Pistons - Jeep cherokee 2000 Service Manual

21 - 234 AW–4 AUTOMATIC TRANSMISSION
DESCRIPTION AND OPERATION (Continued)
brake. The modulator valve operates when the shift
lever and manual valve are in the 1–2 position.
Fig. 27 Low Coast Modulator Valve
1 – LINE PRESSURE (FROM 2–3 SHIFT VALVE)
2 – LOW COAST MODULATOR PRESSURE TO FIRST/REVERSE
BRAKE
ACCUMULATOR CONTROL VALVE
The accumulator control valve (Fig. 28) cushions
the transmission clutch and brake applications. This
is achieved by reducing back pressure to the accumu-
lators when throttle opening is small. The valve is
operated by line and throttle pressure.
ACCUMULATORS
Four accumulators are used to cushion clutch and
brake application. The accumulators (Fig. 29), consist
of spring loaded pistons. The pistons dampen the ini-
tial surge of apply pressure to provide smooth
engagement during shifts.
Control pressure from the accumulator control
valve is continuously applied to the back pressure
side of the accumulator pistons. This pressure plus
spring tension holds the pistons down. As line pres-
sure from the shift valves enters the opposite end of
the piston bore, control pressure and spring tension
momentarily delay application of full line pressure to
cushion engagement. The accumulators are all
located in the transmission case (Fig. 29).
TRANSMISSION VALVE BODY SOLENOIDS
Three solenoids are used (Fig. 30). The No. 1 and 2
solenoids control shift valve operation by applying or
releasing line pressure. The signal to apply or release
Fig. 28 Accumulator Control Valve
1 – ACCUMULATOR CONTROL PRESSURE (TO
ACCUMULATORS)
2 – THROTTLE PRESSURE
3 – LINE PRESSURE (FROM PUMP)
pressure is provided by the transmission control
module.
The No. 3 solenoid controls operation of the torque
converter clutch. The solenoid operates in response to
signals from the transmission control module.
When the No. 1 and 2 solenoids are activated, the
solenoid plunger is moved off its seat opening the
drain port to release line pressure. When either sole-
noid is deactivated, the plunger closes the drain port.
The No. 3 solenoid operates in reverse. When the
solenoid is deactivated, the solenoid plunger is moved
off its seat opening the drain port to release line
pressure. When the solenoid is activated, the plunger
closes the drain port.

PISTONS

DESCRIPTION
There are several sizes and types of pistons used in
an automatic transmission. Some pistons are used to
apply clutches, while others are used to apply bands.
They all have in common the fact that they are
round or circular in shape, located within a smooth
walled cylinder, which is closed at one end and con-
verts fluid pressure into mechanical movement. The
fluid pressure exerted on the piston is contained
within the system through the use of piston rings or
seals.
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