Transfer Case Description and Operation
The Borg Warner (BW) model 4477, transfer case power take-off unit (PTU) is a 1-speed active transfer case.
An on demand torque biasing friction clutch is tuned through software calibrations. This allows features such as flexible clutch preload torque levels. This allows for vehicle speed dependent clutch torque levels to enhance system performance. For example, the system is calibrated to provide low clutch torque during low speed, low engine torque operation, and predetermined higher torque at greater speeds. This prevents crow-hop and binding at low speeds and provides higher torque biases at higher vehicle speeds, to enhance stability.
The BW 4477 clutch assembly contains an armature, predetermined selectable pressure plate, friction plates and reaction plates; serviced as an assembly. The clutch is controlled by pulse width modulated voltage through a coil, ball and cam apply system. Performance variation is reduced further through a selectable external in-line resistor. Matched to clutch system performance, the resistor fine tunes apply rates and maximum front wheel torque outputs.
The BW 4477 case halves are die-cast aluminum. Ball bearings support the input shaft, and front output shaft. An oil pump, driven by a 1-piece input/output shaft, pumps fluid through the shaft oil gallery for the clutch and bearing lubrication. This transfer case requires specific lubrication. Refer to Sealers, Adhesives, and Lubricants .