Chevrolet Silverado / GMC Sierra. Service manual — part 2956

Fig. 64: Transfer Case Two/Four Wheel Drive Actuator Assembly
Courtesy of GENERAL MOTORS CORP.

The transfer case two/four wheel drive actuator assembly consists of a permanent magnet DC
motor and gear reduction assembly. It is located on the left hand side, driver's side, of the transfer
case. When activated, it turns the shift detent lever shaft of the transfer case, clockwise or
counterclockwise, to shift the transfer case. The motor/encoder is controlled with a PWM signal
by the transfer case shift control module. This circuit consists of a driver on both the Motor A and
Motor B circuits. The encoder motor is bi-directional in order to allow the motor to shift the
transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.

Transfer Case Actuator Shaft Position Sensor Assembly

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

Fig. 65: Transfer Case Actuator Shaft Position Sensor Assembly
Courtesy of GENERAL MOTORS CORP.

This sensor works in conjunction with the incremental sensor and shows transfer case position in
degrees.

Transfer Case Motor Lock

The transfer case motor lock is used to provide a 2HI, 4HI, and 4LO lock-up feature. When the
lock circuit is energized, the transfer case encoder motor is allowed to turn. When the transfer
case is placed 2HI, 4HI, or 4LO the motor lock circuit is de-energized and the lock is applied.
This assures that the transfer case remains in the current gear position until a new gear position is

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

requested. When AUTO 4WD is selected, the motor lock remains applied until an adaptive mode,
torque being applied to the front propshaft, is required. During an adaptive mode the motor lock
circuit is energized and the motor lock is released, enabling the encoder motor to turn and apply
or release torque at the front propshaft.

Transfer Case Shift Control Module

Fig. 66: Transfer Case Shift Control Module
Courtesy of GENERAL MOTORS CORP.

The transfer case shift control module uses the vehicle identification number (VIN) information
for calculations that are required for the different calibrations used based on axle ratio,
transmission, tire size, and engine. The system does not know which calibration to use without
this information. When the vehicle is in the AUTO 4WD mode, the transfer case shift control
module monitors the speed of the front and rear propshaft, in order to detect wheel slippage.
When wheel slippage is detected, the module applies a clutch pack contained inside the transfer

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

case. This clutch pack is used to lock-in and apply the front propshaft, transferring torque to the
front wheels. The clutch pack is applied by a motor/encoder assembly. When slip is no longer
detected by the transfer case shift control module, the clutch is no longer applied.

Vehicle Speed Sensor (VSS)

There is a VSS mounted to the transfer case on the rear output shaft. The VSS is a PM generator.
The PM generator produces a AC voltage. The AC voltage and frequency increases as speed
increases. The VSS is an input to the powertrain control module (PCM). The PCM sends this
information to the transfer case shift control module via the GM LAN serial data bus.

SPECIAL TOOLS AND EQUIPMENT

SPECIAL TOOLS

Illustration

Tool Number/Description

DT-48021

Seal Installer

DT-48022

Output Bushing Installer

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado