Automatic transmission control apparatus

An automatic transmission control apparatus has a clutch, a hydraulic servo for the clutch, a fluid pressure control unit, a range shift detector, and an electronic control unit. The fluid pressure control unit has a manual valve, a pressure regulating valve device, and a changeover valve device. The electronic control unit controls the changeover valve device and the pressure regulating valve device. Immediately after a shift to the forward driving range, the electronic control unit holds the changeover valve device in such a state that a forward driving range pressure is supplied without passing through the pressure regulating valve device. Then, the electronic control unit shifts the changeover valve device to such a state that a regulated pressure is supplied from the pressure regulating valve device, which has been switched to a pressure regulating state by the electronic control unit. This control operation precisely sets the fluid pressure changeover timing and the initial fluid pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an automatic transmission control apparatus and, 
more particularly, to a control apparatus for controlling engagement of a 
clutch for transmitting rotation from an engine to a speed changing 
apparatus. 
2. Description of the Related Art 
A typical automatic transmission employs a clutch for transmitting rotation 
from an engine to a speed changing apparatus. Such a clutch is operated to 
engage by shifting the automatic transmission from the neutral range to 
the driving range. Since this engaging operation must be quickly performed 
without producing a shock, technology, such as disclosed in Japanese 
patent application laying-open No. HEI 3-28571, is used. This technology 
temporarily rapidly raises the fluid pressure supply to the hydraulic 
servo for engaging the clutch to a high fluid pressure (shelf pressure), 
and maintains the high fluid pressure for a period until the clutch starts 
to engage, that is, during the piston stroke of the hydraulic servo, in 
order to quickly complete the piston stroke, that is, to shorten the time 
taken until the engagement starts, and then rapidly lowers the fluid 
pressure to a low fluid pressure level before an initial period of the 
engagement, and gradually raises the fluid pressure as the engagement 
progresses, in order to reduce the engaging shocks. In this technology, 
the fluid pressure supply to the hydraulic servo is regulated by a 
pressure regulating valve on the basis of a signal pressure from a 
solenoid valve during the entire supply period. Thus, the conventional 
technology shortens the engaging time while reducing the engaging shocks. 
However, in a case where a pressure regulating valve is employed to output 
a regulated fluid pressure, although the electrical signal to the solenoid 
valve for controlling the pressure regulating valve may be rapidly 
changed, it is normally difficult to cause the pressure regulating valve 
to follow the rapid change of the electrical signal. Thus, the fluid 
pressure actually output from the pressure regulating valve is inevitably 
delayed to some extent in response to the electrical signal, and becomes 
blunt. Viewed from this aspect, the above-described conventional 
technology in which the pressure regulating valve regulates the fluid 
pressure supply to the hydraulic servo, it is considered that the fluid 
pressure actually output from the pressure regulating valve will delay, as 
indicated by a broken line in FIG. 9, relative to an expected output fluid 
pressure (instructed value) indicated by a solid line. It is also of 
concern that a high fluid pressure may linger into a clutch engagement 
initial period during which the fluid pressure must be rapidly reduced to 
remove the shelf pressure. If this occurs, the clutch will rapidly engage, 
producing an engaging shock as illustrated by the output shaft torque 
(To), as shown by the lower of FIG. 9 broken line. Considering that the 
value of fluid pressure occurring at the start of the engagement has a 
greater effect on the engaging shocks than the fluid pressure 
characteristics over the rising process from the initial engagement period 
to the completion of the engagement, the excessively high fluid pressure 
at the start of the engagement is a disadvantage with regard to engaging 
shocks. Thus, the conventional art needs improvements in the reduction of 
actual engaging shocks. 
SUMMARY OF THE INVENTION 
Accordingly, it is a first object of the invention to reduce the clutch 
engaging time and further reduce the clutch engaging shocks by controlling 
the fluid pressure supply to a hydraulic servo for engaging the clutch 
that transmits rotation from an engine to a speed changing apparatus, with 
high precision in accordance with the realities, during a shift from the 
non-driving range to the driving range of the automatic transmission and, 
more particularly, at the start of the engagement of the clutch. 
