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

Principles of Operation

The Navigator is equipped with an automatic, electronic-shift system that allows the operator to choose (by pressing the mode select buttons) between 3 different modes: 4x2, 4x4 AUTO and 4x4. The operator can switch between 4x2 and 4x4 AUTO mode or 4x4 mode at any speed below 88 km/h (55 mph). The operator can switch between 4x4 AUTO mode and 4x4 mode at any speed.

Wheel slip is sensed by monitoring the average front and rear wheel speeds. Based on steering wheel angle (SWA), Accelerator Pedal Position (APP) and wheel slip, a Pulse-Width Modulation (PWM) signal is transmitted to the transfer case as torque is required. The 4X4 control module will provide the anti-lock brake system (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:

^ Occurrences of brake events and failures.
^ Steering wheel angle status.
^ Wheel speeds.

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

The Accelerator Pedal Position (APP) output signal is provided to the 4X4 control module from the PCM. This signal is used by the 4X4 control module in controlling the transfer case clutch.

The 4X4 control module sends a duty cycle command to the clutch coil which activates the clutch within the transfer case.

The 4X4 control module varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the 4X4 control module activates the transfer case clutch a 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 4WD control module will increase the duty cycle in order to prevent or control slip:

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

Feature inputs:

^ Mode select switch (MSS)
^ Accelerator Pedal Position APP (from the PCM)
^ Steering Wheel Angel SWA (from the ABS module)
^ Transmission Range TR (from the transmission control module [TCM])
^ Transmission output torque (from the PCM)
^ Wheel speed signal (transmitted from the ABS module)


Feature outputs:

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