Variable pressure gain control system for a CVT

For achieving a high accuracy line pressure control and a high line pressure, a valve control system outputs a first signal pressure to a primary regulator valve as a second signal pressure when the first signal pressure is low. The valve control system outputs a gain pressure higher than the first signal pressure to the primary regulator valve as the second signal pressure when the first signal pressure is high. A change amount of the line pressure from the primary regulator valve concerning with a change amount of the first signal pressure is a gain. The gain is a low gain which increases the accuracy of the line pressure control when the first signal pressure is low, and the gain is a high gain with which the high line pressure is output when the first signal pressure is high.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The invention relates to a hydraulic control system for an automatic 
transmission. More specifically, it relates to the hydraulic control 
system, which controls an output pressure with a different amplification 
factor related to a signal pressure, for an automatic transmission. 
2. Description of Related Art 
Generally, in a conventional hydraulic control system for an automatic 
transmission, a linear solenoid valve outputs a signal pressure, the 
signal pressure is input to a primary regulator valve, and the primary 
regulator valve regulates a line pressure to an output pressure properly 
based on the signal pressure. Then, many kinds of controls are performed 
based on the output pressure. A change rate of the output pressure from 
the primary regulator valve related to a change in the signal pressure 
output from the linear solenoid valve is made using an amplification 
factor, which is called a gain from now on. The gain is set so that the 
line pressure is regulated to the output pressure needed for the hydraulic 
controls within the range of the signal pressure which is output from the 
linear solenoid valves. That is to say, the gain is set so that the change 
range of the output pressure is achieved within the change range of the 
signal pressure. 
For example, in a continuously variable transmission which performs 
non-stage shift by changing a pulley ratio between two pulleys joined with 
a belt, an unusually high output pressure is needed. That is to say, in 
such a continuously variable transmission, the pulley ratio is changed by 
increasing and decreasing a tension on the belt by a fixed sheave and a 
movable sheave that comprise each pulley. The tension on the belt needs to 
change over a wide range. Therefore, the output pressure from the primary 
regulator valve for achieving the tension on the belt over the wide range 
needs to change over a wide range. As a way for achieve the wide change 
range of the output pressure, the change range of the signal pressure may 
be made large. In this case, the linear solenoid valve needs to be large 
to achieve the wide change range for the signal pressure. Therefore, a 
large space for positioning the large linear solenoid valve is needed and 
costs increase. 
Another way to achieve the wide change range of the output pressure 
considering the aforementioned disadvantages, it is possible to make the 
gain large. In this case, when the gain is large, the change in the output 
pressure is large related to the change in the signal pressure. Then, 
however, the dispersion of the output pressure becomes large due to 
vibration in the signal pressure. Therefore, it is difficult to achieve 
high accuracy control for the output pressure. That is, in the case where 
the change range of the signal pressure is stable, when the gain is set to 
be large, the change range of the output pressure becomes large and the 
high output pressure is achieved, but high accuracy control for the output 
pressure is difficult. When the gain is set to be small, high accuracy 
control for the output pressure is achieved, but the change range of the 
output pressure becomes small and the necessary high output pressure is 
not obtained. 
Actually, in a vehicle mounting the aforementioned continuously variable 
transmission, the continuously variable transmission needs the large 
output pressure at, for example, the vehicle start because the pulley 
ratio must be greatly changed. And, for example, during a settled speed 
driving, the continuously variable transmission needs a small output 
pressure for the purpose of, for example, minimizing fuel consumption and 
maintaining a high accuracy control for the output pressure. That is to 
say, in the continuously variable transmission, when the continuously 
variable transmission needs low output, a small gain is proper. When the 
continuously variable transmission needs high output, a large gain is 
proper. 
The linear solenoid valves and regulator valves have dispersions caused by, 
for example, manufacturing accuracy and assembly accuracy of the valve 
body and spools, and the spring load of the spring loaded spools in each 
assembly. Therefore, the signal pressure from the linear solenoid valves 
and control pressures from the regulating valves have a dispersion related 
to the same current value in each hydraulic circuit. That is, the control 
pressure as output, related to the current value as input, has a degree of 
dispersion. 
For reducing the dispersion in each hydraulic circuit, an adjusting 
mechanism is arranged in the linear solenoid valve. Used as the adjusting 
mechanism, for example, is an adjusting screw for increasing and 
decreasing the spring load of the spring loaded spool. In this case, the 
spring load can be increased and decreased by the adjusting screw in the 
each hydraulic circuit and, then, the signal pressure and the control 
pressure related to the signal pressure are regulated. The proper control 
pressure is thereby achieved. 
FIG. 12 shows an example for regulating the control pressure in the related 
art. FIG. 12 shows linearly changing the control pressure P.sub.L, with 
the dispersion found in various hydraulic circuits related to the current 
values I of the linear solenoid valves. In FIG. 12, solid line A shows a 
target value, upper side alternating long and two short dash line a.sub.1 
shows a maximum value of the dispersion before adjustment, a lower side 
alternate long and two short dash line a.sub.2 shows a minimum value of 
the dispersion before adjustment, an upper side alternate long and short 
dash line b.sub.1 shows a maximum value of the dispersion after 
adjustment, and a lower side alternate long and short dash line b.sub.2 
shows a minimum value of the dispersion after adjustment. 
The control pressure P.sub.L is adjusted as follows. 
In the related art, an adjusting point (basic point) is set on a low 
pressure side of the control pressure P.sub.L, and the control pressure 
P.sub.L is regulated so that the control pressure P.sub.L is a 
predetermined control pressure P.sub.A. Actually, the current value of the 
linear solenoid is maintained at a predetermined value. Then, while 
watching the control pressure output from the regulator valve, a thrust 
amount of the adjusting screw is adjusted, and the adjustment is ended 
when the control pressure P.sub.L reaches the predetermined control 
pressure P.sub.A. 
When adjusting from the upper side alternate long and two short dashes line 
a.sub.1, the thrust amount of the adjusting screw is increased and the 
left end of the line a.sub.1 is set to the predetermined control pressure 
P.sub.A. When adjusting from the lower side alternate long and two short 
dashes line a.sub.2, the thrust amount of the adjusting screw is decreased 
and the left end of the line a.sub.2 is set to the predetermined control 
pressure P.sub.A. By this process, the lines a.sub.1, a.sub.2 are 
displaced in parallel and changed to the lines b.sub.1, b.sub.2. 
By performing the adjustment for the each hydraulic circuit, the 
predetermined control pressure P.sub.A is achieved for the predetermined 
current value of the linear solenoid valve in each hydraulic circuit. That 
is, the predetermined control pressure is set to correspond to the 
predetermined current value. 
But in the related art, the dispersion after adjustment is large when the 
control pressure P.sub.L is high because the adjusting point is set for 
the low pressure side of the control pressure P.sub.L. Therefore, the 
hydraulic pressure that may be output is too great compared with the 
target value A shown in FIG. 12. As a result, it is necessary to consider 
the strength and durability of the hydraulic circuit when such a high 
pressure results. 
