Jeep XJ. Manual — part 424
When the TCM activates solenoid No. 1, line pres-
sure at the top of the 2–3 valve is released through
the solenoid drain port. Spring tension moves the
valve up to hold the valve in second gear position. As
the solenoid is deactivated, line pressure then moves
the valve down exposing the direct clutch feed port
for the shift to third gear.
Fig. 23 1–2 Shift Valve
Fig. 24 2–3 Shift Valve
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DESCRIPTION AND OPERATION (Continued)
3–4 SHIFT VALVE
The 3–4 shift valve (Fig. 25) is operated by the No.
2 solenoid and by line pressure from the manual
valve, 2–3 valve and primary regulator valve.
Energizing the No. 2 solenoid causes line pressure
at the top of the 3–4 valve to be released through the
solenoid valve drain port. Spring tension moves the
valve up exposing the overdrive clutch accumulator
feed port to apply the clutch.
De–energizing the solenoid causes the drain port to
close. Line pressure then moves the valve down
exposing the overdrive brake accumulator feed port
for the shift to fourth gear.
In the 1–2 or 3 gearshift lever positions, line pres-
sure from the 2–3 shift valve is applied to the lower
end of the 3–4 valve. This holds the valve upward,
closing off the overdrive brake feed port preventing a
shift into fourth gear.
SECOND COAST MODULATOR VALVE
The second coast modulator valve (Fig. 26) momen-
tarily reduces line pressure from the 1–2 shift valve.
This cushions application of the second coast brake.
The valve is operative when the shift lever and man-
ual valve are in the 3 position.
LOW COAST MODULATOR VALVE
The low coast modulator valve (Fig. 27) momen-
tarily reduces line pressure from the 2–3 shift valve;
this action cushions application of the first/reverse
Fig. 25 3–4 Shift Valve
1 – TO OVER DRIVE BRAKE ACCUMULATOR
2 – LINE PRESSURE (FROM PRIMARY REGULATOR VALVE)
3 – LINE PRESSURE (FROM PRIMARY REGULATOR VALVE)
4 – TO OVERDRIVE CLUTCH ACCUMULATOR
Fig. 26 Second Coast Modulator Valve
1 – LINE PRESSURE (FROM 1–2 SHIFT VALVE)
2 – LOW COAST MODULATOR PRESSURE SECOND COAST
BRAKE
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DESCRIPTION AND OPERATION (Continued)
brake. The modulator valve operates when the shift
lever and manual valve are in the 1–2 position.
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
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.
Fig. 27 Low Coast Modulator Valve
1 – LINE PRESSURE (FROM 2–3 SHIFT VALVE)
2 – LOW COAST MODULATOR PRESSURE TO FIRST/REVERSE
BRAKE
Fig. 28 Accumulator Control Valve
1 – ACCUMULATOR CONTROL PRESSURE (TO
ACCUMULATORS)
2 – THROTTLE PRESSURE
3 – LINE PRESSURE (FROM PUMP)
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DESCRIPTION AND OPERATION (Continued)
OPERATION
The principal which makes this operation possible
is known as Pascal’s Law. Pascal’s Law can be stated
as: “Pressure on a confined fluid is transmitted
equally in all directions and acts with equal force on
equal areas.”
PRESSURE
Pressure (Fig. 31) is nothing more than force (lbs.)
divided by area (in or ft.), or force per unit area.
Given a 100 lb. block and an area of 100 sq. in. on
the floor, the pressure exerted by the block is: 100
lbs. 100 in or 1 pound per square inch, or PSI as it is
commonly referred to.
PRESSURE ON A CONFINED FLUID
Pressure is exerted on a confined fluid (Fig. 32) by
applying a force to some given area in contact with
the fluid. A good example of this is a cylinder filled
with fluid and equipped with a piston that is closely
fitted to the cylinder wall. If a force is applied to the
piston, pressure will be developed in the fluid. Of
course, no pressure will be created if the fluid is not
confined. It will simply “leak” past the piston. There
must be a resistance to flow in order to create pres-
sure.
Piston
sealing
is
extremely
important
in
hydraulic operation. Several kinds of seals are used
to accomplish this within a transmission. These
include but are not limited to O–rings, D–rings, lip
seals, sealing rings, or extremely close tolerances
between the piston and the cylinder wall. The force
exerted is downward (gravity), however, the principle
remains the same no matter which direction is taken.
Fig. 29 Accumulators
1 – OVERDRIVE CLUTCH
2 – SECOND BRAKE
3 – DIRECT CLUTCH
4 – OVERDRIVE BRAKE
Fig. 30 Transmission Valve Body Solenoids
Fig. 31 Force and Pressure Relationship
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DESCRIPTION AND OPERATION (Continued)
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