A second object of the invention is to further reduce the engaging shocks 
by setting the fluid pressure at the start of the engagement, which is a 
most important factor to reduce the engaging shocks as mentioned above, in 
accordance with the input torque to the clutch. 
If the timing of starting to raise the fluid pressure in an engagement 
initial period significantly delays from the engagement start timing of 
the clutch, it takes a long time to complete the engagement. Conversely, 
if the raise start timing becomes too early, a rapid engagement will occur 
producing a considerable engaging shock. Accordingly, a third object of 
the invention is to further reduce the engaging time and the engaging 
shocks by starting to raise the fluid pressure synchronously with the 
timing at which the clutch engagement starts. 
It is desirable for reduction of the engaging time that the period for 
supplying the forward driving range pressure to the hydraulic servo 
coincides with the period of the piston stroke of the hydraulic servo. 
However, even a slight overlap of the forward driving range pressure 
supplying period with the engagement start timing will increase the 
engaging shocks. Accordingly, a fourth object of the invention is to 
minimize the engaging time and reduce the engaging shocks by detecting the 
start of engagement of the clutch and supplying the forward driving range 
pressure for a maximized period before the start of the engagement. 
However, even with the control wherein the start of engagement of the 
clutch is detected as mentioned above, a detection delay, which may occur 
for some cause, will result in a high fluid pressure lingering at the 
start of the engagement, causing engaging shocks. Accordingly, a fifth 
object of the invention is to reduce the engaging time as much as possible 
while allowing some detection errors or delay by supplying the forward 
driving range pressure for a period lasting until the elapse of a length 
of time predetermined to start when a shift to the forward driving range 
is performed and to end immediately before the clutch engagement starts. 
According to the invention, there is provided an automatic transmission 
control apparatus comprising: a clutch which, when a forward driving range 
is selected, is engaged to transmit rotation from an engine to a speed 
changing apparatus; a hydraulic servo for selectively engaging and 
releasing the clutch; fluid pressure control means for controlling fluid 
pressure supply to the hydraulic servo; range shift detecting means for 
detecting shift from a non-driving range to the forward driving range; and 
electronic control means for controlling the fluid pressure control means 
on the basis of a signal from the range shift detecting means. The fluid 
pressure control means comprises: a manual valve for outputting a forward 
driving range pressure when the forward driving range is selected; 
pressure regulating means for regulating the forward driving range 
pressure on the basis of a signal from the electronic control means to 
output a regulated fluid pressure; and changeover means able to be 
selectively changed over between a first position for supplying the 
forward driving range pressure to the hydraulic servo, and a second 
position for supplying the regulated fluid pressure to the hydraulic 
servo, on the basis of a signal from the electronic control means. The 
electronic control means comprising: holding means for outputting a signal 
to the changeover means to hold the changeover means in the first position 
for a predetermined time period after the range shift detecting means has 
detected shift to the forward driving range, and to shift the changeover 
means to the second position when the predetermined time period elapses; 
and fluid pressure raising means for outputting a signal to the pressure 
regulating means to cause the pressure regulating means to output an 
initial fluid pressure that is lower than the forward driving range 
pressure, before or when the holding means shifts the changeover means to 
the second position, and to cause the pressure regulating means to 
gradually raise fluid pressure from the initial fluid pressure, when or 
after the holding means shifts the changeover means to the second 
position. 
In a second structure of the invention, the control apparatus of the 
invention further comprises input torque detecting means for detecting 
input torque inputted from the engine to the speed changing apparatus, the 
electronic control means controls the fluid pressure control means on the 
basis of signals from the input torque detecting means and the range shift 
detecting means, and the fluid pressure raising means outputs a signal to 
the pressure regulating means to cause the initial fluid pressure to be a 
fluid pressure in accordance with the input torque detected by the input 
torque detecting means. 
In a third structure of the invention, the control apparatus of the 
invention further comprises engagement detecting means for detecting the 
start of engagement of the clutch, and the fluid pressure raising means 
outputs a signal to the pressure regulating means to gradually raise fluid 
pressure from the initial fluid pressure when engagement of the clutch is 
detected on the basis of a signal from the engagement detecting means. 