Conversely, when the adjusting point is set at the high pressure side of 
the control pressure P.sub.L, the dispersion is reduced at the adjusting 
point on the high pressure side, but the dispersion is increased at the 
low pressure side. Therefore, it is necessary to set the lowest control 
pressure high so that the control pressure is enough for the large 
dispersion at the low pressure side. 
When the adjusting point of the control pressure P.sub.L is set on the low 
pressure side, the dispersion is increased at the high pressure side, and 
when the adjusting point of the control pressure P.sub.L is set on the 
high pressure side, the dispersion is increased at the low pressure side. 
That is to say, the dispersion of the control pressure P.sub.L is 
increased in a side which is far away from the adjusting point because the 
adjustment by the adjusting screw does not change the slope of the lines 
in FIG. 12, it just displaces them in parallel. 
SUMMARY OF THE INVENTION 
Therefore, an object of the invention is to provide a hydraulic control 
system, which achieves both a high accuracy control for an output pressure 
and a high output pressure by changing an amplification factor, also 
called a gain, related to a change in a signal pressure, for an automatic 
transmission. 
An another object of the invention is to reduce the dispersion in the 
output pressure, which is a control pressure, by setting two adjusting 
points. 
In order to achieve the aforementioned objects, a hydraulic control system 
for an automatic transmission comprises a signal pressure output device 
which outputs a first signal pressure, an amplification factor changing 
device which receives the first signal pressure from the signal pressure 
output device and outputs a second signal pressure based on the first 
signal pressure, and a regulating device which receives the second signal 
pressure from the amplification factor changing device and regulates an 
output pressure based on the second signal pressure. The amplification 
factor changing device changes the amplification factor, which is the 
change rate of the output pressure with respect to the change in the first 
signal pressure, to a low amplification factor and a high amplification 
factor within a change range of the first signal pressure. The 
amplification factor changing device comprises a gain control valve which 
outputs a gain pressure regulated based on the first signal pressure and a 
selecting device which selects the second signal pressure from the first 
signal pressure and the gain pressure based on their strength. 
The selecting device comprises a check ball which selects the higher 
pressure from the first signal pressure and the gain pressure as the 
second signal pressure, and outputs the second signal pressure to the 
regulating device. 
The first signal pressure is input to the regulating device as the second 
signal pressure and the amplification factor is changed to the low 
amplification factor when the first signal pressure from the signal 
pressure output device is lower than a predetermined value, and the gain 
pressure higher than the first signal pressure is input to the regulating 
device as the second signal pressure and the amplification factor is 
changed to the high amplification factor when the first signal pressure 
from the signal pressure output device is higher than the predetermined 
value. 
The gain control valve regulates the output pressure regulated by the 
regulating device to the gain pressure. The gain control valve regulates 
the gain pressure from low to high when the first signal pressure is 
changed from low to high. 
The hydraulic control system for an automatic transmission comprises a 
first adjusting mechanism which regulates the first signal pressure by 
adjusting the signal pressure output device and a second adjusting 
mechanism which regulates the gain pressure by adjusting the gain control 
valve. The first adjusting mechanism regulates the first signal pressure 
based on a basic point on a low pressure side of the output pressure from 
the regulating device, and the second adjusting mechanism regulates the 
gain pressure based on a basic point on a high pressure side of the output 
pressure from the regulating device. 
The adjustment for the first signal pressure by the first adjusting 
mechanism is performed earlier than the adjustment for the gain pressure 
by the second adjusting mechanism. 
In another way for achieving the aforementioned objects, a hydraulic 
control system for an automatic transmission comprises a signal pressure 
output device which outputs a first signal pressure, an amplification 
factor changing device which receives the first signal pressure from the 
signal pressure output device and outputs a second signal pressure based 
on the first signal pressure and a regulating device which receives the 
second signal pressure from the amplification factor changing device and 
regulates an output pressure based on the second signal pressure. The 
amplification factor changing device changes an amplification factor, 
which is the change rate of the output pressure with respect to the change 
of the first signal pressure, to a low amplification factor and a high 
amplification factor within a change range of the first signal pressure. 
The amplification factor changing device comprises a gain control valve 
which outputs a gain pressure regulated based on the first signal 
pressure. The amplification factor changing device outputs the first 
signal pressure as the second signal pressure to the regulating device 
when the first signal pressure is lower than a predetermined value, and 
the amplification factor changing device outputs both of the first signal 
pressure and the gain pressure as the second signal pressure to the 
regulating device when the first signal pressure is higher than or equal 
to the predetermined value. 
The gain control valve regulates the output pressure regulated by the 
regulating device to the gain pressure. The gain control valve regulates 
the gain pressure from low to high when the first signal pressure is 
changed from low to high. 
The hydraulic control system for an automatic transmission comprises a 
first adjusting mechanism which regulates the first signal pressure by 
adjusting the signal pressure output device and a second adjusting 
mechanism which regulates the gain pressure by adjusting the gain control 
valve. The first adjusting mechanism regulates the first signal pressure 
based on a basic point on a low pressure side of the output pressure from 
the regulating device, and the second adjusting mechanism regulates the 
gain pressure based on a basic point on a high pressure side of the output 
pressure from the regulating device. The adjustment for the first signal 
pressure by the first adjusting mechanism is performed earlier than the 
adjustment for the gain pressure by the second adjusting mechanism. 
According to the invention, the amplification factor (gain) of the output 
pressure related to the first signal pressure is changed to the low gain 
by the amplification factor changing device. Therefore, the change of the 
output pressure related to the first signal pressure is reduced. As a 
result, the accuracy of the output pressure related to the first signal 
pressure is increased. Further, the amplification factor (gain) of the 
output pressure related to the first signal pressure is changed to the 
high gain by the amplification factor changing device. Therefore, the 
change of the output pressure related to the first signal pressure is 
increased. As a result, the high output pressure is achieved. In this 
case, one pressure is selected from the first signal pressure and the gain 
pressure based on their height, and the selected pressure is output as the 
second signal pressure to the regulating device by the gain control valve 
and the selecting device. Then, the gain, which is achieved when the first 
signal pressure is input to the regulating device, and the gain, which is 
achieved when the gain pressure is input to the regulating device, differ. 
Therefore, two different gains, that is to say, the low gain and the high 
gain, are achieved. 
The higher pressure is easily selected from the first signal pressure and 
the gain pressure by the check ball. Therefore, the structure for 
selecting is simplified. 
In the case where the first signal pressure is increased, the first signal 
pressure is selected as the second signal pressure and is output to the 
regulated device, and the gain is changed to the low gain when the first 
signal pressure is lower than the predetermined value, and the gain 
pressure higher than the first signal pressure is selected as the second 
signal pressure and is output to the regulated device with the gain 
changed to the high gain when the first signal pressure is higher than the 
predetermined value. That is to say, in the case the first signal pressure 
is increased, the gain is automatically changed from the low gain to the 
high gain based on the predetermined value. The gain pressure is 
stabilized because the gain pressure is regulated from the output pressure 
regulated by the regulating device. 
When the first signal pressure is increased, the gain pressure is 
increased. The first signal pressure is regulated based on the basic point 
on the low pressure side of the low output pressure from the regulating 
device. The gain pressure is regulated based on the basic point on the 
high pressure side of the high output pressure from the regulating device. 
That is to say, the output pressure from the regulating device is 
regulated based on the two basic points on the low pressure side and the 
high pressure side, respectively, by the first adjusting mechanism and the 
second adjusting mechanism. Therefore, the dispersion of the output 
pressure from the regulating device is reduced at the low pressure side 
and the high pressure side, and the accuracy of the output pressure is 
increased. 
The gain control valve is controlled by the signal pressure output device. 
Therefore, the gain pressure is influenced by the first signal pressure. 
As a result, if the gain control valve is adjusted by the second adjusting 
mechanism before the signal pressure output device is adjusted by the 
first adjusting mechanism, the accuracy of the control is reduced. To 
address the problem, the signal pressure output device is adjusted by the 
first adjusting mechanism before the gain control valve is adjusted by the 
second adjusting mechanism. Therefore, the accuracy of the control is 
increased because the first signal pressure is not influenced by the gain 
pressure. 
According to another structure of the invention, the amplification factor 
(gain) of the output pressure related to the first signal pressure is 
changed to the low gain by the amplification factor changing device. 
Therefore, the change of the output pressure related to the first signal 
pressure is reduced. As a result, the accuracy of the output pressure 
related to the first signal pressure is increased. Further, the 
amplification factor (gain) of the output pressure related to the first 
signal pressure is changed to the high gain by the amplification factor 
changing device. Therefore, the change of the output pressure related to 
the first signal pressure is increased. As a result, the high output 
pressure is achieved. In this case, the first signal pressure is input to 
the regulating device as the second signal pressure when the first signal 
pressure is lower than the predetermined value, and both of the first 
signal pressure and the gain pressure are input to the regulating device 
as the second signal pressure when the first signal pressure is higher 
than the predetermined value. Then, the gain, which is achieved when the 
first signal pressure is input to the regulating device, and the gain, 
which is achieved when the first pressure and the gain pressure are input 
to the regulating device, differ. Therefore, two different gains, that is 
to say, the low gain and the high gain, are achieved. Further, the gain is 
changed smoothly because the first signal pressure is always input to the 
regulating device. 
The gain pressure is stabilized because the gain pressure is regulated from 
the output pressure regulated by the regulating device. When the first 
signal pressure is increased, the gain pressure is increased. 
The first signal pressure is regulated based on the basic point on the low 
pressure side of the low output pressure from the regulating device. The 
gain pressure is regulated based on the basic point on the high pressure 
side of the high output pressure from the regulating device. That is to 
say, the output pressure from the regulating device is regulated based on 
the two basic points on the low pressure side and the high pressure side, 
respectively, by the first adjusting mechanism and the second adjusting 
mechanism. Therefore, the dispersion of the output pressure from the 
regulating device is reduced at the low pressure side and the high 
pressure side, and the accuracy of the output pressure is increased. 
The gain control valve is controlled by the signal pressure output device. 
Therefore, the gain pressure is influenced by the first signal pressure. 
As a result, if the gain control valve is adjusted by the second adjusting 
mechanism before the signal pressure output device is adjusted by the 
first adjusting mechanism, the control accuracy is reduced. To address the 
problem, the signal pressure output device is adjusted by the first 
adjusting mechanism before the gain control valve is adjusted by the 
second adjusting mechanism. Therefore, the control accuracy is increased 
because the first signal pressure is not influenced by the gain pressure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention will become more apparent from a detailed description of 
preferred embodiments with reference to the accompanying drawings. 
Throughout this description words like upper, lower, up, and down are used 
to convey directions. These words are relative to the figures and do not 
reflect an absolute direction. Terms such as first direction, opposite 
direction, and second direction could also be used but would have required 
the addition of arrows to the figures and additional explanation. Thus, 
for simplicity, words related to the visual representation are used and 
are not meant to be limiting. 
The first embodiment of the invention will be described in the following 
order: 
(1) a structure of a continuously variable transmission supplied with a 
hydraulic control system for an automatic transmission. 
(2) a structure of a hydraulic circuit which is a basis of the invention. 
(3) operations of the continuously variable transmission and the hydraulic 
circuit. 
(4) regulation of a line pressure P.sub.L. 
(5) a structure of the hydraulic control system for an automatic 
transmission. 
(6) an operation of the hydraulic control system. 
(7) adjusting valves in the hydraulic control system. 
The structure of a continuously variable transmission supplied with a 
hydraulic control system for an automatic transmission of the invention 
will be described with reference to FIG. 1. The figure shows an outline 
structure of a continuously variable transmission for a vehicle supplied 
with a hydraulic control system for an automatic transmission. 
As shown in FIG. 1, the continuously variable transmission 1 comprises a 
CVT 2 (which is a belt-type continuously variable transmission mechanism), 
a forward/reverse mode selecting device 3, a torque converter 6 equipped 
with a lock-up clutch 5, a counter shaft 7, and a differential device 9. 
These devices are covered by a partitioned case. 
The torque converter 6 comprises a pump impeller 11 connected to an engine 
output shaft 10 via a front cover 17, a turbine runner 13 connected to an 
input shaft 12, and a stator 16 supported on the transmission case via a 
one-way clutch 15. A lock-up clutch 5 is interposed between the input 
shaft 12 and the front cover 17. A damper spring 20 is interposed between 
the lock-up clutch plate and the input shaft 12. An oil pump 21 is 
connected to and driven by the pump impeller 11. 
The CVT 2 comprises a primary pulley 26, a secondary pulley 31, and a metal 
belt 32 wrapped around the pulleys 26, 31. The primary pulley 26 comprises 
a fixed sheave 23 fixed to a primary shaft 22 and a movable sheave 25 
axially slidably supported by the primary shaft 22. The secondary pulley 
31 comprises a fixed sheave 29 fixed to a secondary shaft 27 and a movable 
sheave 30 axially slidably supported by secondary shaft 27. 
