allowed to be 
established. Thus, the D range snow pattern is suitable for starting the 
automotive vehicle from a standstill on road with low friction 
coefficient. 
The control unit 300 is programmed to perform a gear shift control in 
accordance with the D range economy pattern shown in FIG. 6 when the 
manual selector valve 58 is placed at the D range position in the 
automatic mode, while in the snow mode, the D range snow pattern shown in 
FIG. 7 is used for the gear shift control when the manual selector valve 
58 is placed at the D range position. 
Referring to FIGS. 1, 2 and 3, let us now consider how the first speed or 
gear ratio is established after placing the manual selector valve 58 to 
the D range position from the neutral position in the automatic mode. In 
the N range position, hydraulic fluid under the line pressure is supplied 
via a line pressure circuit 400 to and blocked by the manual selector 
valve 58 as shown in FIG. 3. For ease of control, both of the shift 
solenoids 64 and 66 are energized in the N and P range positions. Placing 
the manual selector valve 58 to the D range position from the N range 
position causes the control unit 300 to keep energizing both of the shift 
solenoids 64 and 66 since the shift control in accordance with the D range 
economy pattern shown in FIG. 6 is performed when the A/T mode switch 340 
takes the automatic mode position. With energization of both of the shift 
solenoids 64 and 66, the associated drain circuits for the pilot pressure 
are closed to allow supply of pilot pressure to one end of the shift 
valves 60 and 62, respectively. The pilot pressure generated by the pilot 
valve 48 is distributed via a pilot pressure circuit 402 to the shift 
solenoid 64 and 66. 
Since the line pressure circuit 400 is allowed to communicate with a first 
gear circuit 404, the hydraulic fluid is supplied via the first gear 
circuit 404 to a servo motor, not shown, of the forward clutch 22. The 
flow rate of hydraulic fluid supplied to the servo motor of the forward 
clutch 22 is determined by the magnitude of the line pressure generated by 
the pressure regulator valve 40. During a precharge period of time, a 
servo piston of the servo motor moves toward a position at which the 
frictional elements of the forward clutch 22 are about to engage, and then 
the engagement progresses toward the full complete engagement owing to an 
increase in pressure within the servo motor. The term "a coupling period 
of time" is used herein to mean a period of time from a moment when a 
command for establishing a gear ratio is issued to a moment when 
engagement of one or a plurality of hydraulically actuable and 
frictionally engageable couplings is completed to establish the gear 
ratio. The first gear ratio circuit 404 distributes hydraulic fluid to the 
shift valve 60, 4-2 relay valve and overrunning clutch control valve 80. 
Via the shift valve 68, the hydraulic fluid is supplied to the shift valve 
62 and the accumulator control valve 94. 
Next, let us consider how the second speed or gear ratio is established 
after placing the manual selector valve 58 to the D range position from 
the neutral position in the snow mode. Placing the manual selector valve 
58 to the D range position from the N range position causes the control 
unit 300 to deenergize the shift solenoid 64 with the shift solenoids 66 
kept energized since the shift control in accordance with the D range snow 
pattern shown in FIG. 7 is performed when the A/T mode switch 340 takes 
the snow mode position. Upon deenergization of the shift solenoid 64 with 
the shift solenoid 66 energized, the drain circuit for the pilot pressure 
associated with the shift solenoid 64 is opened, while the drain circuit 
for the pilot pressure associated with the shift solenoid 66 is closed. 
Thus, the pilot pressure does not act on the one end of the shift valve 
60, while the one end of the shift valve 62 is subject to the pilot 
pressure. Since the shift valve 60 is not subject to the pilot pressure, 
the first gear circuit 404 is in fluid communication with a second gear 
circuit 406. The second gear circuit 406 leads to the second speed apply 
chamber 28a of the band brake 28 via the 1-2 accumulator 86, and also to 
the first shuttle valve 54 and servo charger valve 68. 
Since the line pressure circuit 400 is allowed to communicate with the 
first gear circuit 404, the hydraulic fluid is supplied via the first gear 
circuit 404 to a servo motor, not shown, of the forward clutch 22, and the 
hydraulic fluid is supplied via the second gear circuit 406 to the second 
speed apply chamber 28a of the band brake 28. The flow rate of hydraulic 
fluid supplied to the servo motor of the forward clutch 22 and to the 
second speed apply chamber 28a of the band brake 28 is determined by the 
magnitude of the line pressure generated by the pressure regulator valve 
40. 