In a fourth structure of the invention, the control apparatus of the 
invention further comprises engagement detecting means for detecting the 
start of engagement of the clutch, and the predetermined time period 
during which the holding means holds the changeover means to the first 
position is a period starting when shift to the forward driving range is 
performed and ending when the start of engagement of the clutch is 
detected by the engagement detecting means. 
In a fifth structure of the invention, the predetermined time period during 
which the holding means holds the changeover means to the first position 
is a period lasting until the elapse of a length of time predetermined to 
start when shift to the forward driving range is performed and to end 
immediately before the clutch is started to engage. 
With the above-described basic structure of the invention, when the shift 
to the forward driving range causes the manual valve to output the forward 
driving range pressure, the forward driving range pressure is supplied 
directly to the hydraulic servo without passing through the pressure 
regulating means because, in this occasion, the changeover means is 
shifted to the first position for supplying the forward driving range 
pressure to the hydraulic servo by the holding means. The changeover means 
is maintained in the first position for a predetermined time period 
following the shift to the forward driving range, during which period the 
piston stroke of the hydraulic servo is quickly performed, thus enabling 
reduction of the engaging time. After the predetermined period elapses, 
the changeover means is shifted to the second position for supplying the 
hydraulic servo with a regulated pressure from the pressure regulating 
means. Since the pressure regulating means is set into a state for 
outputting an initial fluid pressure not later than the elapse of a 
predetermined length of time, the fluid pressure supply to the hydraulic 
servo is switched from the forward driving range pressure to the initial 
fluid pressure, which is lower than the forward driving range pressure, 
without delay. Thus, unlike the conventional art, the apparatus of the 
invention eliminates a delay in reducing the fluid pressure, preventing 
engaging shocks. Following the shifting of the changeover means to the 
second position, the fluid pressure supply to the hydraulic servo is 
gradually increased to gradually engage the clutch. According to the 
invention, since the changeover means switches from the state where the 
forward driving range pressure is supplied without passing through the 
pressure regulating means, to the state where a regulated fluid pressure 
is supplied from the pressure regulating means, the fluid pressure supply 
to the hydraulic servo will not delayed in response to the signal from the 
electronic control means. The invention thus reduces the engaging time 
while preventing the engaging shocks. 
The second structure described above prevents the engaging shocks with 
enhanced reliability because of the setting of the initial fluid pressure 
in accordance with the input torque, in addition to optimization of the 
timing of switching the fluid pressure supply to the hydraulic servo. 
The third structure described above further reduces the engaging time and 
the engaging shocks, since the engagement detecting means actually detects 
the start of engagement of the clutch and the fluid pressure raise is 
performed synchronously with the start of engagement of the clutch. 
The fourth structure described above minimizes the engaging time while 
reducing the engaging shocks, since the engagement detecting means 
actually detects the start of engagement of the clutch so that the period 
for supplying the forward driving range pressure can be set to a period 
that ends immediately before the clutch starts to engage. 
With the fifth structure described above, since the forward driving range 
pressure is supplied during a period lasting until elapse of a length of 
time predetermined to start when the shift to the forward driving range is 
performed and to end immediately before the clutch starts to engage, there 
is no possibility that the forward driving range pressure will continue to 
be supplied to the hydraulic servo when the clutch starts to engage. Thus, 
this structure further reduces the possibility of the occurrence of 
engaging shocks.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A preferred embodiment of the invention will be described hereinafter with 
reference to the accompanying drawings. 
Referring to FIG. 1, an automatic transmission control apparatus according 
to a first embodiment comprises: a clutch C1 which, when a forward driving 
range is selected, is engaged to transmit rotation from an engine to a 
speed changing apparatus; a hydraulic servo C-1 for selectively engaging 
and releasing the clutch C1; fluid pressure control means 3 for 
controlling fluid pressure (P.sub.C1) to be supplied to the hydraulic 
servo C-1; range shift detecting means Sn5 for detecting shift from a 
non-driving range to a forward driving range; and electronic control means 
5 for controlling the fluid pressure control means 3 on the basis of a 
signal from the range shift detecting means Sn5. 
