Transfer Case: Testing and Inspection

Principles of Operation

Transfer Case
The four-wheel drive electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4WD HIGH, and 4WD LOW. The system mode is selected by the operator through the Mode Select Switch (MSS) on the instrument panel. The operator is informed which mode the system is in by two instrument cluster indicators: one for 4WD HIGH which appears as 4WD, and one for 4WD LOW, which appears as 4WD LOW. Shifts into 4WD HIGH can be made at any speed. When shifting into 4WD HIGH with the vehicle stationary, tooth blockage may occur preventing shift completion. When the vehicle is driven above 8 km/h (5 mph) the shift will complete. When shifting in or out of 4WD LOW, the four-wheel drive (4WD) control module requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal be applied, and the transmission be in NEUTRAL (automatic transmission) or the clutch pedal be depressed (manual transmission).

The gearmotor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and range fork within the transfer case between the 4WD HIGH, 4WD LOW, and 2WD range positions.

The four-wheel drive (4WD) control module controls the gearmotor encoder assembly that shifts between 4WD HIGH, 4WD LOW, and 2WD modes.

System Function
Feature inputs:
^ Brake ON/OFF switch
^ mode select switch (MSS)
^ digital transmission range (TR) sensor
^ vehicle speed signal transmitted from the powertrain control module (PCM)
^ contact plate position inputs A, B. C, D Feature outputs:
^ 4WD LOW indicator (ground when activated, open circuit when deactivated)
^ 4WD indicator (ground when activated, open circuit when deactivated)
^ 4WD shift motor outputs Shifts between 2WD and 4WD HIGH can be made at any speed. Listed are the inputs and outputs needed by the 4WD control module to execute a change between any of these modes.

Feature inputs:
^ vehicle speed signal
^ mode select switch

Feature outputs:
^ 4WD cluster indicator (ground when activated, open circuit when deactivated)

When shifting into or out of LOW range, the 4WD control module requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.

Feature inputs:
^ mode select switch
^ contact plate position inputs A, B. C, D
^ vehicle speed signal
^ brake ON/OFF switch input (battery voltage when brake is depressed, open circuit when not activated)
^ digital TR sensor (ground when transmission is in NEUTRAL, open circuit otherwise)

Feature outputs:
^ 4WD shift motor outputs
^ 4WD LOW cluster indicator (ground when activated, open circuit when deactivated)