Jeep Cherokee

 Service Manual

Transmission and transfer case » Automatic transmission-30RH » Description and operation
Servos

DESCRIPTION

KICKDOWN (FRONT) SERVO

The kickdown servo (Fig. 49) consists of a two-land piston with an inner piston, a piston rod and guide, and a return spring. The dual-land piston uses seal rings on its outer diameters and an O-ring for the inner piston.

LOW/REVERSE (REAR) SERVO

The rear (low/reverse) servo consists of a single stage or diameter piston and a spring loaded plug.

The spring is used to cushion the application of the rear (low/reverse) band.

OPERATION

KICKDOWN (FRONT) SERVO

The application of the piston is accomplished by applying pressure between the two lands of the piston.

The pressure acts against the larger lower land to push the piston downward, allowing the piston rod to extend though its guide against the apply lever.

Release of the servo at the 2-3 upshift is accomplished by a combination of spring and line pressure, acting on the bottom of the larger land of the piston.

Fig. 49 Front Servo

1 - VENT
2 - INNER PISTON
3 - PISTON
4 - SPRING
5 - RELEASE PRESSURE
6 - APPLY PRESSURE
7 - PISTON ROD

The small piston is used to cushion the application of the band by bleeding oil through a small orifice in the larger piston. The release timing of the kickdown servo is very important to obtain a smooth but firm shift. The release has to be very quick, just as the front clutch application is taking place. Otherwise, engine runaway or a shift hesitation will occur. To accomplish this, the band retains its holding capacity until the front clutch is applied, giving a small amount of overlap between them.

LOW/REVERSE (REAR) SERVO

While in the de-energized state (no pressure applied), the piston is held up in its bore by the piston spring. The plug is held down in its bore, in the piston, by the plug spring. When pressure is applied to the top of the piston, the plug is forced down in its bore, taking up any clearance. As the piston moves, it causes the plug spring to compress, and the piston moves down over the plug. The piston continues to move down until it hits the shoulder of the plug and fully applies the band. The period of time from the initial application, until the piston is against the shoulder of the plug, represents a reduced shocking of the band that cushions the shift.

30RH automatic transmission

Fluid

Torque converter

Oil pump

Valve body

Accumulator

Pistons

Front clutch

Rear clutch

Overrunning clutch

Planetary gearset

Bands

Servos

Governor

Gearshift mechanism

Converter drainback valve

Brake transmission shift interlock mechanism