The fluid pressure control means 3 has: a manual valve 32 for supplying a 
forward driving range pressure (P.sub.L) to the hydraulic servo C-1 when 
the forward driving range is selected; pressure regulating means 33, 34 
for regulating the forward driving range pressure (P.sub.L) on the basis 
of a signal from the electronic control means 5 to output a regulated 
fluid pressure (P.sub.A); and changeover means 35, 36 able to be 
selectively changed over between a first position for supplying the 
forward driving range pressure (P.sub.L) to the hydraulic servo C-1, and a 
second position for supplying the regulated fluid pressure (P.sub.A) to 
the hydraulic servo C-1. 
The electronic control means 5 comprises: holding means for outputting a 
signal to the changeover means 35, 36 to hold the changeover means 35, 36 
in the first position for a predetermined time period after the range 
shift detecting means Sn5 has detected a shift to the forward driving 
range, and to shift the changeover means 35, 36 to the second position 
when the predetermined time period elapses; and fluid pressure raising 
means for outputting a signal to the pressure regulating means 33, 34 to 
cause the pressure regulating means 33, 34 to output an initial fluid 
pressure (P.sub.AI) that is lower than the forward driving range pressure 
(P.sub.L) , before or when the holding means shifts the changeover means 
35, 36 to the second position, and to cause the pressure regulating means 
33, 34 to gradually raise the fluid pressure from the initial fluid 
pressure (P.sub.AI), when or after the holding means shifts the changeover 
means 35, 36 to the second position. 
Various components will be described in detail. As illustrated in FIG. 2, 
the automatic transmission T according to this embodiment has a gear train 
of four forward speeds and one reverse speed for a front-engine front 
wheel-drive vehicle. The automatic transmission T comprises: a torque 
converter 12 having a lockup clutch denoted by an abbreviation L/C in the 
drawing; a biaxial gear train having a main speed changing unit 14 and an 
under drive-type planetary gear unit 15; the differential unit 16; a fluid 
pressure control unit 3, provided as the fluid pressure control means, for 
controlling the gear train and the lockup clutch L/C; and an electronic 
control unit (indicated by an abbreviation ECU in the drawing) 5, provided 
as the electronic control means, for controlling the fluid pressure 
control unit 3. 
FIG. 2 further shows an engine E/G, a throttle sensor Sn1 for detecting 
throttle opening (.theta.) of the engine E/G; an engine speed sensor Sn2 
for detecting engine speed (N.sub.E); a clutch C1 rotational speed sensor 
Sn3 for detecting rotational speed (N.sub.C1) of the clutch C1 provided as 
the first friction engaging means of the automatic transmission T; a 
vehicle speed sensor Sn4 for detecting vehicle speed (V) based on output 
speed; and a neutral start switch Sn5 provided as range shift detecting 
means for detecting shift position. The electronic control unit 5 is a 
control computer that outputs control signals to the on-off solenoid valve 
and linear solenoid valve of the fluid pressure control unit 3 on the 
basis of information from the various sensors, mainly in accordance with 
the throttle opening (.theta.) and the vehicle speed (V), following the 
installed program. 
The main speed changing unit 14 of the gear train of the automatic 
transmission T comprises a single pinion-type planetary gear unit and a 
double pinion-type planetary gear unit. The two gear units are combined by 
using an integrated sun gear S1 and integrating with one set of pinion 
gears P1 of the double pinion-type gear unit with the planetary gears of 
the single pinion-type gear unit, and by connecting a carrier Cr1 
supporting the pinion gears P1 and a carrier Cr2 supporting the other set 
of pinion gears P2 of the double pinion-type gear unit. The sun gear S1, 
common to the two gear units of the main speed changing unit 14, is 
fixable to a transmission casing 10 by a brake B1, and also fixable to the 
casing 10 by a one-way clutch F1 and a brake B2 that are disposed in 
series. A ring gear R1 of the single pinion-type gear unit is connected to 
an input shaft 101 by the clutch C1 according to the present invention. 
