Sierra Classic 2500 4WD V8-6.0L (2007)
SERVICE Indicator (4WD)
The SERVICE indicator (4WD) is an integral part of the cluster and cannot be serviced separately. This indicator is used to inform the driver of
malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD) is controlled by the transfer case shift control module
via a Class 2 message or by a service indicator control circuit.
Transfer Case Motor/Encoder
The transfer case encoder motor consists of a permanent magnet (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 scan tool.
The encoder motor also has a built in encoder. This device is a linear encoder. The TCCM sends a 5 volt reference signal to the encoder. The TCCM
reads the voltage back based on the exact position of the sector shaft of the transfer case. The TCCM knows if a shift has been completed and if the
shift is somewhere in between.
The encoder motor on the NVG 263 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
The TCCM has been standardized for all K trucks. The TCCM 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 TCCM memory. The TCCM comes with the appropriate calibrations out
of the factory, but new calibrations must be flashed into the TCCM during service. This is especially important if a new TCCM is installed during
service. Calibration files may be introduced irregularly. See your technical service bulletins.