A hydraulic actuator 33, comprising a double piston, is disposed behind the 
movable sheave 25 on the primary side. A hydraulic actuator 35, comprising 
a single piston, is disposed behind the movable sheave 30 on the secondary 
side. The hydraulic actuator 33 on the primary side comprises a cylinder 
member 36 and a reaction support member 37 fixed to the primary shaft 22 
and a piston member 40 and a cylindrical member 39 fixed to the movable 
sheave 25. A first hydraulic chamber 41 is formed from the cylindrical 
member 39, the reaction support member 37, the primary shaft 22, and the 
back surface of the movable sheave 25. A second hydraulic chamber 42 is 
formed from the cylinder member 36, the piston member 40, and the reaction 
support member 37. The first hydraulic chamber 41 and the second hydraulic 
chamber 42 are made continuous via a continuous hole 37a. As a result of 
the combination of equal hydraulic pressures in hydraulic chambers 41, 42, 
a force in the axial direction is generated that is roughly double that of 
a secondary-side hydraulic actuator 35. The secondary-side hydraulic 
actuator 35 comprises a reaction support member 43 fixed to the secondary 
shaft 27 and a cylindrical member 45 fixed to the back surface of the 
movable sheave 30. A single hydraulic chamber 46 is formed by these 
members and the secondary shaft 27. A pre-loading spring 47 is interposed 
and compressed between the movable sheave 30 and the reaction support 
member 43. 
The forward/reverse mode selecting device 3 comprises a double pinion 
planetary gear 50, a reverse brake B1, and a direct clutch C1. In the 
double pinion planetary gear 50, a sun gear S is connected to the input 
shaft 12, a carrier CR supporting a first pinion P1 and a second pinion P2 
is connected to the primary-side fixed sheave 23, a ring gear R is 
connected to the reverse brake B1, and the direct clutch C1 is interposed 
between the carrier CR and the ring gear R. 
A large gear 51 and a small gear 52 are fixed to the counter shaft 7. The 
large gear 51 meshes with a gear 53 fixed to the secondary shaft 27. The 
small gear 52 meshes with a gear 55 of the differential device 9. In the 
differential device 9, the rotation from a differential gear 56 supported 
by a differential case 66, which contains the gear 55, is transferred via 
left and right side gears 57, 59 to left and right axles 60, 61. 
Multiple irregularity portions 23a are formed with an equal spacing on the 
outer portion of the primary-side fixed sheave 23 by gear cutting. An 
electromagnetic pick-up 62 is fixed on a position, which faces to the 
irregularity portions 23a, of a case. Further, multiple irregularity 
portions 29a are formed with an equal spacing on the outer portion of the 
secondary-side fixed sheave 29 by gear cutting. An electromagnetic pick-up 
63 is fixed on a position, which faces to the irregularity portions 29a, 
of the case. The electromagnetic pick-ups 62, 63 are arranged so that the 
detecting surfaces of the electromagnetic pick-ups 62, 63 are closed to 
the irregularity portions 23a, 29a. The electromagnetic pick-up 62 
provides a primary (input) rotation speed sensor for detecting the 
irregularity portions 23a. The electromagnetic pick-up 63 provides a 
secondary (output) rotation speed sensor (vehicle speed sensor) for 
detecting the irregularity portions 29a. An electromagnetic pick-up 65 is 
arranged close to the front cover 17. The electromagnetic pick-up 65 
provides an engine rotation speed sensor. 
An input torque is calculated as follows. An engine torque is obtained from 
a map based on a throttle opening and an engine rotation speed. Such maps 
are known by those skilled in the art. A speed ratio is calculated based 
on an input rotation speed and an output rotation speed of the torque 
converter 6. A torque ratio is obtained from a map based on the speed 
ratio. Such maps are also known to those skilled in the art. The input 
torque is calculated by multiplying the torque ratio and the engine 
torque. 
The structure of the hydraulic circuit of the continuously variable 
transmission 1 will be described with reference to FIG. 2. In the 
invention, to be discussed below, an amplification factor changing device, 
for example, a gain control valve 110 in FIG. 4, is added to the hydraulic 
circuit shown in FIG. 2. 
In FIG. 2, there is shown an oil pump 21, an oil pump control valve 70, and 
a pump solenoid valve S2 for the oil pump control valve. There is also 
shown a primary regulator valve 72, a secondary regulator valve 73, a line 
pressure linear solenoid SLT for controlling the line pressure, a lock-up 
linear solenoid valve SLU for a lock-up control, a ratio linear solenoid 
valve SLR for a ratio control, and a modulator valve 76 for controlling 
the solenoid valves. 
A manual valve 77 is operated manually to switch a modulated pressure 
modulated by a clutch modulator valve 79 from a port 1 to a port 2 or a 
port 3 as shown in the table in FIGS. 2 or 4. In FIG. 2, there is also 
shown a C1 control valve 80, a neutral relay valve 81, a reverse inhibit 
valve 82, and a solenoid S1 for controlling forward/reverse. There is also 
shown a hydraulic servo C1 for the direct clutch C1, a hydraulic servo B1 
for the reverse brake B1, an accumulator 90 for the hydraulic servo B1, 
and an accumulator 91 for the hydraulic servo C1. There is also shown a 
ratio control valve 92, a primary-side hydraulic actuator 33, and a 
secondary-side hydraulic actuator 35. Also shown is a lock-up control 
valve 95, a lock-up relay valve 96, and a solenoid valve S3 for changing 
the lock-up state. In FIG. 2, EX shows a drain port. There is also shown a 
bypass control valve 97, a secondary control pressure modulator valve 99, 
and a cooler 100. 
Operation of the continuously variable transmission 1 and the hydraulic 
circuit will now be described. A predetermined pressure is obtained by 
operation of the oil pump 21 based on the engine rotation. The 
predetermined pressure is regulated to a line pressure P.sub.L by A5 the 
primary regulator valve 72 based on the line pressure linear solenoid 
valve SLT which is controlled by a signal, the signal calculated based on 
the pulley ratio and the input torque, from a control unit. The line 
pressure P.sub.L is regulated to a secondary pressure P.sub.S by the 
secondary regulator valve 73 to be discussed below. When a high line 
pressure is not needed, for example, when a vehicle is stopped, the pump 
solenoid valve S2 is controlled based on a signal from the control unit so 
that the oil pump control valve 70 is moved to right half side position, 
as shown in FIG. 2 and the predetermined pressure from the oil pump 21 is 
circulated. 
When the manual valve 77 is in D (drive) range or L (low) range, the 
hydraulic pressure from the port 1 is applied, via the port 2, to the 
hydraulic servo C1 for the direct clutch C1 and the direct clutch C1 is 
engaged. In this state, the rotation from the engine output shaft 10 is 
transferred to the primary pulley 26 via the torque converter 6, the input 
shaft 12, and the planetary gear 50 which is in a direct connecting state 
as a result of the engagement of the direct clutch C1. The rotation is 
transferred to the secondary shaft 27 via the CVT 2, when the CVT2 is 
modified properly, and is then transferred to the left and right axles 60, 
61 via the counter shaft 7 and the differential device 9. 
When the manual valve 77 is moved to R (reverse) range, the hydraulic 
pressure from he port 1 is applied to the hydraulic servo B1 for the brake 
B1 via the port 3. In this state, a ring gear R of the planetary gear 50 
is engaged, while the rotation of a sun gear S from input shaft 12 is 
converted to a reverse rotation by the carrier CR, and the reverse 
rotation is transferred to the primary pulley 26. 
In the CVT 2, the line pressure P.sub.L from the primary regulator valve 72 
is applied to the hydraulic actuator 35 of the secondary pulley 31 so that 
a belt grasping force related to the input torque and the shift ratio is 
applied. The ratio linear solenoid valve SLR, for the ratio control, is 
controlled based on the shift signal from the control unit, and the ratio 
control valve 92 is controlled by the signal pressure from the ratio 
linear solenoid valve SLR. The regulated pressure from the output port of 
the ratio control valve 92 is applied to the hydraulic actuator 33, which 
comprises the double piston, for the primary pulley 26. Then, the 
transmission ratio of the CVT 2 is controlled properly. 
The torque of the engine output shaft 10 is transferred to the input shaft 
12 via the torque converter 6. Especially, when drive of a vehicle is 
started, the torque is converted to be high by the torque converter 6 and 
is transmitted to the input shaft 12. Then, a vehicle starts smoothly. The 
torque converter 6 comprises the lock-up clutch 5. At high speed 
stabilized driving, the lock-up clutch 5 is engaged, then the engine 
output shaft 10 is connected to the input shaft 12 directly so that a 
power loss based on oil in the torque converter is reduced. 
Regulation of the line pressure P.sub.L will be described with reference to 
FIG. 3. A spring 72b is compressed and arranged in a first end chamber 1 
of the primary regulator valve 72. A control pressure output from the 
output port m of the line pressure linear solenoid valve SLT is input to 
the first end chamber 1 via an orifice 101. The line pressure P.sub.L is 
input to a second end chamber n of the primary regulator valve 72 via an 
orifice 102. Therefore, a spool 72a is operated by the control pressure 
input to the first end chamber 1 and the feedback pressure input to the 
second end chamber n. Then, a hydraulic pressure applied from the oil pump 
21 to a port o of the primary regulator valve 72 is regulated by 
connecting the port o to a drain port EX and a secondary port q at a 
predetermined rate. Then, the line pressure P.sub.L calculated based on 
the input torque and the transmission ratio of the CVT 2 is applied to an 
oil path h. 
A spring 73b is compressed and arranged in a first end chamber r of the 
secondary regulator valve 73. A control pressure output from the output 
port s of the secondary control pressure modulator valve 99 is input to 
the first end chamber r via an orifice 103. The secondary pressure P.sub.S 
is input to a second end chamber t of the secondary regulator valve 73 via 
an orifice 105. Therefore, a spool 73a is operated by the control pressure 
input to the first end chamber r and the feedback pressure input to the 
second end chamber t. Then, a hydraulic pressure applied from the port q 
of the primary regulator valve 72 to a port u of the secondary regulator 
valve 73 is regulated by communicating the port u to a drain port EX and a 
lubricating oil port v at a predetermined rate. Then, the secondary 
pressure P.sub.S regulated based on the control pressure from the output 
port s of the secondary control pressure modulator valve 99 is applied to 
an oil path p. A lubricating pressure is applied from the lubricating oil 
port v of the secondary regulator valve 73 to a lubricating device 107 via 
an orifice 109. 
A spring 99b is compressed and arranged in a first end chamber w of the 
secondary control pressure modulator valve 99. The control pressure output 
from the output port s of the secondary control pressure modulator valve 
99 is input to a second end chamber x via an orifice 106. The secondary 
control pressure modulator valve 99 comprises the output ports, a drain 
port EX, and an input port y applied with the control pressure from the 
line pressure linear solenoid valve SLT for controlling the line pressure 
P.sub.L via an orifice 104. The output port s is connected to the input 
port y and the drain port EX at a predetermined rate. A V-shaped notch y' 
is formed at the input port y. 
A spool 99a is operated based on the control pressure as a feedback 
pressure input to the second end chamber x and a biasing force of the 
spring 99b in the first end chamber w. When the control pressure from the 
line pressure linear solenoid valve SLT is lower than a predetermined 
pressure, the spool 99a is maintained on right-half side position of FIG. 
5, because the biasing force of the spring 99b is higher than the feedback 
pressure input to the second end chamber x, then the control pressure 
input to the input port y is output from the output port s. When the 
control pressure from the line pressure linear solenoid valve SLT is 
higher than the predetermined pressure, the spool 99a is operated with the 
feedback pressure input to the second end chamber x and the biasing force 
of the spring 99b in the first end chamber w. Therefore, at that time, if 
the control pressure from the line pressure linear solenoid valve SLT is 
increased, the control pressure from the output port s is maintained at a 
settled value. 
Therefore, the line pressure linear solenoid valve SLT for controlling the 
line pressure P.sub.L regulates a modulator pressure P.sub.M based on the 
control signal, which is output from the control unit based on the input 
torque and the transmission ratio of the CVT 2, and outputs the regulated 
pressure as the control pressure from the output port m. Then, the primary 
regulator valve 72 outputs the line pressure P.sub.L in proportion to the 
input torque between an U/D (under drive) state and an O/D (over drive) 
state of the CVT 2 based on the control pressure, which is regulated, 
applied to the first end chamber 1 of the primary regulating valve 72. 
When the control pressure from the line pressure linear solenoid valve SLT 
is lower than the predetermined pressure, the secondary control pressure 
modulator valve 99 outputs the control pressure, which is not regulated, 
from the output port s. Then, the secondary regulator valve 73 outputs the 
secondary pressure P.sub.S in proportion to the input torque between the 
U/D (under drive) state and the O/D (over drive) state of the CVT 2 based 
on the control pressure, which is not regulated, applied to the first end 
chamber r of the secondary regulating valve 73. The secondary pressure 
P.sub.S at the O/D state is set to a needed pressure which is needed for 
the torque converter 6. Therefore, the secondary pressure P.sub.S is 
enough at the U/D state. 
The control pressure from the output ports of the secondary control 
pressure modulator valve 99 is maintained at the settled value when the 
control pressure from the line pressure linear solenoid valve SLT is 
higher than the predetermined pressure. Therefore, the line pressure 
P.sub.L is increased in proportion to the control pressure from the line 
pressure linear solenoid valve SLT, but the secondary pressure P.sub.S is 
limited to the settled value based on the 15 settled control pressure from 
the output port s. The upper limitation of the secondary pressure P.sub.S 
is lower than a limit pressure of the torque converter 6 and is almost the 
same with a needed highest pressure. The limit pressure is a minimum 
pressure at which the torque converter 6 becomes inoperable. The needed 
highest pressure is a maximum pressure related to the maximum input 
torque. 
The hydraulic control system for an automatic transmission comprises a 
signal pressure output device, an amplification factor changing device, 
and a regulating device, and causes changes between a low gain and a high 
gain. In the invention, as shown in FIGS. 4 and 5, the amplification 
factor changing device is interposed between the line pressure linear 
solenoid valve SLT, as the signal pressure output device, and the primary 
regulator valve 72, as the regulating device, when compared with the 
hydraulic circuit as shown in FIGS. 2 and 3. In FIG. 4, elements having 
the same structures and operations as the elements in FIG. 2 have the same 
reference numbers and letters as in FIG. 2. Some elements which are not 
related to the invention of FIG. 4 do not correspond to elements in FIG. 
2. For example, the lock-up control valve 95 in FIG. 2 is omitted in FIG. 
4. 
The line pressure linear solenoid valve SLT, as the signal pressure output 
device is the same valve mentioned with reference to FIGS. 2 and 3. That 
is to say, the line pressure linear solenoid valve SLT regulates the 
modulator pressure P.sub.M from the solenoid modulator valve 79 based on 
the control signal, which is based on the input torque and the shift ratio 
of the CVT 2, from the control unit and outputs the regulated pressure as 
a first signal pressure P.sub.10 from the output port m. 
In FIGS. 4 and 5 showing the first embodiment, the amplification factor 
changing device comprises a gain control valve 110 and a check ball 111 as 
a selecting device. As shown in FIG. 5, the gain control valve 110 
comprises a hydraulic chamber a to which the first signal pressure 
P.sub.10 output from the line pressure linear solenoid valve SLT is input, 
an input port c to which the line pressure P.sub.L output from the primary 
regulator valve 72 is input, and an output port b from which a gain 
pressure P.sub.G regulated from the line pressure P.sub.L on the basis of 
the first signal pressure P.sub.10 is output. The gain control valve 110 
further comprises a spool 110a comprising lands L.sub.1, L.sub.2. The 
spool 110a is biased upward by a spring 110b. 
The check ball 111 comprises two entrances 111a, 111b and one exit 111c. 
The entrance 111a provides the input of the first signal pressure P.sub.10 
from the line pressure linear solenoid valve SLT. The entrance 111b 
provides the input of the gain pressure P.sub.G from the gain control 
valve 110. The higher pressure is selected from the first signal pressure 
P.sub.10 and the gain pressure P.sub.G by a ball 111d. Then, the selected 
pressure is output as a second signal pressure P.sub.20 from the exit 
111c. The second signal pressure P.sub.20 is input to the first end 
chamber 1 of the primary regulator valve 72 via the orifice 101. The line 
pressure P.sub.L as an output pressure is regulated based on the second 
signal pressure input to the first end chamber 1. The explanation of the 
structure and the operation of the primary regulator valve 72 as the 
regulating device is omitted because there is no substantial difference 
from the previously described structure and operation. 
The operation of the hydraulic control system for an automatic transmission 
will be described with reference to FIGS. 5 and 7. When the oil pump 21 is 
operated, the hydraulic pressure P.sub.L from the oil pump 21 is applied 
to the input port o of the primary regulator valve 72 and applied to the 
second end chamber n, which is on upper side of the spool 72a as shown in 
the figures, via the orifice 102. The spool 72a is pushed down toward the 
spring 72b by the hydraulic pressure input to the second end chamber n, 
and the spool 72a is maintained in the left-half position shown in FIG. 5. 
The spool 110a of the gain control valve 110 is pushed up by the spring 
110b, and the spool 110a is maintained in the left-half position in FIG. 
5. Therefore, the input port c of the gain control valve 110 is closed by 
the land L.sub.2, and the line pressure P.sub.L is not input to the gain 
control valve 110. That state is shown with a position of min in FIG. 7. 
The reference numeral (P.sub.20) on the vertical axis in FIG. 7 shows that 
the second signal pressure P.sub.20 is changed with respect to the change 
of the first signal pressure P.sub.10. 
In that state, when the line pressure linear solenoid valve SLT outputs the 
first signal pressure P.sub.10, the first signal pressure P.sub.10 is 
input to the hydraulic chamber a of the gain control valve 110, and the 
first signal pressure P.sub.10 is input to the entrance 111a of the check 
ball 111. At that time, the gain pressure P.sub.G regulated from the line 
pressure P.sub.L is not input to the opposing entrance 111b of the check 
ball 111. Therefore, the ball 111d is pushed to the left, in FIG. 5, by 
the first signal pressure P.sub.10 input to the entrance 111a. As a 
result, the first signal pressure P.sub.10 is output as the second signal 
pressure P.sub.20 from the exit 111c of the check valve 111. As a result, 
the second signal pressure P.sub.20 (currently equal to the first signal 
pressure P.sub.10) is input to the first end chamber 1 of the primary 
regulator valve 72 via the orifice 101. 
When the first signal pressure P.sub.10 is increased, the spool 110a is 
pushed down gradually by the first signal pressure P.sub.10, and the 
second signal pressure P.sub.20 is increased gradually. Then, the line 
pressure P.sub.L from the primary regulator valve 72 is also increased. 
The change rate of the line pressure P.sub.L related to the change rate of 
the first signal pressure P.sub.10, which is the gain G, is shown as the 
low gain G.sub.1 in FIG. 7. The low gain G.sub.1, which has a small grade, 
in FIG. 7, is continued to a changing point to be discussed. 
When the spool 110a has descended, by increasing the first signal pressure 
P.sub.10, so the upper surface of the land L.sub.2 passes the upper end of 
the input port c, the line pressure P.sub.L is input to the input port c 
of the gain control valve 110, and is output as the gain pressure P.sub.G 
from the output port b. The gain pressure P.sub.G is input to the entrance 
111b of the check ball 111. At that time, the line pressure P.sub.L pushes 
up the spool 110a because the pressure area of the lower surface of the 
land L.sub.1 of the spool 110a is larger than the pressure area of the 
upper surface of the land L.sub.2. The force pushing up the spool 110a is 
calculated by multiplying the area difference between the pressure area of 
the lower surface of the land L.sub.1 and the pressure area of the upper 
surface of the land L.sub.2 subject to the gain pressure P.sub.G. The gain 
pressure P.sub.G is regulated by the force and the first signal pressure 
P.sub.10 input to the hydraulic chamber a. The gain pressure P.sub.G from 
the output port b is regulated by setting the difference between the 
pressure area of the lower surface of the land L.sub.1 and the pressure 
area of the upper surface of the land L.sub.2 properly. For example, when 
the difference between the pressure areas is small, the gain pressure 
P.sub.G become high with respect to the aforementioned first signal 
pressure P.sub.10. Therefore, the gain G can be higher than a high gain 
G.sub.2 discussed below. 
When the first signal pressure P.sub.10 input to the hydraulic chamber a is 
increased, the first signal pressure P.sub.10 input to the entrance 111a 
of the check ball 111 and the gain pressure P.sub.G input to the entrance 
111b are increased. Thus, when the first signal pressure P.sub.10 is low, 
the first signal pressure P.sub.10 is output as the second signal pressure 
P.sub.20 from the exit 111c because the first signal pressure P.sub.10 is 
higher than the gain pressure P.sub.G and the ball 111d of the check ball 
111 is pushed to the left. 
