Principles of Operation
The Expedition is equipped with an automatic electronic-shift system that allows the operator to choose between two-wheel drive (2WD) and 3 different Four-Wheel Drive (4WD) modes. The operator can switch between 2WD (2H) mode and 4X4 AUTO (4A) mode or 4X4 (4H) mode at any speed below 88 km/h (55 mph). The operator can switch between 4X4 AUTO (4A) mode and 4WD (4H) mode at any speed. To engage or disengage 4X4 LOW (4L) mode, 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 VEHICLE DYNAMIC SYSTEMS article.
- Steering wheel angle status. Refer to VEHICLE DYNAMIC SYSTEMS article.
- Wheel speeds. Refer to VEHICLE DYNAMIC SYSTEMS article.
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 shift motor sense plate, an integral part of the shift motor 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 (2H, 4A and 4H) and LOW range (4L) positions.
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 driveshaft 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
- Heavy acceleration (throttle position)
- Straight-ahead steering wheel angle
Feature inputs:
- Mode Select Switch (MSS)
- 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 (transmitted from the ABS module)
- APP (from the PCM)
- Transfer case shift motor contact plate position inputs A, B, C, D
Feature outputs:
- PWM signal to the transfer case clutch
- 4WD message center indicators
- Transfer case shift motor outputs
Shifts between 4X4 AUTO (4A) and 4X4 (4H) 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:
- Wheel speed signal
- APP (from the PCM)
- MSS
Feature outputs:
- Transfer case shift motor outputs
- PWM signal to the transfer case clutch
- 4WD message center indicators
When shifting into or out of 4X4 LOW (4L) 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 (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 the transfer case clutch