Chevrolet Silverado / GMC Sierra. Manual — part 2836
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
Circuit Testing
Connector Repairs
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
Circuit/System Verification
1. Place the vehicle in 4WD install a scan tool and using engine data 2 look at the display for
4WD signal.
2. Verify that the scan tool indicates that the vehicle is in 4WD.
If the scan tool indicates that the vehicle is in 2WD go to circuit system testing.
Circuit/System Testing
1. Check the axle switch signal circuit for an open or high resistance.
If everything checks normal then go to the next step.
2. Check the front axle actuator control circuit for an open or high resistance with the front
axle actuator disconnected and the mode selection switch disconnected.
If everything checks normal reconnect the mode selection switch and retest. If the
circuit still shows an open or high resistance then replace the switch. If the circuit
indicates no open or high resistance then refer to front axle actuator and axle testing.
Front Axle Actuator and Axle Testing
Remove the actuator and attempt shift from 2WD to 4WD, if the actuator moves check the front
axle actuator plunger and fork assembly.
If the actuator does not function properly then replace the front axle actuator.
Repair Procedures
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Front Drive Axle Actuator Replacement
Transfer Case Gear Indicator Switch Replacement
Front Drive Axle Clutch Fork Inspection
FOUR-WHEEL DRIVE INDICATOR ALWAYS ON
Diagnostic Instructions
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
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 4WD indicator lamp is located in the instrument cluster. Ignition 3 voltage is supplied to the
instrument panel cluster (IPC) when the ignition switch is in the START or RUN position. When
the vehicle is in START or RUN and the Engine control module (ECM) receives the axle switch
signal from the front axle actuator, the ECM sends a CAN signal to the instrument cluster turning
on the 4WD indicator. The 4WD indicator will stay on as long as the axle switch signal input is
present.
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
Circuit/System Verification
1. Place the vehicle in 2WD install a scan tool and using engine data 2 look at the display for
2WD signal.
2. Verify that the scan tool indicates that the vehicle is in 2WD.
If the scan tool indicates that the vehicle is in 4WD go to circuit system testing.
Circuit/System Testing
1. Check the axle switch signal circuit for a short to voltage.
If everything checks normal then go to the next step.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
2. Check the front axle actuator control circuit pin B at the front axle actuator connector or pin
C at the transfer case shift control switch for a short to ground.
If everything checks normal then short the axle actuator control circuit to ground this
should cause the actuator to engage. Remove the short to ground and then short the
circuit to ground again, this should cause the actuator to disengage. If the actuator does
not operate properly then refer to front axle will not engage or front axle will not
disengage. If everything checks normal go to ECM and IPC functionality check.
Instrument Panel Cluster (IPC) and Engine Control Module (ECM) Functionality
If all circuits referenced above check good use the scan tool under engine data 2 to see if the scan
tool still indicates the vehicle is still in 4WD.
If the scan tool indicates the vehicle is in 4WD then replace the engine control module, if
not then replace the IPC.
Repair Procedures
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Instrument Cluster Replacement (without RPO SLT)
Engine Control Module Replacement for the 4.3L engine
Engine Control Module Replacement for the 4.8L, 5.3L, 6.0L, and 6.2L engine
Engine Control Module Replacement for the 6.6L engine
Control Module References for ECM replacement, setup, and programming
Four-Wheel Drive Does Not Disengage
Four-Wheel Drive Does Not Engage
FOUR-WHEEL DRIVE INDICATOR 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
Diagnostic Procedure Instructions provides an overview of each diagnostic category
Circuit/System Description
The 4WD indicator lamp is located in the instrument cluster. Ignition 3 voltage is supplied to the
instrument panel cluster (IPC) when the ignition switch is in the START or RUN position. When
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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