The increasing rate of the gain pressure P.sub.G input to the entrance 111b 
is larger than the increasing rate of the first signal pressure P.sub.10 
input to the entrance 111a. The gain pressure P.sub.G is regulated based 
on the first signal pressure P.sub.10. The difference between the pressure 
area of the lower surface of the land L.sub.1 and the pressure area of the 
upper surface of the land L.sub.2 is set (manufactured) to be smaller than 
the pressure area for the first signal pressure P.sub.10. 
Therefore, when the first signal pressure P.sub.10 is increased gradually, 
the gain pressure P.sub.G becomes greater than the first signal pressure 
P.sub.10 at a point, designated the changing point, shown in FIG. 7. As a 
result, the ball 111d of the check ball 111 is pushed to the right, the 
gain pressure P.sub.G is output as the second signal pressure P.sub.20 
from the exit 111c, and the second signal pressure P.sub.20 is input to 
the primary regulator valve 72. The change rate, which is the gain, of the 
line pressure P.sub.L from the primary regulator valve 72 related to the 
change rate of the first signal pressure P.sub.10 input to the hydraulic 
chamber a of the gain control valve 110 is changed to the high gain 
G.sub.2 which is larger than the low gain G.sub.1. In this case, the words 
high and low in the low gain G.sub.1 and the high gain G.sub.2 are 
relative words and are not used as high and low in the absolute sense. 
The high gain G.sub.2 is continued till the maximum value max of the line 
pressure P.sub.L, corresponding to the maximum value of the first signal 
pressure P.sub.10 is reached. In FIG. 7, the grade of the graph shows the 
gain G, and the gain G is the low gain G.sub.1 when the grade is small and 
the gain G is the high gain G.sub.2 when the grade is large. 
In this embodiment, the needed change range of the line pressure P.sub.L, 
which is between min and max in FIG. 7, is achieved within the change 
range of the first signal pressure P.sub.10 output from the line pressure 
linear solenoid SLT. 
For example, in the continuously variable transmission 1 shown in FIG. 1, 
the high accuracy control for the line pressure P.sub.L and the high line 
pressure P.sub.L based on the first signal pressure P.sub.10 are required 
at different times or conditions. The low gain G.sub.1 is needed for the 
high accuracy control, and the high gain G.sub.2 is needed for achieving 
the high line pressure P.sub.L. Therefore, when the gain G is fixed at a 
certain value as in the related art, it is difficult to achieve both of 
the high accuracy control and the high line pressure P.sub.L. 
That is to say, when the change range of the first signal pressure P.sub.10 
is settled, in the case the gain G is set as the low gain G.sub.1 for 
achieving the high accuracy control, as shown in FIG. 7, with the 
alternating long and two short dash line, the needed maximum value of the 
line pressure P.sub.L is not achieved. In the case the gain G is set as 
the high gain G.sub.2 for achieving the high line pressure P.sub.L, as 
shown in FIG. 7 with an alternate long and short dash line, high accuracy 
control for the line pressure P.sub.L is difficult. 
Based on the changing point within the change range of the first signal 
pressure P.sub.10, in the case where the first signal pressure P.sub.10 is 
low and the high accuracy control is needed, the gain G is set as the low 
gain G.sub.1. Then, in the case where the first signal pressure P.sub.10 
is high and the high line pressure P.sub.L is needed, the gain G is set as 
the high gain G.sub.2. Therefore, both of the high accuracy control for 
the line pressure P.sub.L and the high line pressure P.sub.L are achieved 
at the appropriate time or under the appropriate conditions. 
As the way for achieving the described result, in the first embodiment, the 
amplification factor changing device comprising the gain control valve 110 
and the check ball 111 selects the second signal pressure from the first 
signal pressure P.sub.10 and the gain pressure P.sub.G, the second signal 
pressure P.sub.20 is output to the primary regulator valve 72, and the 
line pressure P.sub.L is controlled based on the second signal pressure 
P.sub.20. 
An adjusting mechanism and an adjusting way will now be described. In the 
hydraulic control system shown in FIG. 9, a first adjusting mechanism 120 
and a second adjusting mechanism 130 are added to the hydraulic control 
system shown in FIG. 5. 
The first adjusting mechanism 120 is structured on the lower end portion of 
the line pressure linear solenoid valve SLT integrally, and comprises a 
female screw portion 120a formed on the valve body and an adjusting screw 
120b engaged with the female screw portion 120a. The spring 140b is 
compressed and arranged between the adjusting screw 120b and the spool 
140a. Therefore, the spring load of the spring 140b toward the spool 140a 
is increased when the screw amount of the adjusting screw 120b is 
increased, and is decreased when the screw amount of the adjusting screw 
120b is decreased. 
The second adjusting mechanism 130 has the same structure as the first 
adjusting mechanism 120. That is to say, the second adjusting mechanism 
130 is structured on the upper end portion of the gain control valve 110 
integrally, and comprises a female screw portion 130a formed on the valve 
body and an adjusting screw 130b engaged with the female screw portion 
130a. The spring 110b is compressed and arranged between the adjusting 
screw 130b and the spool 110a. Therefore, the spring load of the spring 
110b toward the spool 110a is increased when the screw amount of the 
adjusting screw 130b is increased, and is decreased when the screw amount 
of the adjusting screw 130b is decreased. 
The line pressure P.sub.L as related to a current value I shown in FIG. 10 
has the dispersion shown dependent on the hydraulic circuit. The maximum 
and minimum values of the dispersion before adjustment are shown with the 
alternate long and two short dashes lines. 
In the first embodiment, the gain pressure P.sub.G is influenced by the 
first signal pressure P.sub.10. Therefore, the gain pressure P.sub.G is 
regulated after the first signal pressure P.sub.10 is regulated. 
At first, the current value I applied to the linear solenoid 140c of the 
line pressure linear solenoid valve SLT is a minimum value, and the line 
pressure P.sub.L, which is the lowest pressure of the line pressure 
P.sub.L, with respect to the minimum current value, is measured. When the 
lowest pressure of the line pressure P.sub.L is higher than an adjusting 
point, which is a basic point, on the low pressure side, the screw amount 
of the adjusting screw 120b of the first adjusting mechanism 120 is 
increased. Therefore, the spring load of the spring 140b is increased. As 
a result, the first signal pressure P.sub.10 is decreased and the lowest 
pressure of the line pressure P.sub.L is coincided with the adjusting 
point on low pressure side. When the lowest pressure of the line pressure 
P.sub.L is lower than the adjusting point, which is the basic point, on 
the low pressure side, the screw amount of the adjusting screw 120b of the 
first adjusting mechanism 120 is decreased. Therefore, the spring load of 
the spring 140b is decreased. As a result, the first signal pressure 
P.sub.10 is increased and the lowest pressure of the line pressure P.sub.L 
is coincided with the adjusting point on low pressure side. 
This adjustment is substantially the same as the related art. Thus, as in 
the related art, at this time, the line pressure P.sub.L has a large 
dispersion at the high pressure side. 
The invention provides for an adjustment to be performed by the second 
adjustment mechanism 130 at the high pressure side. 
