Chevrolet Silverado / GMC Sierra. Service manual — part 2905

Fig. 58: 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 PM DC motor and gear
reduction assembly. It is located on the left side, drivers side, of the transfer case. When
activated, it turns the sector shaft of the transfer case clockwise or counter clockwise to shift the
transfer case. The encoder motor is controlled by the transfer case shift control module. This
circuit consists of a driver on both the motor control A and motor control B circuits. The encoder
motor is bi-directional, to allow the motor to shift the transfer case from 2HI or 4HI to
NEUTRAL and 4LO positions. The transfer case encoder motor can be fully driven clockwise or
counterclockwise using a scan tool. You may also monitor motor control A and B circuits using a

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

scan tool.

The two/four wheel drive actuator assembly also has a built in incremental sensor. This device is
a linear encoder. The transfer case shift control module sends a 5 volt reference signal to the
encoder. The transfer case shift control module reads the voltage back based on the exact position
of the sector shaft of the transfer case. The transfer case shift control module knows if a shift has
been completed and if the shift is somewhere in between.

The encoder motor on the MP 1625/1626 does not have an internal brake like the NVG 246. The
encoder motor is not serviceable and must be replaced as an assembly.

Transfer Case Shift Control Module

Fig. 59: Transfer Case Shift Control Module
Courtesy of GENERAL MOTORS CORP.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

The transfer case shift control module has been standardized for all K trucks. The transfer case
shift control module contains the same hardware, boot software, and operational software found
in other GM K trucks. The only difference is the calibration file downloaded into the transfer
case shift control module memory. The transfer case shift control module comes with the
appropriate calibrations out of the factory, but new calibrations must be flashed into the transfer
case shift control module during service. This is especially important if a new transfer case shift
control module is installed during service. Calibration files may be introduced irregularly. See
your technical service bulletins.

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 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

DT-48209

Bushing Remover

DT-48211

Drive Sprocket Bearing Installer

DT-48212

Input Shaft Pilot Bearing Installer

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado