Hydraulic control

:
Main valves and main pressures of the
system
hydraulic control system
The hydraulic control system consists
of the oil pump, which generates the
hydraulic pressure; various valves having
various functions; and fluid passages and
tubes, which deliver the transmission
fluid to the clutches, brakes, and other
components of the hydraulic control
system. Most valves in the hydraulic
control system are housed in the valve
body assembly under the planetary
gears.
Roles of hydraulic control system
 Supplying transmission fluid to the
torque converter.
Regulating the hydraulic pressure
generated by the oil pump.
Converting the engine load and vehicle
speed into hydraulic "signals".
Applying hydraulic pressure to the
clutches and brakes to control planetary
gear operation.
Lubricating rotating parts with fluid.
Cooling the torque converter and
transmission with fluid.
VALVES FUNCTION
Primary Regulates the hydraulic pressure generated by the oil pump, creating line pressure,
regulator becomes the basis for various other pressures: governor pressure, lubrication pressure,
valve throttle pressure, etc.
Secondary Creates converter pressure and lubrication pressure.
regulator
valve
Manual valve Operated by the selector lever; opens the oil passage to the appropriate valve for each
gear.
Throttle valve Creates hydraulic pressure (throttle pressure) which matches the angle of the
accelerator pedal.
Throttle When the throttle pressure rises above a predetermined pressure, this valve reduces
modulator the line pressure generated by the primary regulator valve.
valve
Governor Creates hydraulic pressure (governor pressure) which matches the vehicle speed.
valve
Cut-back When the governor pressure rises, this valve reduces the throttle pressure
valve (generated by the throttle valve) by the appropriate amount.
Shift valves Select passages (1st – 2nd ), (2nd – 3rd ) and (3rd - OD) for line pressure which
(1-2, 2-3, 3- acts on the planetary gear unit.
4)
Lock-up Determines the lock-up clutch on-off timing and transmits the results to the lock- up relay
signal valve.
valve
Lock-up relay Selects passages for the converter pressure, which turns the torque converter's lock-up
valve clutch on and off.
Accumulators Reduce the shock generated when the Co, C1,C2 or B2 pistons operate.
FLUID FUNCTION
PRESSURE
Regulated by the primary regulator valve, this is the most basic and important
pressure used in the automatic transmission, because it is used to operate all of
Line pressure the clutches and brakes in the transmission, and also because it is the source of
all other pressures (e.g., governor pressure, throttle pressure, etc.) used in the
automatic transmission.
Converter and Generated by the secondary regulator valve, this is used to supply the torque
lubrication converter with fluid, to lubricate the transmission case and bearings, etc., and to
pressure send oil to the oil cooler.
Throttle Throttle pressure (regulated by the throttle valve) corresponds to increases or
pressure decreases in the amount the accelerator pedal is depressed. Governor pressure
(regulated by the governor valve) corresponds to the vehicle's speed. The
Governor
balance between these two pressures is a factor in determining shift points; for
pressure
this reason, both of these pressures are very important.
A si mplif ied hyd ra ulic co ntrol s yste m
The hydraulic control system converts the engine load (throttle valve opening angle)
and vehicle speed to various hydraulic pressures, which in turn determine the shift
timing. A simplified hydraulic control system is shown in figure 36 below.
This system consists of an oil pump, a
governor valve, and a valve body. The oil
pump's drive gear is meshed with the pump
impeller of the torque converter. It always
rotates at the same speed as the engine. The
governor valve is driven by the output shaft
(drive pinion), and converts the rotation
(speed) of the drive pinion shaft to hydraulic
signals, which are sent to the valve body. The
valve body contains many passages through
which the transmission fluid flows. Many
valves are built into these passages, and they
open up and close off these passages, sending
or cutting off hydraulic shift "signals" to the
various components of the planetary gear unit.
 Oil pump
One type of Automatic transmissions pump is a gear
pump. The pump is usually located in the cover of
the transmission. It draws fluid from a sump in the
bottom of the transmission and feeds it to the
hydraulic system. It also feeds the transmission
cooler
The and
inner theof
gear torque converter.
the pump hooks up to the housing of
the torque converter, so it spins at the same speed as
the engine. The outer gear is turned by the inner gear,
and as the gears rotate, fluid is drawn up from the
sump on one side of the crescent and forced out into
the hydraulic system on the other side.
 Valve Body and main valves
 The valve body contains many of the hydraulic
valves, such as the pressure regulator, shift valves,
manual valve, and others used in an automatic
transmission.
 The valve body bolts to the bottom of the
transmission case and is housed in the transmission
pan.
 A filter or screen is attached to the bottom of the
valve body. Passages in the valve body route fluid
from the pump to the valves and then into the
transmission case.
 Passages in the transmission case carry fluid to
other hydraulic components.
 Manual Valve
A manual valve operated by the shift
lever mechanism, allows the operator to
select park “P”, neutral “n”, reverse “R”,
or different forward drive ranges (“D”,
“2”, “L”). Depending on which gear is
selected, the manual valve feeds
hydraulic circuits to the appropriate
passages.
