Chevrolet Silverado / GMC Sierra. Manual — part 2924
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.
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
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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
Inspect for aftermarket devices which could affect the operation of the automatic transfer
case system. Refer to Checking Aftermarket Accessories .
Inspect the easily accessible or visible system components for obvious damage or conditions
which could cause the symptom.
Inspect the automatic transfer case for the proper fluid level.
Intermittent
Faulty electrical connections or wiring may cause 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:
Front Axle Will Not Engage
Front Axle Will Not Disengage
IMPORTANT:
Perform the Diagnostic System Check - Vehicle before using
the symptom tables in order to verify the following
statements are true:
There are no diagnostic trouble codes (DTCs) set.
The control modules can communicate via the serial
data link.
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 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
Transfer Case Shift Control Switch Inoperative
No Correction for Slip in All-Wheel Drive Mode
Transfer Case Shift Control Switch Indicator Always On - Two or More
Transfer Case Shift Control Switch Indicator Inoperative - One or More
Service Indicator Always On
Transfer Case High/Low Clutch Reset
Transfer Case Popping Noise
Transfer Case Whine or Rumble Noise
Transfer Case Growl or Grinding Noise
Transfer Case Grating Noise
Transfer Case Clunk in 2HI Only
Transfer Case Clunk in 4HI
Transfer Case Clunk in 4LO
Transfer Case Clunk during Acceleration and Deceleration
Transfer Case Shudder or Binding
Transfer Case Will Not Shift
Transfer Case Jumps Out of Gear
Transfer Case Leak Diagnosis
FRONT AXLE WILL NOT ENGAGE
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 motor consists of a permanent magnetic motor, a worm
gear controlled plunger, a front axle switch and an electronic control circuit within the actuator
assembly.
The front axle actuator consists of the following circuits:
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
The front axle control circuit, which is also connected to the transfer case shift control
module
The front axle switch circuit, which is also connected to the transfer case shift control
module
An ignition 3 voltage circuit
A ground circuit
When a shift to AUTO 4WD, 4HI, or 4LO is requested, the transfer case shift control module
engages the front axle by grounding the front axle control circuit through a current limiting
driver.
Reference Information
Schematic Reference
Transfer Case Control Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
Scan Tool Reference
Scan Tool Data List
Scan Tool Data Definitions
Circuit/System Testing
1. If this is occurring without DTC C0379 setting it is likely related to a mechanical issue.
Refer to Four-Wheel Drive Does Not Engage .
2. If no mechanical issue is found replace the front axle actuator.
Repair Procedures
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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