The sun gear S1 is connected to the input shaft 101 by a clutch C2. The 
input shaft 101 is connected to a turbine output shaft of the torque 
converter 12. The carrier Cr1 supporting the pinion gears P1 meshed with 
the sun gear S1, the ring gear R1 and the pinion gears P2 and the carrier 
Cr2 supporting the pinion gears P2 meshed with the pinion gears P1 and a 
ring gear R2 are connected by a counter gear 102. The ring gear R2 of the 
double pinion-type gear unit is fixable to the transmission casing 10 by a 
brake B3 and a one-way clutch F2 that are disposed in parallel. 
A ring gear R3 of the under drive planetary gear unit 15 is an input 
element connected to the main speed changing unit 14 by the counter gears 
102, 103. A carrier Cr3 and a sun gear S3 are connected by a clutch C3. 
The sun gear S3 is fixable to the transmission casing 10 by a one-way 
clutch F3 and a band brake B4 that are disposed in parallel. The carrier 
Cr3 is connected to an output gear 104 for transmission to the 
differential unit 16. 
In the gear train structured as described above, under an under drive 
rotational state where the sun gear S3 of the under drive planetary gear 
unit 15 is fixed with the clutch C3 released and the brake B4 engaged, so 
as to achieve input to the ring gear R3 and output from the carrier Cr3, 
the first speed is achieved when the input transmitted to the ring gear R1 
by engagement of the clutch C1 of the main speed changing unit 14 is 
outputted as rotation of the carriers Cr1, Cr2 by reaction force support 
of the ring gear R2 provided by engagement of the one-way clutch F2. The 
second speed is achieved when the input to the ring gear R1 is output as 
rotation of the carriers Cr1, Cr2 while the sun gear S1 is fixed by the 
engagement of the brake B2. The third speed is achieved when the main 
speed changing unit 14 is in the direct-coupled state where the clutch C2 
is additionally engaged to rotate the ring gear R1 and the sun gear S1 at 
the same speed, so that the input rotation is directly output from the 
carriers Cr1, Cr2. In a direct-coupled state of the under drive planetary 
gear unit 15, achieved by release of the brake B4 and engagement of the 
clutch C3, the fourth speed is achieved. The reverse is reached when the 
clutch C2 is engaged and the brake B3 is engaged to achieve input to the 
sun gear S1 and the fixation of the ring gear R2 so that the rotation of 
the carrier Cr2 reverses. 
FIG. 3 is a table showing the relationship between the operation of the 
clutches, the brakes and the one-way clutches and the thereby-achieved 
gear speeds, that is, the first speed to the fourth speed. In the table, 
"R" indicates the reverse drive shift position; "N" indicates the neutral 
position; "D" indicates the forward drive shift positions; "O" indicates 
engagement; "X" indicates release; and (O) indicates engagement effected 
only during engine braking. 
Referring to FIG. 4, the hydraulic circuit for controlling the gear train 
as described above comprises, similar to the hydraulic circuit of a 
conventional fluid pressure control apparatus, an oil pump (PUMP) as a 
fluid pressure source incorporated in the speed changing mechanism, 
various pressure regulating valves that include a primary regulator valve 
31 for regulating the pressure from the oil pump to a predetermined stable 
line pressure (P.sub.L) in accordance with the vehicle speed and the 
throttle opening to output a secondary pressure and a secondary regulator 
valve (not shown) for reducing the secondary pressure to a regulated 
torque converter pressure supply and outputting the remaining pressure as 
a lubrication pressure, the manual valve 32, various solenoid valves, 
various shift valves, orifices and check valves disposed in the passages 
connecting the various valves. 
FIG. 4 is illustrates only those portions of the above-described hydraulic 
circuit that are relevant to the present invention. This circuit comprises 
the manual valve 32, a C-1 control valve 33 and the linear solenoid valve 
34 for controlling the C-1 control valve 33, the C-1 control valve 33 and 
the linear solenoid valve 34 being provided as the pressure regulating 
means, and a C-1 changeover valve 35 and the solenoid valve 36 for 
controlling the C-1 changeover valve 35, which are provided as the 
changeover means. The manual valve 32 is connected to a line pressure 
supply fluid passage 301 and a range pressure output fluid passage 302 so 
as to output a D range pressure, that is, a line pressure (P.sub.L) in 
accordance with current throttle opening (.theta.), when the forward 
driving range, that is, the D range, is selected. 