Primary regulator valve
Uncontrolled pump pressure would
change with engine speed. Oil from the
pump flows directly to a regulator valve
that limits pressure.
The regulator valve adjusts the hydraulic
pressure (line pressure) to each element
in conformity with the engine power. Oil
pressure controlled by the primary
regulator valve is called line pressure.
 Throttle valve
The throttle valve produces throttle
pressure in response to accelerator
pedal angle (engine output power).
There are two different ways that this is
done. Some cars have a simple cable
linkage connected to a throttle valve in
the transmission. Other cars use a
vacuum modulator to apply pressure to
the throttle valve. The modulator senses
the manifold pressure, which drops when
the engine is under a greater load. The
vacuum controlled diaphragm is used to
 Cut-Back Valve
This valve regulates the cut-back pressure acting
on the throttle valve, and is actuated by
governor pressure and throttle pressure. Applying
cut-back pressure to the throttle valve in this manner
lowers the throttle pressure to prevent unnecessary
power loss
Governor from the
pressure oil on
acts pump.
the upper portion of this
valve, and as the valve is pushed downward, a
passage from the throttle valve is opened and throttle
pressure is applied. Because of the difference in the
diameters of the valve pistons, the cutback valve is
pushed upward as a result, and the balance between
the downward force due to governor pressure and the
pressure becomes the cut-back pressure.
In some automatic transmissions, the governor
pressure is first reduced by the governor modulator
valve before acting on the cut-back valve.
Governor Valve
The governor valve senses vehicle speed
(transmission output shaft speed) to help
control gear shifting. The faster the car
goes, the more the governor valve opens
and the higher the governor pressure.
The governor valve is driven (turned) by
the governor driven gear, which is meshed
with the differential drive pinion
(Automatic transaxle), and produces fluid
pressure (governor pressure) in response
to the drive pinion rpm (vehicle speed).
 Accumulator Control Valve and
accumulators
This valve reduces shifting shock by
lowering the back pressure of the direct
clutch (C2) accumulator and 2nd brake
(B2) accumulator when the-throttle
opening angle is small. Since the torque
produced by the engine is small if the
throttle opening angle is small,
accumulator control pressure, and
therefore the initial pressure used to
operate the clutches and brakes, are
both reduced, preventing the shock
that would otherwise occur when the
Low Modulator Valve
The low modulator valve reduces the line pressure from the manual valve (low
modulator pressure) to reduce shock when the transmission is shifted into the "L"
range. Low modulator pressure pushes down the low coast shift valve and acts on
the 1st and reverse brake (B3) to buffer the shock. It also causes low modulator
pressure to act on the primary regulator valve to raise line pressure, thus increasing
torque in order to prevent the clutches and brakes from slipping.
Second modulator valve
In the "2" range, this valve reduces line pressure from the intermediate
shift valve (2nd modulator pressure). 2nd modulator pressure acts on
the 2nd coast brake (B1) through the 1-2 shift valve to reduce shifting
shock.
Shift Valves
Shift valves supply hydraulic pressure to the clutches and bands to
engage each gear. The valve body of the transmission contains
several shift valves. The shift valve determines when to shift from one
gear to the next. For instance, the 1- 2 shift valve determines when to
shift from first to second gear. The shift valve is pressurized with fluid
from the governor on one side, and the throttle valve on the other.
Each shift valve responds to a particular pressure range; so when the
car is going faster, the 2 - 3 shift valve will take over, because the
pressure from the governor is high enough to trigger that valve.
1-2 shift valve
This valve controls shifting between 1st and 2nd gears in accordance
with governor and throttle pressures. To improve the valve sliding, a 3-
piece valve is used. When governor pressure is low but throttle
pressure high, this valve is pushed down by throttle pressure. Since
this causes the 2nd brake circuit to close, the transmission shifts into
first gear.
When governor pressure is high and throttle pressure low, the valve is
pushed up by governor pressure and the circuit to the 2nd brake piston
opens so the transmission will shift into second gear
SHIFT VALVE
This valve performs shifting between
2nd and 3rd gears. Control is
accomplished by opposing throttle
pressure and spring tension against the
governor pressure. When governor
pressure is high, this valve is pushed up
against the resistance of the throttle
pressure and spring tension to open the
passage to the direct clutch (C2) piston,
thus effecting shifting into 3rd gear.
When governor pressure is low, this
valve is pushed down by throttle
pressure and spring tension to close the
Automatic transmission fluid (ATF)
A special high-grade petroleum-based mineral
oil mixed with several special additives is used
to lubricate the automatic transmission.
This oil is called automatic transmission fluid
(abbreviated "ATF") to discriminate it from
other types of oil.
The specified type of ATF must always be used
for the automatic transmission. Use of a non-
specified ATF, or use of the proper ATF mixed
with a non-specified ATF, will degrade
automatic transmission performance.
To ensure the correct functioning of the
automatic transmission, the fluid level is also
important.
Roles of ATF
• Transmission of torque in
the torque converter.
•Control of the hydraulic
control system, as well as of
clutch and brake operation in
the transmission section.
• Lubrication of the planetary
gears and other moving parts.
• Cooling of moving parts.