Scheme 3
Principles of Operation
The constantly activated, automatic, all-wheel drive transfer case has no external controls. A two-piece aluminum case houses the assembly. The unit is chain-driven. A non-repairable viscous coupling provides torque distribution to the front and rear transfer case outputs. The viscous coupling contains lotted alternating plates through which a high viscosity transmission fluid flows. The resistance of he fluid shear causes the plates to transmit at the approximate ratio of 35 percent to the front output and 65 percent to the rear output.
Torque flows through the input shaft to the front planetary gear assembly outward to the ring gear to the upper output shaft. Torque also flows through the front planetary gear assembly to the overdrive sun gear outward to the upper drive sprocket. The torque flow continues from the upper drive sprocket through the drive chain to the lower drive sprocket to the front output shaft. The viscous coupling provides the connection between the ring gear and the overdrive sun gear.
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.
Normal Operation
With the key in the ON position, the four-wheel drive (4WD) control module receives the operator's choice of four-wheel drive selection from the mode select switch (MSS). The 4WD control module then controls the electric clutch and shift motor. If the transfer case is not responding to the operator's intentions, systematically check all inputs and outputs to the 4WD control module. Check all circuits for opens and shorts to ground.
The 4WD control module communicates with the diagnostic tool through the universal asynchronous receiver-transmitter (UART)-based protocol (UBP) communications network circuit 693 (OG). If the 4WD control module loses power it will not be able to communicate. Check power circuit 972 (BK/WH) and ground circuit 1203 (BK/LB) for open. If circuits prove out, refer to Section MODULE COMMUNICATIONS NETWORK for further diagnostics.
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 transmitted through the sun gear from the input shaft turns the front planetary gear set assembly. The front planetary gear set assembly, now engaged, provides transfer case speed reduction. Certain criteria, such as vehicle speed and transmission range selection, must be met before this shift can occur. Systematically check all inputs and related circuits. If the inputs prove out, check the transfer case shift motor and clutch along with the associated circuitry.