The control unit 300 is programmed such that the second speed or gear ratio 
is established after placing the manual selector valve 58 has been placed 
to the 2 range position from the N range position. 
The manner of the line pressure control is explained below in principle 
with reference to the signal diagram shown in FIG. 10. 
At the top of FIG. 10 is shown a change in the output signal of the select 
position switch 304 from the N range position indicative signal to the D 
range position indicative signal or 2 range position indicative signal. 
Let us now assume that the manual selector valve 58 has been placed to the 
D range position in the automatic mode selected by the A/T mode switch 
340. Upon or after occurrence of a change in the output signal of the 
select position switch 304 from the N range position indicative signal to 
the D range position indicative signal, a target line pressure P.sub.L 
indicative signal, i.e., a control signal on which the line pressure 
solenoid 44 operates, is subject to a pulse-like change or increase with a 
height corresponding to a first precharge level P.sub.CH1 and a first 
duration of time corresponding to a first precharge period of time 
T.sub.1, then to a gradual increase at a first rate DP.sub.L1 until a 
first coupling period of time T.sub.2 is expired, and then to a drop to a 
normal line pressure level P.sub.L (TH) that is a function of the throttle 
opening degree TH. The fully drawn line illustrates the change in the 
target line pressure P.sub.L indicative signal. In accordance with this 
change in the target line pressure P.sub.L indicative signal, the pressure 
regulator valve 40 generates the line pressure under the control of the 
line pressure solenoid 44. Thus, the line pressure in rapidly increased to 
the first precharge level P.sub.CH1, then subject to a drop. After this 
drop, the line pressure is gradually increased at the rate appropriate for 
smooth engagement of the forward clutch 22, and then dropped to the normal 
line pressure level that is determined in response to the throttle opening 
degree. 
Let us now assume that the manual selector valve 58 has been placed to the 
D range position in the snow mode selected by the A/T mode switch 340. In 
this case, the second speed or gear ratio is established owing to 
engagement of the forward clutch 22 and engagement of the band brake 28. 
Upon or after occurrence of a change in the output signal of the select 
position switch 304 from the N range position indicative signal to the D 
range position indicative signal, the target line pressure P.sub.L 
indicative signal is subject to a pulse-like change or increase with a 
height corresponding to a second precharge level P.sub.CH2 and a second 
duration of time corresponding to a second precharge period of time 
T.sub.12, then to a gradual increase at a second rate DP.sub.L2 until a 
second coupling period of time T.sub.22 is expired, and then to a drop to 
the normal line pressure level P.sub.L (TH). The broken line illustrates 
this change in the target line pressure P.sub.L indicative signal. The 
second precharge level P.sub.CH2 is higher than the first precharge level 
P.sub.CH1, and the second rate DP.sub.L2 is different from the first rate 
DP.sub.L1. In this embodiment, the second precharge period of time 
T.sub.12 is slightly longer than the first precharge period of time 
T.sub.1, but the second coupling period of time T.sub.22 is substantially 
the same as the first coupling period of time T.sub.2. Owing to this line 
pressure control policy, the second speed or gear ratio is established 
smoothly and as quickly as the first speed or gear ratio is. The line 
pressure control in establishing the second gear ratio after placing the 
manual selector valve 58 to the 2 range position from the N range position 
is substantially the same as the line pressure control policy just 
discussed in connection with the broken line shown in FIG. 10. 
The flow diagram shown in FIG. 8 shows an example of a routine for the line 
pressure control explained with reference to FIG. 10. The flow diagram 
shown in FIG. 9 is a timer routine executed at regular intervals. In the 
execution of the timer routine, increment of a timer T is carried out at a 
block 500. 
