Principles Of Operation

Four Wheel Drive (4WD) Systems - 2-Speed Torque-On-Demand

Principles of Operation

The vehicle is equipped with an automatic electronic-shift system that allows the operator to choose between 3 different Four-Wheel Drive (4WD) modes. The operator can switch between 4X4 AUTO and 4X4 HIGH at any speed. To engage or disengage 4X4 LOW modes, the vehicle speed must be less than 3 km/h (2 mph) and the transmission must be in NEUTRAL.

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 4X4 control module 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:

- 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 will affect operation of the 4WD system.

The 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 transfer case shift motor sense plate, an integral part of the assembly, informs the 4X4 control module of the transfer case position.

The electric shift motor is mounted externally to the transfer case. It drives a rotary cam which moves the range fork within the transfer case between the HIGH range (4X4 AUTO and 4X4 HIGH modes) and low range (4X4 LOW) positions.

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 4X4 control module will increase the duty cycle in order to prevent or control slip.

- Slip is detected
- Transfer case shift motor contact plate position inputs A, B, C, D

Feature inputs:

- Mode Select Switch (MSS)
- Steering wheel angle
- Transmission range indicator (transmitted from the Transmission Control Module (TCM))
- Wheel speed (transmitted from the ABS module)
- APP information (transmitted from the PCM)

Feature outputs:

- PWM signal to transfer case clutch
- 4WD message center indicators
- Transfer case shift motor outputs

Shifts between 4X4 AUTO mode and 4X4 HIGH modes can be made at any speed. Listed below are the inputs and outputs needed by the 4X4 control module to execute a change between any of these modes.

Feature inputs:

- APP information (transmitted from the PCM)
- MSS
- Transfer case shift motor encoder contact plate position inputs A, B, C, D
- Wheel speed (transmitted from ABS module)

Feature outputs:

- Transfer case shift motor outputs
- 4WD message center indicators
- PWM signal to transfer case clutch

When shifting into or out of 4X4 LOW mode, the 4X4 control module requires that the vehicle speed is less than 3 km/h (2 mph) and the transmission is in NEUTRAL.

Feature inputs:

- APP information (transmitted from the PCM)
- MSS
- Transfer case shift motor encoder contact plate position inputs A, B, C, D
- Wheel speed signal (transmitted from ABS module)

Feature outputs:

- Transfer case shift motor outputs
- 4WD message center indicators
- PWM signal to transfer case clutch

Neutral Tow

The neutral tow feature is a program embedded in the 4X4 control module, which must be activated by a dealer technician. The neutral tow feature disengages the wheels from the powertrain by shifting the transfer case to the NEUTRAL position. This allows the vehicle to be towed with all 4 wheels on the ground. To activate the neutral tow feature, the dealer technician must access the 4X4 control module through the scan tool and data link connector. Once communication is achieved, the technician can request the neutral tow feature be enabled. The technician must also install the neutral tow indicator lamp (and neutral position sensor for vehicles equipped with 4.6L), in order for the feature to be activated.