Principles of Operation

Four-Wheel Drive Systems Electronic Shift
Refer to Wiring Diagrams for schematic and connector information. Electrical Diagrams

Principles of Operation

Transfer Case
The 4-wheel drive electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4x4 HIGH and 4x4 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 2 instrument cluster indicators: one for 4x4 HIGH which appears as "4x4", and one for 4x4 LOW, which appears as "4x4 LOW". Shifts into 4x4 HIGH can be made at any speed. When shifting into 4x4 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 into or out of 4x4 LOW, the 4-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 encoder assembly is mounted externally and is integral to the transfer case shift motor. The transfer case shift motor 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.
The 4WD control module accomplishes shifts by interpreting inputs from the:
^ Mode select switch (MSS)
^ Vehicle speed signal (VSS)
^ Encoder plate position
^ Brake ON/OFF (BOO) switch
^ Digital transmission range (TR) sensor
^ Ignition switch Based on these inputs the 4WD control module controls the shifts into 2WD, 4x4 HIGH or 4x4 LOW with these outputs:
^ Transfer case shift motor (clockwise)
^ Transfer case shift motor (counterclockwise)

Neutral Tow
Neutral tow is a dealer installed option that allows the vehicle to be towed with the 4 road wheels without damaging the transmission. Neutral tow operates by disengaging the transmission from the rear differential.
When neutral tow is requested, the 4WD control module checks that the neutral tow feature is enabled, the battery voltage is between 9 and 16 volts, the vehicle is 2WD and the ignition switch is in the OFF position. Once these conditions are met, the 4WD control module checks for the presence of the neutral tow indicator lamp. If the indicator lamp is detected, the 4WD control module will request the transfer case shift motor position the transfer case into NEUTRAL. If any of the previous conditions are not met, the neutral tow feature will be disabled until the next neutral tow request.
Once all of the doors are closed, the neutral tow indicator lamp will stay on until the battery saver relay enables the battery saver function. If a door is opened after the battery saver function is enabled, the lamp will come back on until the transfer case is shifted out of NEUTRAL or until the battery saver feature times out.
To activate the neutral tow system, the operator must:
^ turn the ignition switch to the ON position.
^ place the mode select switch (MSS) in 2WD.
^ apply the brake pedal.
^ place the transmission in NEUTRAL.
^ turn the ignition switch to the OFF position.
^ hold the brake pedal down until the NEUTRAL indicator illuminates (approximately 4 seconds).
^ release the brake pedal.