Four Wheel Drive (4WD) Systems - Electronic Shift

Four Wheel Drive (4WD) Systems - Electronic Shift

Principles of Operation

The Electronic Shift-On-The-Fly (ESOF) system has 3 modes of operation: 2H (4X2), 4H (4X4 HIGH) and 4L (4X4 LOW). Integrated Wheel Ends (IWEs) are used at each front wheel to reduce rolling resistance when 2H is selected. The Transfer Case Control Module (TCCM) sends a duty cycle command to the synchronization coil which activates the clutch within the transfer case. Refer to Transfer Case.

Engagement and disengagement of the front wheels is done by the IWEs (Integrated Wheel Ends). The IWEs (Integrated Wheel Ends) are normally engaged (no vacuum). Vacuum is supplied to the IWEs (Integrated Wheel Ends) by actuation of the IWE (Integrated Wheel End) solenoid when commanded by the TCCM (Transfer Case Control Module). When actuated, the IWE (Integrated Wheel End) solenoid allows manifold vacuum to pass through to each IWE (Integrated Wheel End). The IWEs (Integrated Wheel Ends) disengage and allow the front wheel hubs to be turned freely from the front axle.

The electric shift motor is mounted externally to the transfer case. It drives a rotary cam which moves the 2 shift forks in the transfer case to change between the 2H and 4H modes and between the 4H and 4L modes. For information on transfer case mechanical operation, refer to Transfer Case. The TCCM (Transfer Case Control Module) directly controls the electric shift motor and can reverse motor polarity to reverse rotary cam/shift fork direction. The shift motor sense plate, an integral part of the shift motor assembly, informs the TCCM (Transfer Case Control Module) of the transfer case position.

The transfer case is equipped with an electronically controlled clutch which is located inside the case. This clutch is used to synchronize the speed of the front driveline with the rear driveline during 2H to 4H shifts. When the Mode Select Switch (MSS) is switched between 2H and 4H modes, the TCCM (Transfer Case Control Module) energizes the synchronization clutch. When the transfer case front and rear output shafts are synchronized, the spring-loaded lockup collar mechanically engages (4H) or disengages (2H) the mainshaft hub to the drive sprocket. Finally, the front axle IWEs (Integrated Wheel Ends) are engaged (4H) or disengaged (2H) and the synchronization clutch is deactivated.

Shifts from 2H mode to 4H mode can be made at any speed. The synchronization clutch is applied to equalize output shaft speeds before 4H mode is engaged. The following are the inputs and outputs needed by the TCCM (Transfer Case Control Module) to execute a change between these modes.

Feature inputs:

- Ambient air temperature from the PCM

- MSS (Mode Select Switch)

- Wheel speed signal from ABS module

- APP (Accelerator Pedal Position) from PCM

- Transfer case shift motor contact plate position inputs A, B, C and D

Feature outputs:

- Transfer case electric shift motor outputs

- PWM (Pulse-Width Modulation) signal to transfer case clutch

- 4WD (Four-Wheel Drive) message center and indicators

When shifting into or out of 4L mode, the TCCM (Transfer Case Control Module) requires the vehicle speed to be less than 5 km/h (3 mph) and the transmission in NEUTRAL. The following are the inputs and outputs needed by the TCCM (Transfer Case Control Module) to execute a change into or out of 4L mode.

Feature inputs:

- MSS (Mode Select Switch)

- Wheel speed signal from ABS module

- TR (Transmission Range) position from PCM

- Transfer case shift motor contact plate position inputs A, B, C and D

Feature outputs:

- Transfer case electric shift motor outputs

- PWM (Pulse-Width Modulation) signal to transfer case clutch

- 4WD (Four-Wheel Drive) message center and indicators