In FIG. 8, there is an interrogation at a block 502 whether or not there 
has occured a change from the N (or P) range position to the D (or 2 or 1 
or R) range position. If this is the case, there is another interrogation 
at a block 504 whether or not a flag F.sub.TR is set equal to 1. Since the 
flag F.sub.TH is equal to 0 initially, the routine proceeds to blocks 506 
and 508. In the block 506, the timer T is reset equal to 0 (zero), while 
in the block 508, the flag F.sub.TR is set equal to 1. After the block 
508, the routine proceeds to a block 510 where there is an interrogation 
whether or not the 2 range indicative signal is present. If this is not 
the case, the routine proceeds to a block 512 where there is an 
interrogation whether or not the snow mode is selected. If this is not the 
case, the routine proceeds to block 514 where there is an interrogation 
whether or not the timer T is greater than the first coupling period of 
time T.sub.2 (see FIG. 10). Since this is not the case, the routine 
proceeds to a block 516 where there is an interrogation whether or not the 
timer T is greater than the first precharge period of time T.sub.1 (see 
FIG. 10). Since this is not the case, the routine proceeds to a block 518 
where the target line pressure P.sub.L is set equal to the first precharge 
level P.sub.CH1. After this block 518, the routine proceeds back to the 
block 504. Since F.sub.TR is equal to 1, the routine proceeds to a block 
520 where the timer T is fetched. After this block 520, the routine 
proceeds to the blocks 510, 512, 514, 516, 518, 504 and back to 510. This 
loop is maintained until the timer T reaches the first precharge period of 
time T.sub.1. After the timer T has become greater than the first 
precharge period of time T.sub.1, the routine proceeds from the block to a 
block 522. In the block 522, there is an interrogation whether or not a 
flag FF is set equal to 1. Initially this flag FF is reset equal to 0 
(zero), the routine proceeds a block 524 where the target line pressure 
P.sub.L is decreased by M.sub.1 and then to a block 526 where the flag FF 
is set equal to 1. After this step 526, the routine proceeds to a block 
528 where the target line pressure P.sub.L is increased by the first rate 
DP.sub.L1 before proceeding back to the block 504. The execution along a 
loop along the blocks 504, 520, 510, 512, 514, 516, 522 and 528 is 
repeated until the timer T reaches the first coupling period of time 
T.sub.2. After the timer T has become greater than the first coupling 
period of time T.sub.2, the routine proceeds from the block 514 where the 
flags F.sub.TR and FF are reset to 0 (zero), respectively. After this 
block 530, the routine proceeds to a block 532 where the target line 
pressure P.sub.L is set equal to the normal line pressure P.sub.L (TH). 
If the result of the interrogation at the block 502 is negative (NO), the 
routine proceeds to the block 532. 
Let us now assume that there has occurred a change from the N range 
position to the D range position in the snow mode. 
In this case, the routine proceeds along the blocks 502, 504, 506, 508, 
510, 512 down to a block 534. In the block 534, there is an interrogation 
whether or not the timer T is greater than the second coupling period of 
time T.sub.22. Since this is not the case, the routine proceeds to a block 
536 where there is another interrogation whether or not the timer is 
greater than the second precharge period of time T.sub.12. Since this is 
not the case, the routine proceeds to a block 538 where the target line 
pressure P.sub.L is set equal to the second precharge level P.sub.CH2 
before proceeding back to the block 504. After the timer T has increased 
and becomes greater than the second precharge period of time T.sub.12, the 
routine proceeds from the block 534 to a block 540 where there is an 
interrogation whether or not the flag FF is set equal to 1. Since this 
flag FF is initially equal to 0 (zero), the routine proceeds to blocks 542 
and 544 where the target line pressure P.sub.L is decreased by M.sub.2 and 
the flag FF is set equal to 1, respectively. After the block 544, the 
routine proceeds to a block 546 where the target line pressure P.sub.L is 
increased by the second rate DP.sub.L2 before proceeding back to the block 
506. Thereafter, the execution of a loop along the blocks 504, 520, 510, 
512, 534, 536, 540 and 546 is repeated until the timer T reaches the 
second coupling period of time T.sub.22. After the timer T has become 
greater than T.sub.22, the routine proceeds from the block 534 to the 
block 530 and then to the block 532. 
Assuming that there is a change from the N range position to the 2 range 
position, the routine proceeds along the blocks 502, 504, 506, 508 and 510 
down to the block 534. Thus, the execution repeated subsequently is the 
same as the case when there has occurred a change from the N range 
position to the D range position in the snow mode. 
Line pressure control along the target line pressure P.sub.L determined is 
not specifically shown in this routine for ease of simplicity of 
explanation. Such line pressure control may be effected in a different 
routine. 
From the preceding description, it will now be appreciated that a desired 
speed or gear ratio is established in a smooth and quick manner after 
placing the manual valve from the non-drive range position to any one of 
the drive range positions.