Here's some background on the 4WD system and how it operates:

The four wheel drive system is an electronic shift 4X4 system that allows the operator to choose between three different modes. The operator can switch 4X4 HIGH modes at any speed. To engage or disengage 4X4 LOW the vehicle speed must be less than 5 kph (3 mph), the brake depressed,
and the transmission must be in NEUTRAL.

The shift motor sense plate, an integral part of the electronic shift motor, informs the generic electronic module (GEM) of the transfer case shift motor and contact plates A, B, C and D position.

The digital transmission range (TR) sensor is located on the LH side of the transmission, and informs the GEM when the transmission is in the NEUTRAL position.

The electronic shift motor is mounted externally at the rear of the transfer case. It drives a rotary cam which moves the mode fork and range fork within the transfer case between the 4X4 HIGH, 4X4 LOW and 2WD range positions.

When the GEM verifies the completion of the transfer case shift from 2WD to 4X4 HIGH (reading the contact plate positions), the GEM will then energize both pulse vacuum hublock (PVH) solenoids (supplying ground for 45 seconds). The PVH solenoid will allow a HIGH flow of intake manifold vacuum to the "wheel end hublocks", engaging the wheel end hubs to the front differential and transfer case to achieve four wheel drive.

When the GEM verifies the completion of the transfer case shift from 4X4 HIGH to 2WD (reading the contact plate positions), the GEM will then energize only one (low vacuum solenoid) of the PVH solenoids (supplying a ground for 10 seconds). The PVH solenoid will allow a "LOW" flow of intake manifold vacuum to the wheel end vacuum hublocks, disengaging the wheel end hubs from the front differential and transfer case to achieve two wheel drive.

The 4X4 shift motor uses a module containing two relays which, under control of the GEM, shifts the transfer case shift motor between 4X4 HIGH, 4X4 LOW and 2WD modes.


*The solid state clutch relay is used to activate the clutch within the transfer case which aids in shifting to 4X4.

Feature inputs:

- 4WD mode switch.
- Vehicle speed signal (transmitted from the ABS module [4WABS only] or GEM [RABS only])
- Output shaft (OSS) speed sensors.
- Contact plate position inputs A, B, C, D.

Feature outputs:

- Solid state clutch relay (pulse width modulated signal: grounded when the relay is on, battery potential when relay is off).


*Shifts into 4X4 HIGH can be made at any speed. Listed below are the inputs and outputs needed by the GEM to execute a change between any of these modes.

Feature inputs:

- 4X4 mode switch.
- Contact plate position inputs A, B, C, D.

Feature Outputs:

- 4X4 shift motor relay outputs.
- Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).
- Cluster indicators (ground when activated, open circuit when deactivated).


*When shifting into or out of LOW range, the GEM requires that the vehicle speed is less than 5 kph (3 mph), the brake is applied, and the transmission is in NEUTRAL.

Feature inputs:

- 4X4 mode switch.
- Contact plate position inputs A, B, C, D.
- Vehicle speed signal (transmitted from the ABS module [4WABS only] or GEM [RABS only]).
- Brake input (battery voltage when brake is depressed, open circuit when not activated).
- Digital TR sensor (ground when transmission is in NEUTRAL, open circuit otherwise).
- Start/clutch depressed input.

Feature outputs:

- 4X4 shift motor relay outputs.
- Cluster indicators (ground when activated, open circuit when deactivated).


Problem: The Vehicle Does Not Shift Between 2WD and 4WD Modes Properly. Possible causes:

- 4WD mode switch.
- Electric shift relay.
- Electronic shift control module.
- GEM.
- Circuitry.
- Transfer case shift motor.
- Transfer case electric clutch.
- Transfer case mechanism.