The C-1 control valve 33 is a secondary pressure-operated pressure reducing 
valve that receives at the opposite ends of the spool the throttle signal 
pressure (P.sub.th) and the feedback pressure of the regulated fluid 
pressure (P.sub.A) to the clutch C1 to adjust the openings of the output, 
input and drain ports. The input port is connected to the range pressure 
fluid passage 302. The output port is connected to an apply pressure 
output fluid passage 305. The signal port is connected to a throttle 
signal pressure fluid passage 306. The feedback port is connected to the 
apply pressure output fluid passage 305 through an orifice. The linear 
solenoid valve 34 outputs the throttle signal pressure (P.sub.th) provided 
by reducing the line pressure (P.sub.L) as the basic pressure, on the 
basis of the signal from the electronic control unit 5. The throttle 
signal pressure (P.sub.th) is supplied to the primary regulator valve 31 
and the C-1 control valve 33 through the throttle signal pressure fluid 
passage 306. Thus, the C-1 control valve 33 and the linear solenoid valve 
34 perform the function of the pressure regulating means for regulating 
the line pressure (P.sub.L) on the basis of the signal from the electronic 
control unit 5 and for outputting the regulated fluid pressure (P.sub.A). 
The C-1 changeover valve 35 is a spool-type changeover valve that 
selectively communicates the output port with the input port connecting to 
the apply pressure output fluid passage 305 or the input port connecting 
to the range pressure fluid passage 302. The C-1 changeover valve 35 is 
switched by a solenoid signal pressure opposing the spring force acting on 
an end of the spool. The solenoid valve 36 is a normal open-type on-off 
valve that closes upon receiving the solenoid signal from the electronic 
control unit 5. The solenoid valve 36 drains the fluid pressure from a 
solenoid signal pressure fluid passage 307 connecting to the line fluid 
passage 301 through an orifice and stops draining the pressure. Thus, the 
C-1 changeover valve 35 and the solenoid valve 36 perform the function of 
the changeover means able to be selectively changed over between a first 
position (indicated in the lower half of the illustration of each of the 
valves in the drawing) for supplying the line pressure (P.sub.L) to the 
hydraulic servo C-1 of the clutch C1 through the fluid passage 304 and 
second position (indicated in the upper half of the illustration of each 
of the valves in the drawing) for supplying the regulated fluid pressure 
(P.sub.A) thereto, by the signal from the electronic control unit 5. 
The thus-structured hydraulic circuit as shown in FIG. 4 is controlled by 
the electronic control unit 5. Referring to the flowchart shown in FIG. 5, 
step S-1 determines whether the shift from the neutral range to the drive 
range (hereinafter, referred to as "N-to-D shift") has been performed, on 
the basis of the signal from the neutral start switch Sn5. An affirmative 
determination is followed by step S-2 where the magnetizing signal S.sub.L 
1 to the solenoid valve 36 is turned off. The fluid pressure of the fluid 
passage 307 is thus drained, so that the C-1 changeover valve 35 takes the 
first position, that is, the position of the spool indicated in the lower 
half the illustration of the valve in the drawing. Thereby, the line 
pressure (P.sub.L) of the range pressure fluid passage 302 starts to be 
supplied to the hydraulic servo C-1 via the C-1 changeover valve 35 and 
the fluid passage 304. The following step S-3 sets the current rotational 
speed (N.sub.C1) of the clutch C1 as the clutch rotational speed 
(N.sub.C1R) at the time of the N-to-D shift, and the current engine speed 
(N.sub.E) as the engine speed (N.sub.ER) at the time of the N-to-D shift, 
and also resets a timer T. In step S-4, the C-1 control valve 33 is 
supplied with the signal pressure (P.sub.th) from the linear solenoid 
valve 34 in accordance with the input torque, and the C-1 control valve 33 
outputs a regulated fluid pressure (P.sub.A) in accordance with the signal 
pressure (P.sub.th). According to this embodiment, the signal pressure 
(P.sub.th) is set in accordance with the engine speed (N.sub.ER) set in 
step S-3. Step S-5 determines whether the value of the timer T has become 
T1. According to this embodiment, the timer value T1 is preset to a time 
point slightly preceding the time point to start engaging the clutch C1. 