The gain pressure, which regulates the line pressure P.sub.L at the high 
pressure side, is regulated. The current value I to the linear solenoid 
140c of the line pressure linear solenoid valve SLT is increased to a 
maximum value, and the line pressure P.sub.L, which is the highest 
pressure of the line pressure P.sub.L, with respect to the maximum current 
value, is measured. When the highest pressure of the line pressure P.sub.L 
is higher than an adjusting point, which is a basic point, on the high 
pressure side, the screw amount of the adjusting screw 130b of the second 
adjusting mechanism 130 is increased. Therefore, the spring load of the 
spring 110b is increased. As a result, the gain pressure P.sub.G is 
decreased and the highest pressure of the line pressure P.sub.L is 
coincided with the adjusting point on the high pressure side. When the 
highest pressure of the line pressure P.sub.L is lower than the adjusting 
point, which is the basic point, on the high pressure side, the screw 
amount of the adjusting screw 130b of the second adjusting mechanism 130 
is decreased. Therefore, the spring load of the spring 110b is decreased. 
As a result, the gain pressure P.sub.G is increased and the highest 
pressure of the line pressure P.sub.L is coincided with the adjusting 
point on the high pressure side. 
The dispersion of the line pressure P.sub.L with respect to the current 
value I in each hydraulic circuit is decreased by regulating the line 
pressure P.sub.L at two points that are the adjusting point on the low 
pressure side and the adjusting point on the high pressure side. The 
dispersion after the adjustments is shown with an oblique line portion 
between the alternate long and short dash lines in FIG. 10. The lowest 
pressure of the line pressure P.sub.L as the adjusting point on low 
pressure side and the highest pressure of the line pressure P.sub.L as the 
adjusting point on high pressure side are related to the current value I 
with a high degree of accuracy. 
Therefore, the need to consider the strength and durability of the 
hydraulic circuit and of setting the lowest control pressure to be high is 
removed. 
The second embodiment will now be described. In the second embodiment, when 
the first signal pressure P.sub.10 is low, the first signal pressure 
P.sub.10 is output as the second signal pressure P.sub.20 to the primary 
regulator valve 72. When the first signal pressure P.sub.10 is high, both 
of the first signal pressure P.sub.10 and the gain pressure P.sub.G are 
output as the second signal pressure P.sub.20 to the primary regulator 
valve 72. 
The second embodiment is shown in FIG. 6. The elements having the same 
structure and the same operations are identified with the same reference 
numbers or numerals as the previous embodiment. Where the explanations are 
the same, they are omitted, and only the portions that differ from the 
first embodiment will be described. 
In FIG. 6, the line pressure linear solenoid valve SLT, as the signal 
pressure output device, and the gain control valve are the same as those 
in FIG. 5. The primary regulator valve 72A, as the regulating device, is 
expanded at one end adding a hydraulic chamber d as compared with the 
regulator valve 72 of FIGS. 5 and 9. 
In the second embodiment, an amplification factor changing device comprises 
the hydraulic chambers 1, d of the primary regulator valve 72A and the 
gain control valve 110. 
The first signal pressure P.sub.10 output from the line pressure linear 
solenoid valve SLT is input to the hydraulic chamber 1 of the primary 
regulator valve 72A and pushes up the spool 72a gradually. Further, the 
first signal pressure P.sub.10 is input to the hydraulic chamber a of the 
gain control valve 110 and pushes down the spool 110a gradually. 
In this state, as shown in FIG. 8, the gain G is set as the low gain 
G.sub.1 and the low gain G.sub.1 is maintained until the first signal 
pressure P.sub.10 reaches the changing point. 
As the first signal pressure P.sub.10 from the line pressure linear 
solenoid valve SLT, input to the hydraulic chamber a of the gain control 
valve 110, gradually pushes down the spool 110a, the gain pressure P.sub.G 
is output from the output port b and input to the hydraulic chamber d of 
the primary regulator valve 72A. The spool 72a is divided into two 
portions. One of them is an upper portion which is arranged so that the 
upper portion is pushed up by the first signal pressure P.sub.10. The 
other one is an lower portion which is arranged so that the lower portion 
is pushed up by the gain pressure P.sub.G and is pushed down by the first 
signal pressure P.sub.10. In the lower portion, the pressure area for the 
first signal pressure on the upper surface and the pressure area for the 
gain pressure on the lower surface of the lower portion are same. The 
upper portion and the lower portion are in contact with each other at the 
position on left-half side in FIG. 6. When the gain pressure P.sub.G is 
lower than the first signal pressure P.sub.10, the lower portion is pushed 
down by the first signal pressure P.sub.10. Therefore, the gain pressure 
P.sub.G does not push up the upper portion of the spool 72a of the primary 
regulator valve 72A via the lower portion. Then, the upper portion is 
pushed up by the first signal pressure P.sub.10. When the first signal 
pressure P.sub.10 is increased above the changing point, the gain pressure 
P.sub.G is higher than the first signal pressure P.sub.10. Then the upper 
portion is pushed up directly by the first signal pressure P.sub.10 and is 
also pushed up by the gain pressure P.sub.G via the lower portion. That is 
to say, the spool 72a is pushed up by both of the first signal pressure 
P.sub.10 and the gain pressure P.sub.G. Therefore, in the second 
embodiment, the first signal pressure P.sub.10 operates as the second 
signal pressure P.sub.20 which pushes up the spool 72a of the primary 
regulator valve 72A when the first signal pressure P.sub.10 is not 
increased to the changing point, and both of the first signal pressure 
P.sub.10 and the gain pressure P.sub.G operate as the second signal 
pressure P.sub.20 when the first signal pressure is increased above the 
changing point. 
In the first embodiment, the higher pressure is selected from the first 
signal pressure P.sub.10 and the gain pressure P.sub.G and operates as the 
second signal pressure P.sub.20. But in the second embodiment, as shown in 
FIG. 8, a pressure shown with a oblique line portion is added to the first 
signal pressure P.sub.10 by the gain pressure P.sub.G. Therefore, the 
change from the low gain G.sub.1 to the high gain G.sub.2 and the change 
from the high gain G.sub.2 to the low gain G.sub.1 are performed smoothly. 
FIG. 11 shows the hydraulic control system in which the first adjusting 
mechanism 120 and the second adjusting mechanism 130 are added to the 
hydraulic control system shown in FIG. 6. 
The hydraulic control system shown in FIG. 11 has the same effect as the 
hydraulic control system shown in FIG. 9. That is to say, the line 
pressure P.sub.L is regulated by controlling the primary regulator valve 
72A with the first adjusting mechanism 120 of the line pressure linear 
solenoid valve SLT and the second adjusting mechanism 130 of the gain 
control valve 110 at the adjusting point on the low pressure side and the 
adjusting point on the high pressure side. Therefore, the dispersion of 
the line pressure P.sub.L based on the current value I in each hydraulic 
circuit is decreased. The dispersion after the adjustments is shown with 
an oblique line portion between the alternate long and short dash lines in 
FIG. 10.