Transfer Case Description and Operation

Fig 1: Transfer Case

Courtesy of GENERAL MOTORS COMPANY

The Borg Warner (BW) model 4474, transfer case is a 1-speed active design.

The BW 4474 transfer case allows vehicle speed dependent flexible clutch preload torque levels for enhanced system performance. An on-demand torque biasing friction clutch is tuned through software calibrations. When a driver enters a turn, clutch torque is decreased based on the various CAN inputs, such as steering angle, to prevent crowhop or driveline binding from occurring at low speeds. If the driver were to accelerate hard enough in a turn to induce rear wheel slip the transfer case would immediately correct it and then reduce torque to prevent crowhop or binding.

The system is fully automatic, requires no driver intervention, and has its own control module integrated with the vehicle chassis control system via the vehicle's CAN bus. The torque split between the front and rear axle is not fixed as in an open center differential transfer case meaning the active transfer case has the ability to transfer up to 100% of available torque to the front axle as needed.

The case halves are die-cast aluminum. Ball bearings support the input shaft and front output gear. Tapered bearings support the idler gear. Power from the input shaft is transferred to the front output shaft via a helical gear set. A gerotor type oil pump is driven by a shaft located into the end of the idler gear assembly. Oil is pulled from the sump through the filter and screen, pumped through the oil distribution hub, and into the input shaft oil gallery for the clutch and bearing lubrication.

Fig 2: Clutch Housing/Pack

Courtesy of GENERAL MOTORS COMPANY

Torque is transferred to the front axle through an active wet clutch within the transfer case. The clutch pack is engaged by an electromagnetic clutch coil and ball ramp assembly. By varying the amount of current, torque output to the front axle can be adjusted.

Whenever the coil (1) is energized as determined by vehicle operating conditions and commanded by TCCM, the armature (7) in the clutch housing is magnetically drawn toward the rotor (2). Torque is generated through friction between the armature and rotor. Friction torque is multiplied through the ball (3) cam (4) mechanism and applied to the clutch pack (8). The clutch pack assembly contains a predetermined selectable pressure plate, friction, and reaction plates that are to be serviced as a clutch pack assembly. Clutch activation is controlled by a pulse width modulated voltage signal. If the clutch pack assembly is replaced, the TCCM must be recalibrated using the service calibration. Refer to Transfer Case Information Label .

External Components 

Fig 3: Transfer Case (1 Of 3)

Courtesy of GENERAL MOTORS COMPANY

Fig 4: Transfer Case (2 Of 3)

Courtesy of GENERAL MOTORS COMPANY

Fig 5: Transfer Case (3 Of 3)

Courtesy of GENERAL MOTORS COMPANY

1. Front Output Flange
2. Vent
3. Input Shaft
4. Identification Label
5. Rear Output Flange
6. Oil Distribution Tube
7. Oil Drain Plug
8. Fluid Temperature Sensor
9. Two/Four Wheel Drive Inline Wiring Connector
10. Vibration Dampener - Model Dependent
11. Oil Fill Plug

Internal Components 

Fig 6: Cutaway View Of Transfer Case

Courtesy of GENERAL MOTORS COMPANY

1. Front Output Flange
2. Front Output Shaft Gear
3. Oil Pump
4. Two/Four Wheel Drive Clutch Coil
5. Rear Output Flange
6. Front Drive Axle Clutch Assembly
7. Input Gear
8. Input Shaft
9. Idler Gear