The reason for this setting is that if the timer is set so that the 
magnetizing signal S.sub.L 1 to the solenoid is turned on by the start of 
the engagement, detection delay may cause an incident where the hydraulic 
servo C-1 is supplied with the line pressure (P.sub.L) even though the 
clutch C1 has started engaging. The steps described above perform the 
control for causing the hydraulic servo C-1 to rapidly operate its piston 
until the engagement is achieved. 
If step S-5 determines that the value of the timer T has become equal to T1 
after repeating time measurement of the timer T, the following step S-6 
turns on the magnetizing signal S.sub.L 1 to the solenoid valve 36. The 
C-1 changeover valve 35 is thereby switched to supply the hydraulic servo 
C-1 with an apply pressure (P.sub.A) from the C-1 control valve 33 caused 
to output the regulated stable fluid pressure (P.sub.A) in step S-4. Then, 
step S-7 determines that the engagement has started, when the current 
rotational speed (N.sub.C1) of the clutch C1 has become less than the 
rotational speed (N.sub.C1R) of the clutch C1 at the time of the N-to-D 
shift, by at least a predetermined value (.DELTA.N.sub.R1). Based on the 
determination in step S-7, the following step S-8 increases the fluid 
pressure supply (P.sub.C1) to the hydraulic servo C-1 by a predetermined 
amount (.DELTA.P.sub.th) in a cycle of a predetermined length of time. The 
predetermined amount (.DELTA.P.sub.th) is preset in accordance with the 
input torque. More specifically, the predetermined amount 
(.DELTA.P.sub.th) increases with increases in the input torque. The input 
torque is detected on the basis of the throttle opening (.theta.) 
according to this embodiment. Then, step S-9 determines that the control 
is to be ended, when the clutch C1 has substantially completed engagement. 
More specifically, when the difference between the current input 
rotational speed (N.sub.C1) of the clutch C1 and the output rotational 
speed (No.cndot.i) (that is, the rotational speed obtained by multiplying 
the first speed gear ratio i with the output rotational speed (No) 
detected by the vehicle speed sensor Sn4) of the clutch C1 has become a 
predetermined value (.DELTA.N.sub.R2) or less. Based on the determination 
by step S-9, the last step S-10 turns off the signal S.sub.L 1 to the 
solenoid valve 36. The C-1 changeover valve 35 is thereby switched back to 
the state for directly supplying the line pressure (P.sub.L). 
FIG. 6 shows a timing chart corresponding to the flowchart described above. 
As in the throttle signal pressure (P.sub.th), the regulated fluid 
pressure (P.sub.A) regulated by the throttle signal pressure (P.sub.th) is 
regulated to the initial fluid pressure (P.sub.AI) at the time of N-to-D 
shift and maintained thereat, so that the clutch servo C-1 is supplied 
with a precise pressure supply at the initial time of the turning on of 
the solenoid valve 36 and later on. A broken line in FIG. 6 indicates the 
actual fluid pressure inside the hydraulic servo C-1, illustrating that 
the actual pressure inside the hydraulic servo C-1 remains relatively low 
during the piston stroke of the hydraulic servo C-1. As a feature of this 
embodiment, the timer measurement time T1 is preset to elapse slightly 
before the clutch actually starts to engage, so that the pressure supply 
(P.sub.C1) to the hydraulic servo C-1 starts to rise slightly later than 
the timing of switching the C-1 changeover valve. 
