The automatic four-wheel drive (A4WD) system is an electronic shift 4x4 system that utilizes the following components:
transfer case
four-wheel drive (4WD) control module
mode select switch (MSS)
gearmotor encoder assembly (part of the electric shift motor)
electric shift motor
shift motor sense plate (part of the gearmotor encoder assembly)
Principles of Operation
Electric Shift Motor
The externally mounted transfer case shift motor, located at the rear of the transfer case, turns the shift cam assembly. As the shift cam assembly turns, it positions the reduction shift fork for desired transfer case operation. An integral position sensor in the shift motor provides shift motor position information to the 4WD control module.
Mode Select Switch (MSS)
The push button MSS, mounted on the instrument panel, allows the operator to select the 4x4 Auto, 4x4 High, and 4x4 Low transfer case operating modes. The indicator light, on the instrument cluster push button, will illuminate only when the 4x4 Low position is engaged.
Brake Pedal Position (BPP) Switch
The BPP switch is located on the brake pedal. The switch signals the 4WD control module when the operator activates the brake pedal.
Digital Transmission Range (TR) Sensor
The digital TR sensor is located on the outside of the transmission near the transmission shift cable attachment. The digital TR sensor completes the start circuit in PARK and NEUTRAL, the back-up lamp circuit in REVERSE and the neutral sense circuit (4WD) in NEUTRAL. The digital TR sensor also sends a digital output signal to the PCM indicating the manual lever position.
Throttle Position (TP) Sensor
The TP sensor is located on the throttle body. The sensor provides throttle position information to the powertrain control module (PCM). The PCM converts the throttle position signal to a pulse width modulated (PWM) signal, and sends it to the 4WD control module to control and adjust the transfer case clutch duty cycle.
Front and Rear Output Shaft Speed (OSS) Sensors
The externally mounted OSS sensors are located at the rear of the transfer case. The sensors provide the 4WD control module with front and rear driveshaft speeds to control and adjust the transfer case clutch duty cycle.
On vehicles equipped with stability assist, the OSS sensors are present, but not used. The front and rear drive shaft speeds are transmitted to the 4WD control module from the stability assist module. The stability assist module receives the necessary signal from the wheel speed sensors.
Vehicle Speed Sensor (VSS)
The Anti-Lock Brake System (ABS) provides the 4WD control module with the vehicle speed information to control and adjust the transfer case clutch duty cycle.
4WD Control Module
The 4WD control module, located behind the glove compartment, controls the operation of the transfer case in response to the inputs from the shift motor, MSS, BPP switch, digital TR sensor, throttle position output from the PCM, front and rear OSS (on non-stability assist vehicles), stability assist module (if equipped) and the VSS.
Mechanical Operation
The Borg-Warner 44-11 transfer case is available in the Explorer (A4WD) vehicle. The transfer case transfers power from the transmission to the rear axle, and also the front axle when electronically activated.
In 4X4 Auto mode, torque from the transmission is transferred to the input shaft. The input shaft output drives the rear output shaft which, in turn, drives the rear axle assembly. The electromechanical ball ramp clutch assembly drives the drive sprocket after the 4WD control module activates the clutch coil. The drive sprocket turns the drive chain and rotates the front output shaft and the front driveshaft. In the 4x4 High mode, the operation is the same as in the 4X4 Auto mode, except that the 4WD control module consistently activates the electromechanical ball ramp clutch assembly.
The high-low shift occurs when the reduction shift fork moves the high-low collar to lock the planetary gear set to the output shaft. The torque then transmitted through the sun gear from the input shaft turns the front planetary gear set assembly. The front planetary gear set assembly, which is now engaged, provides transfer case speed reduction.
Control-Track® Function
The Control-Track® transfer case uses an electromechanical clutch assembly to control the torque to the front wheels. In Auto mode, while at rest and under cruising conditions, the 4WD control module activates the transfer case clutch at a minimum level. This allows for the slight difference between the front and rear driveshafts which occurs normally when negotiating a corner on dry pavement. Under any of the following conditions, the 4WD control module will increase the torque sent through the clutch to the front wheels in order to prevent or control slip.
Slip is detected by monitoring the front and rear OSS sensors in the transfer case or when slip is indicated by the stability assist module.
Range Function
NOTE: At zero throttle, the duty cycle remains low.
In the 4x4 Low range, with throttle input, the transfer case electromechanical clutch looks at the front and rear driveshafts for maximum 4x4 traction. The transfer case shift motor rotates the shift cam to move the reduction shift fork to the 4x4 Low range position. This low range shift, accomplished through a planetary gear set, changes the torque to the drive shaft from 1:1 to 2.48:1 ratio.
In the 4x4 High range, with throttle input, the 4WD control module signals the transfer case electromechanical clutch to lock the front and rear driveshafts for maximum four wheel drive traction. Never use the 4x4 High range or the 4x4 Low range when driving on dry or merely wet pavement. Use 4x4 High range or the 4x4 Low range only for driving off-road or in severe winter weather.
