Chevrolet Silverado / GMC Sierra. Service manual — part 2835
Engine Controls Schematics
Connector End View Reference
Component Connector End Views
Description and Operation
Transfer Case Description and Operation
Electrical Information Reference
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
1. Raise and support the drive wheels.
2. Start and idle the engine.
3. Place the transmission in DRIVE.
4. Monitor VSS on the scan tool under engine data.
5. The VSS on the scan tool should be greater than 100 RPM.
Circuit/System Testing
1. Measure the resistance between the VSS high signal circuit and the VSS low signal circuit.
The resistance should be between 1,420-2,140 ohms.
If the value is greater than 2,140 ohms, test the VSS high signal circuit and the VSS
low signal circuit for an open.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
If the value is less than 1,420 ohms, test the VSS high signal and the VSS low signal
circuits for a shorted together condition.
2. Measure the resistance of the VSS high signal circuit to ground. The resistance should be
greater than 10M ohms.
If the value is less than 10M ohms, repair the short to ground.
3. Measure the resistance between the VSS low signal circuit and ground. The resistance
should be greater than 10M ohms.
If the value is less than 10M ohms, repair the short to ground.
4. Reconnect the ECM connector. Disconnect the VSS connector. Ignition ON, test the ISS
high signal circuit for a short to voltage.
If all circuits/connections test normal, test or replace the VSS.
5. If all previous steps for circuit checking and VSS replacement have been made and all
component testing has been performed and the condition persists replace the ECM.
Component Testing
1. Verify the VSS resistance is within the specified range.
2. Using a terminal test kit, connect the DMM between the VSS high signal and the VSS low
signal circuits at the VSS connector. Raise and support the drive wheels. While rotating the
drive wheels by hand ensure the AC voltage reading on the DMM is above 0.4 volts.
If the AC voltage is not above 0.4 volts, replace the VSS.
Repair Procedures
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Ensure that the VSS is 500 RPM or greater for 3 seconds and does not change more than
500 RPM or greater for 3 seconds.
Vehicle Speed Sensor Replacement
Control Module References for ECM replacement, setup, and programming.
SYMPTOMS - TRANSFER CASE
Visual/Physical Inspection
IMPORTANT: Review the system operation in order to familiarize yourself with
the system functions. Refer to Transfer Case Description and
Operation.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
Inspect for aftermarket devices which could affect the operation of the manual selectable 4-
wheel drive system. Refer to Checking Aftermarket Accessories .
Inspect the easily accessible or visible system components for obvious damage or conditions
which could cause the symptom.
Ensure all mechanical components are operative before diagnosing the electrical portion of
the system.
In order to determine whether the vehicle is in 2-wheel drive (2WD) or 4-wheel drive
(4WD), lift the vehicle so the wheels can spin freely. With the transfer case control lever in
2HI, turn the rear propeller shaft. If the front propeller shaft does not move with the rear,
the vehicle is in 2HI. If the tires and the front propeller shaft moves, the transfer case is in
4WD.
Ensure the 4WD fuse is not open. If the fuse is open, locate and repair the source of the
overload. Refer to Testing for Short to Ground . Replace the fuse.
Intermittent
Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to
Testing for Intermittent Conditions and Poor Connections .
Symptom List
Refer to a symptom diagnostic procedure from the following list in order to diagnose the
symptom:
Four-Wheel Drive Does Not Disengage
Four-Wheel Drive Does Not Engage
Four-Wheel Drive Indicator Always On
Four-Wheel Drive Indicator Inoperative
Transfer Case Popping Noise
Transfer Case Whine or Rumble Noise
Transfer Case Growl or Grinding Noise
Transfer Case Clunk in 2HI Only
Transfer Case Clunk in 4HI
Transfer Case Clunk in 4LO
Transfer Case Will Not Shift
Transfer Case Jumps Out of Gear
Transfer Case Leak Diagnosis
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
FOUR-WHEEL DRIVE DOES NOT DISENGAGE
Diagnostic Instructions
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
Review Strategy Based Diagnosis for an overview of the diagnostic approach.
Diagnostic Procedure Instructions provides an overview of each diagnostic category.
Circuit/System Description
The front axle control circuit consists of an electric motor actuator which engages and disengages
the front axle. The front axle actuator consists of the following components within the actuator
assembly:
A permanent magnetic motor
A worm gear controlled plunger
A front axle switch
An electronic control circuit
The front axle actuator consists of the following circuits:
An ignition 3 voltage circuit
A ground circuit
An axle actuator control circuit
An axle switch signal circuit
Whenever a shift to 4WD is selected, the transfer case switch engages the front axle by grounding
the axle actuator control circuit.
Reference Information
Schematic Reference
Transfer Case Control Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
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