FIG. 7 is a flowchart according to a second embodiment wherein the control 
manner is modified. According to this embodiment, the timing of turning on 
the signal S.sub.L 1 to the solenoid 36 is preset to coincide with the 
engagement starting timing of the clutch C1, instead of making 
determination based on the time measurement of the timer T as performed in 
the first embodiment. Thus, the resetting of the timer T to "0" is 
eliminated from step S-3' that corresponds to step S-3 according to the 
first embodiment, and the timer measurement operation in step S-5 is also 
eliminated. Since the timing of turning on the solenoid 36 coincides with 
the engagement starting timing of the clutch C1 according to the second 
embodiment, the clutch engagement determining operation in step S-7 of 
determining whether the clutch C1 has been engaged precedes the operation 
of turning on the solenoid 36 in step S-6, unlike the first embodiment. 
The determination and operations in the other steps are substantially the 
same as those according to the first embodiment. The comparable steps are 
denoted by comparable reference characters and will not be described 
again. 
FIG. 8 shows a timing chart corresponding to the flowchart according to the 
second embodiment. This embodiment is distinguished from the first 
embodiment in that the pressure supply (P.sub.C1) to the hydraulic servo 
C-1 starts rising synchronously with the timing of switching the C-1 
changeover valve 35 since the timing of turning on the signal S.sub.L1 to 
the solenoid 36 is set to coincide with the engagement starting time point 
of the clutch C1. With this manner of control, the period during which the 
line pressure (P.sub.L) is supplied to rapidly operate the piston of the 
hydraulic servo C-1 to achieve the engagement can be maximized. Thus, the 
control according to the second embodiment can achieve a shorter clutch 
engaging time than the control according to the first embodiment. 
According to the two embodiments described above, when the N-to-D shift 
causes the manual valve 32 to output the line pressure (P.sub.L), the 
holding means (corresponding to steps S-3 to S-5 according to the first 
embodiment, and steps S-3' to S-7 according to the second embodiment) 
provided in the form of programs installed in the electronic control unit 
5 shifts the changeover means 35, 36 to the first position for supplying 
the line pressure (P.sub.L) to the hydraulic servo C-1, so that the line 
pressure (P.sub.L) from the manual valve 32 is supplied to the hydraulic 
servo C-1. This state where the changeover means 35, 36 is in the first 
position is maintained for a predetermined time period following the 
N-to-D shift, during which the rapid piston stroke of the hydraulic servo 
C-1 is performed, thus enabling a reduction in the engaging time. After 
the predetermined period elapses, the changeover means 35, 36 is shifted 
to the second position for supplying the hydraulic servo C-1 with the 
regulated pressure (P.sub.A) from the pressure regulating means 33, 34. 
Since the pressure regulating means 33, 34 is set into a state for stably 
outputting the initial fluid pressure (P.sub.AI) not later than the elapse 
of a predetermined length of time, the fluid pressure supply (P.sub.C1) to 
the hydraulic servo C-1 is switched from the line pressure (P.sub.L) to 
the initial fluid pressure (P.sub.AI), which is lower than the line 
pressure (P.sub.L), without delay. Thus, unlike the conventional art, the 
embodiments eliminate a delay in reducing the fluid pressure, preventing 
engaging shocks. Following the shifting of the changeover means 35, 36 to 
the second position, the fluid pressure supply (P.sub.C1) to the hydraulic 
servo C-1 is gradually increased from the initial fluid pressure 
(P.sub.AI) to gradually engage the clutch C1, under the control by the 
fluid pressure raising means (corresponding to step S-8 according to the 
first and second embodiments) provided in the form of a program installed 
in the electronic control unit 5. According to the embodiments, since the 
changeover means 35, 36 switches from the state where the line pressure 
(P.sub.L) is supplied without passing through the pressure regulating 
means 33, 34, to the state where the regulated fluid pressure (P.sub.A) is 
supplied from the pressure regulating means 33, 34, the embodiments will 
rapidly reduce the fluid pressure supply (P.sub.C1) to the hydraulic servo 
C-1 to the initial fluid pressure (P.sub.AI) without delay. The 
embodiments thus reduce the engaging time while preventing the engaging 
shocks. 
Although the invention has been described with reference to what are 
presently considered to be preferred embodiments thereof, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
To the contrary, the invention is intended to cover various modifications 
and equivalent arrangements included within the spirit and scope of the 
appended claims.