Principles of Operation
Four-Wheel Drive Systems Electronic ShiftRefer to Vehicle/Diagrams.
Principles of Operation
Transfer Case
The four-wheel drive, electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4WD, 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, and one for 4WD LOW in 4WD LOW both the lamps are on). Shifts into 4WD can be made at any speed. When shifting into 4WD with the vehicle stationary, tooth blockage may occur, preventing shift completion. When the vehicle is in motion, the shift will complete. When shifting in or out of 4WD LOW, the Four-Wheel Drive (4WD) control module or GEM requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal be applied, and the transmission in NEUTRAL. The digital transmission range TR sensor informs the 4WD control module when the automatic transmission is in the NEUTRAL range position.
The gearmotor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and the range fork within the transfer case between the 4WD, 4WD LOW, and 2WD range positions.
The 4WD control module uses two relays to control the gearmotor encoder assembly shift between 4WD, 4WD LOW, and 2WD modes. The 4WD control module controls the pulse vacuum hublock (PVH) solenoid which supplies engagement and disengagement vacuum to control the hublocks.
The 4WD control module accomplishes shifts by interpreting inputs from:
^ Mode Select Switch (MSS)
^ vehicle speed signal (transmitted from the ABS system)
^ gearmotor encoder plate position
^ brake pedal switch
^ digital TR sensor (automatic transmission)
^ Clutch Pedal Position (CPP) switch (manual transmission)
^ ignition switch
^ NEUTRAL position (transmitted from PCM)
Based on these inputs, the 4WD control module controls the shifts into 2WD, 4WD, or 4WD LOW with the following outputs:
^ low to high relay (clockwise)
^ high to low relay (counterclockwise)
^ pulse vacuum hublock (PVH) solenoid
^ gearmotor encoder assembly
The Electronic Shift-On-The-Fly (ESOF) system has a feature which allows the driver to override the vacuum-operated hublocks. When the front hublocks are manually turned to the LOCK position, the hublocks are locked at all times, overriding the vacuum operated system. If the front hublocks are manually turned to the OPEN position, the hublocks can only be locked by turning the MSS to 4WD or 4WD LOW position.
Electronic Shift Vacuum Schematic
Hublock Operation
The 4WD ESOF system uses timed vacuum sequences to lock and unlock the wheel ends. A high vacuum level is used to engage the hublocks, and a lower vacuum level is used to disengage the hublocks. Vacuum is held for a fixed amount of time to ensure hub engagement, after which the vacuum is released and the hublock holds itself in the proper mode. The vacuum signals are supplied to the hublocks by system components, including the 4WD control module, wiring harness, solenoid, vacuum harness and vacuum seals. As the first step in service, eliminate such obvious items as loose wiring connecting, loose vacuum connections, or damaged vacuum lines.
Disengage Time
"Slow" release of the hublocks is not considered abnormal for this system. Anytime vacuum is applied to the hubs, whether for 4WD or 2WD, the hublocks will initially engage or disengage and remain engaged or disengaged. If 4WD was requested, the hublocks will remain engaged, but if 2WD was selected, the internal mechanism will release only after the 4WD control module timers expire and vacuum is vented from the hub. This normally takes 15 seconds, but can take up to two minutes, depending on how the 4WD mode select switch was operated. After the hub mechanism releases, internal springs must work the hublock gears to the disengaged position. Road bumps, vehicle speed, acceleration cycles, or momentary reversal of direction can assist this process, varying the length of time the hublocks remain engaged in each situation. The key is to remove torque from the hubs so that they may engage or disengage freely.
Manual Override
The hublocks have manual override selector dials, which, when rotated to the "LOCK" position, will keep the mechanism locked regardless of the instrument panel 4WD mode select switch position. Verify that both dials are in "OPEN" before evaluating ESOF operation.
Hublock Replacement
Left and right side hublocks are not connected, other than by the common vacuum supply line. If a malfunction in either hublock is diagnosed, it should be installed as an individual unit; there is no need to "balance" an axle with new hublocks on both sides. If both sides appear to be malfunctioning, be sure to verify upstream integrity before installing new hublocks on both sides.
Many system components are involved in the proper operation of the ESOF hubs. Before diagnosing the hublocks themselves as the cause of 4WD concerns, be sure to verify all related system components.
After removing the hublock retaining ring be sure not to use tools other than hands or "grip" gloves to remove the hublock, as damage may occur to either the paint or the function of the hublock. Pliers or locking Channel-Locks should be considered as a last resort, and will usually damage the hublock, making it necessary to install a new hublock.