Transfer Case: Description and Operation

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 pack assembly contains a, predetermined selectable pressure plate, friction 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. If the clutch pack assembly is replaced, a 12.7 k ohm clutch coil resistor should be used in place of the pre-existing resistor. If the resistor is replaced due to loss or damage without replacing the clutch pack, replace with one of the same resistance and refer to Transfer Case Clutch Coil Resistor Replacement Transfer Case Clutch Coil Resistor Replacement.

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 Adhesives, Fluids, Lubricants, and Sealers Fluid Type Specifications.

The BW 4477 for the GMX226, 2 door coupe, application is designed with a second or additional coast side chain snubber within the assembly.

Borg Warner 4477 Build Variations

The Borg Warner 4477 transfer cases are available in many variations, depending on the vehicle, model year, engine, and transmission configurations. The variations allow the transfer case to handle different torque loads and other requirements. When servicing the transfer case, it is important to understand which variation is being serviced because of the different internal components.









First design transfer case assemblies (1) have input shaft splines the full length of the shaft. Second design assemblies (2) have a machined inside diameter (ID) surface at the opening of the shaft.





First design AWD 5/6 speed transmissions, RPOs MV3/MX5/MYA and MYB, can be identified as not having an o-ring and o-ring ring groove at the inner area of the splines. Second design output shafts can be identified by the o-ring (3) located in the groove of the shaft.