Chevrolet Silverado / GMC Sierra. Manual — part 2925
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Control Module References for TCSCM replacement, setup, and programming
Transfer Case High/Low Clutch Reset
Four-Wheel Drive Does Not Engage
Four-Wheel Drive Does Not Disengage
Front Drive Axle Actuator Replacement
FRONT AXLE WILL 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 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:
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
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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 Disengage .
2. If no mechanical issue is found replace the front axle actuator.
Repair Procedures
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Control Module References for TCSCM replacement, setup, and programming
Transfer Case High/Low Clutch Reset
Four-Wheel Drive Does Not Engage
Four-Wheel Drive Does Not Disengage
Front Drive Axle Actuator Replacement
TRANSFER CASE SHIFT CONTROL SWITCH INOPERATIVE
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.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
Diagnostic Procedure Instructions provides an overview of each diagnostic category.
Circuit/System Description
The active transfer case mode switch circuit consists of 5 modes. The transfer case shift control
module supplies 5 volts to the switch through the 5-volt reference circuit. The current travel
through the resistor of the currently active mode. The current is then returned to the transfer case
shift control module through the switch signal circuit.
The transfer case shift control module constantly monitors this signal voltage to determine the
condition of the mode switch circuit.
When each of the modes are selected they complete a circuit through their own specific resistor.
The transfer case shift control module continuously monitors the switch input to determine
whether the Neutral, AWD, 2HI, 4HI, or 4LO mode was selected by the 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
Diagnostic Aids
If the 5-volt reference or the switch signal wire are shorted to ground the scan tool can show up
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
to 7.5 volts.
Circuit/System Testing
1. Review the scan tool data list mode switch selected and select difference mode with the
transfer case shift control switch. Does the mode switch selected parameter on the scan tool
change as you change modes on the switch?
2. If the modes change according to the scan tool replace the transfer case control module,
reprogram, complete the motor learn procedure. If the modes on the scan tool not change
replace the switch.
Repair Procedures
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Transfer Case Shift Control Switch Replacement (Without SLT) or Transfer Case
Shift Control Switch Replacement (With SLT)
Control Module References for TCSCM replacement, setup, and programming
Transfer Case High/Low Clutch Reset
NO CORRECTION FOR SLIP IN ALL-WHEEL DRIVE MODE
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
When transfer case is in auto 4WD mode the transfer case control module monitors the front
wheel speeds and the rear wheel speeds. When a noticeable difference between the front and rear
is seen the transfer case control module sends a PWM voltage to the motor so it can apply the
clutch in order to correct a slip condition. The slip conditions is the difference in speed between
the front and rear axles. The module has a thermal model based system so when the slip
correction can not be made when a large percentage PWM signal has been sent, it will disable the
system to prevent damage to the clutches.
Reference Information
Schematic Reference
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 3023/3024-NQH - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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