Principles of Operation

Four Wheel Drive (4WD) Systems - 1-Speed Torque-On-Demand

Principles of Operation

The Four-Wheel Drive (4WD) has 3 different modes: 2H, AWD and 4X4 High. The operator can switch between 2H, AWD mode or 4X4 High at any speed.

Wheel slip is sensed by monitoring the average front and rear wheel speeds. Based on steering wheel angle, Accelerator Pedal Position (APP) and wheel slip, a Pulse-Width Modulation (PWM) signal is transmitted to the transfer case as torque is required. The Transfer Case Control Module (TCCM) will provide the ABS with current clutch duty cycle and whether or not ABS may command the clutch duty cycle.

The brake subsystem sends the following information signals to the 4WD system over the High Speed Controller Area Network (HS-CAN):

- Occurrences of brake events and failures. Refer to Antilock Brakes / Traction Control Systems.
- Steering wheel angle status. Refer to Antilock Brakes / Traction Control Systems.
- Wheel speeds. Refer to Antilock Brakes / Traction Control Systems.

A concern with any of the above listed brake system signals or the HS-CAN will affect operation of the 4WD system.

The APP output signal is provided to the TCCM from the PCM. This signal is used by the TCCM in controlling the transfer case clutch.

The TCCM sends a duty cycle command to the clutch coil which activates the clutch within the transfer case.

The TCCM varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the TCCM activates the transfer case clutch at minimum duty cycle (percentage of time the clutch is turned on), which allows for the slight difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. Under any of the following conditions, the TCCM will increase the duty cycle in order to prevent or control slip:

- Slip is detected
- Heavy acceleration (throttle position)
- Straight-ahead steering wheel angle

Feature inputs:

- Mode Select Switch (MSS)
- APP (from the PCM)
- Steering wheel angle (from the ABS module)
- Transmission Range (TR) position (from the Transmission Control Module (TCM))
- Transmission output torque (from the PCM)
- Wheel speed signal (from the ABS module)

Feature outputs:

- Solid state PWM signal
- Message center 4WD mode indicators