Late Build
Principles of Operation - A4WD - Late Build
The automatic four-wheel drive (A4WD) system is an electronic-shift 4x4 system that allows the operator to choose between three different 4x4 modes. The operator can switch between 4x4 AUTO and 4x4 HIGH modes at any speed.
To engage or disengage 4x4 LOW range, the vehicle speed must be less than 5 km/h, the brake pedal must be depressed, and the transmission must be in NEUTRAL.
For Non-IVD vehicles, the front and rear shaft speed sensors (Hall-effect) are mounted to the transfer case and detect the speed of the front and rear transfer case driveshafts. This input is used by the four-wheel drive (4WD) control module to control 4WD operation.
For vehicles equipped with IVD, wheel slip is sensed using two axle shaft speed sensors, one located on the front axle and one located on the rear axle. Based on throttle position and wheel slip, a pulse width modulated (PWM) signal is transmitted to the transfer case clutch when a predetermined slip-threshold is exceeded.
The 4WD control module will provide the IVD brake system with current clutch duty cycle and whether or not IVD 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
^ Average rear wheel speed
^ Average front wheel speed
^ Delta front wheel speeds
A concern with any of the above listed brake system signals will affect operation of the 4WD system.
The throttle position output signal is provided to the 4WD control module from the powertrain control module (PCM).
This signal is used by the 4WD control module in controlling the 4WD clutch.
The shift motor sense plate, an integral part of the gearmotor encoder assembly, informs the 4WD control module of the transfer case position.
The digital transmission range (TR) sensor is located on the LH side of the transmission. This sensor informs the4WD control module when the transmission is in NEUTRAL.
The electric shift motor is mounted externally to the gearmotor encoder assembly at the rear of the transfer case. It drives a rotary cam which moves the mode fork and range fork within the transfer case between the HIGH range (4x4 AUTO, 4x4 HIGH) and 4x4 LOW range positions.
The 4x4 shift motor is controlled by the 4WD control module which shifts the transfer case shift motor between HIGH range (4x4 AUTO, 4x4 HIGH), and 4x4 LOW modes.
The solid-state clutch is duty-cycled by the 4WD control module to activate the 4WD clutch within the transfer case.
In the A4WD system, the 4WD control module varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the 4WD 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)
Feature inputs:
^ Brake ON/OFF switch
^ Mode select switch (MSS)
^ Digital TR sensor (automatic transmission only)
^ Clutch pedal position (CPP) switch (manual transmission vehicles only)
^ Vehicle speed signal (transmitted from the ABS module)
^ Front and rear driveshaft speed sensors
^ Throttle position information (transmitted from the PCM)
^ Gearmotor encoder contact plate position inputs A, B, C, D
Feature outputs:
^ Solid state clutch (pulse width modulated signal)
^ 4x4 LOW indicator
^ 4x4 shift motor outputs
Shifts between 4x4 AUTO and 4x4 HIGH can be made at any speed. Listed below are the inputs and outputs needed by the 4WD control module to execute a change between any of these modes.
Feature inputs:
^ Front and rear driveshaft speed sensors
^ Vehicle speed signal
^ Throttle position information
^ MSS
Feature outputs:
^ Solid state clutch (pulse width modulated signal)
^ 4x4 HIGH switch indicator
When shifting into or out of LOW range, the 4WD control module requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature inputs:
^ Throttle position information (transmitted from PCM)
^ MSS
^ Gearmotor encoder contact plate position inputs A, B, C, D
^ Vehicle speed signal (transmitted from ABS module)
^ Brake ON/OFF (BOO) switch input (battery voltage when brake pedal is depressed, open circuit when not activated)
^ Digital TR sensor (ground when transmission is in NEUTRAL, open circuit otherwise)
^ Clutch pedal position (CPP) switch depressed input (manual transmission only)
Feature outputs:
^ 4x4 shift motor outputs
^ 4x4 LOW switch indicator
^ Solid state clutch (pulse width modulated signal)