PRESSURE REGULATOR VALVE
a spool valve cycles back and forth between the open and the exhaust positions,
it is called a regulating valve. An automatic transmission uses a positive
displacement oil pump. As the pump delivers, fluid pressure rises uncontrollably
until the pump stalls from extremely high pressure. This condition is aggravated
by high engine speeds that drive the pump faster to deliver more fluid. To
prevent this stalling, the pump has a pressure regulator valve located in the
valve body that maintains a basic fluid pressure. Fluid pressure flows from the
pump to the pressure regulator valve, which is held in position by coil spring
tension. Pressure regulator valve movement to the exhaust position is controlled
by calibrated coil spring tension.
three steps of pressure regulation operation are charging the torque converter,
exhausting fluid pressure, and establishing a balanced condition.
fluid pressure reaches normal baseline pressure (usually about 60 psi), fluid
enters at the top of the regulator valve, forcing it down against coil spring
tension. The outlet port opens, which permits fluid flow to charge (fill) the
torque converter. The torque converter pressure regulator valve is usually a
spring-loaded spool type. Torque converter pressure is developed from line
pressure at the pressure regulator valve. The torque converter pressure
regulator valve has several spools and lands so when different oil pressures are
applied, converter pressure varies in proportion to torque requirements.
Converter pressure transmits torque and keeps transmission fluid circulating
into and out of the torque converter, which reduces the formation of air bubbles
and aids cooling. The exception to this is when the torque converter is
operating in the lockup mode.
the second stage, the pressure regulator valve is forced down against spring
tension by fluid pressure to uncover the exhaust port. Fluid pressure not needed
for transmission operation is exhausted into the pump inlet circuit.
fluid pressure acting at one end and coil spring tension at the opposite end,
the pressure regulator valve takes a balanced position. While the coil spring
tension controls pressure, the valve adjusts itself automatically to the
pressure, which forces it downward. The pressure regulator valve's new position
might have one of the spool's lands partially covering the inlet port.
are times in the automatic transmission's operation when fluid pressure must be
increased above its baseline pressure. This increase is needed to hold bands and
clutches more tightly and to raise the point at which shifting takes place.
Increasing pressure above normal line pressures allows operational load
flexibility, such as when towing trailers.
are two methods used to monitor vehicle and engine load: vacuum modulation and
throttle pressure. A vacuum modulator measures fluctuating engine vacuum to
sense the load placed on the engine and drivetrain. The modulator is normally a
small canister that is threaded or push-fit into the transmission housing. A
rubber tube connects the canister to the intake manifold of the engine. The
canister itself has two chambers divided by a diaphragm. The chamber closest to
the transmission housing is open to atmospheric pressure, and the second chamber
is closed to atmospheric pressure. The closed chamber contains a coil spring
trapped between the diaphragm and the end of the canister. The open chamber
holds a pushrod connecting the diaphragm to the modulator valve.
Vacuum Modulator Operation
a vehicle is placed under a heavy load, the driver opens the throttle. This
reduces vacuum in the intake manifold and the closed chamber or the vacuum
modulator. With reduced vacuum, the coil spring tension releases to move the
diaphragm, pushrod, and modulator valve. The modulator valve opens, allowing
line pressure to flow through the modulator valve to the booster valve at the
pressure regulator valve. The modulator pressure acting on the booster valve
assists the pressure regulator valve coil spring in pushing the regulator valve
up against the line pressure at the top of the pressure regulator valve. Line
pressure continues to increase until the pressure on the regulator valve
overcomes the pressure regulator valve spring tension and modulator (auxiliary)
pressure. The pressure regulator valve opens the exhaust port at a new boosted
line pressure. This new pressure holds planetary controls tightly to resist
slippage and raise automatic shift points.
Vacuum modulators are found on older vehicles. Newer vehicles use throttle valve
pressure to increase line pressure. Throttle valve pressure develops when line
pressure passes through the throttle valve valley to become throttle pressure.
Throttle pedal movement is carried through the throttle linkage to control the
operation of the throttle valve.
the pedal is depressed (opened), the throttle valve opens to produce throttle
pressure, which is directed to the pressure regulator throttle plug. It helps
the pressure regulator valve spring hold the pressure regulator valve in
position to close the exhaust port. This results in increased pressure.
the pedal is released (closed), the throttle valve partially closes. This
decreases throttle pressure at the throttle valve plug, resulting in a reduction
of line pressure.
direction of line pressure flow is directed by a relay valve. A relay valve is a
spool-type valve with several spools, lands, and reaction areas. It is held in
one position in the valve body core by coil spring tension, auxiliary fluid
pressure, or a mechanical force. Auxiliary fluid pressure or mechanical force
can oppose coil spring tension to move the relay valve to a new position. In the
new position, the valley of the relay valve aligns with interconnecting ports.
Fluid pressure flows from an inlet port across the relay valve valley to the
outlet port. When a relay valve is in one position, it blocks fluid flow. When
moved to an alternative position, fluid is directed through to the outlet port.
Shift Valve One application of the relay valve is a shift valve. A shift
valve usually operates in one of two positions - either downshifted or upshifted.
In operation, throttle pressure, which is high, is acting on one reaction area
of shift valve. Throttle pressure and light coil spring tension act and hold the
valve in the downshifted position. In this position, fluid pressure is blocked
from flowing to one of the planetary gear controls.
vehicle speed increases, a device on the transmission output shaft, called the
governor, develops a pressure, called governor pressure. Governor pressure is
directed to the opposite reaction area of the shift valve. With increasing
vehicle speed, governor pressure increases to overcome coil spring tension and
throttle pressure. Governor pressure moves the shift valve to the upshifted
position. In this position the inlet and outlet ports are in the same valley
area of the shift valve. Fluid pressure flows across the shift valve valley, out
of the outlet port, and through the connecting worm tracks to engage the
planetary controls for the next higher gear.
the vehicle operator pushes down on the throttle pedal, the throttle valve opens
wider and throttle pressure increases. Throttle pressure is higher than governor
pressure. The high throttle pressure and coil spring tension force the shift
valve to move to the downshifted position against governor pressure. The
transmission automatically downshifts to the next lower gear. The shift valve
blocks fluid pressure at the inlet port, which prevents the planetary controls
from operating for an upshift. The automatic downshifting of the transmission is
often referred to as a passing gear or kickdown. A kickdown valve is often used
to generate additional kickdown pressure from line pressure. This ensures that
the downshift is made quickly and positively.
manual valve is a spool valve operated manually by the vehicle operator and gear
selector linkage. When the operator selects the gear position, the gear selector
linkage positions the manual valve in the valve body. The manual valve directs
line pressure to the correct combination of circuits, which produces the range
desired for the driver's requirements. For example, if the drive range is
selected, the gear selector linkage is placed in a position that allows fluid
pressure at the inlet port to flow across the manual valve valley passing
through the outlet port to charge the forward circuit. Control areas of the
forward circuit are the governor, rear clutch, and accumulator. The various
transmission controls and circuitry that drive the vehicle forward make up the
forward circuit. If the gear selector is moved to the reverse position, the
manual valve is moved to open the reverse inlet and outlet ports to charge the
rear servo and front clutch and move the vehicle in reverse.