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Chevrolet Silverado / GMC Sierra. Manual - part 2872
Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC C0379 01
Front Axle System Short to Battery
DTC C0379 02
Front Axle System Short to Ground
DTC C0379 04
Front Axle System Open Circuit
DTC C0379 08
Front Axle System Signal Invalid
Diagnostic Fault Information
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
Circuit
Short to
Ground
Open/High
Resistance
Short to
Voltage
Signal
Performance
Front Axle System
DTC C0379
02
DTC C0379
04
DTC C0379
01
DTC C0379
08
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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.
This DTC detects a short to ground, short to voltage or an open circuit.
Conditions for Running the DTC
The ignition is ON.
System voltage is 10.5-18 volts.
Conditions for Setting the DTC
This DTC sets if the front axle actuator switch shows battery voltage when the actuator is
disengaged, or when the front axle actuator shows no voltage when the actuator is engaged.
The module also monitors the axle actuator control circuit and sets the DTC if a high
voltage is expected (disengaged) and a low voltage is seen. It will also set when a low
voltage is expected (engaged) and a high voltage is seen.
Action Taken When the DTC Sets
The front axle may be engaged at all times or disengaged at all time depending on the failure
type.
The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition
cycle.
Conditions for Clearing the DTC
The transfer case shift control module clears the DTC if the condition for setting the DTC is
not currently present.
A history DTC clears after 40 consecutive warm up cycles without a fault present.
History DTCs can be cleared using a scan tool.
Reference Information
Schematic Reference
Transfer Case Control Schematics
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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
When the circuits operating the lock solenoid are not complete the lock is engaged. When the
circuit is completed the lock is disengaged.
Circuit/System Testing
The Ignition 1 voltage circuit and the axle actuator control circuit must be tested for opens, shorts
to ground, shorts to voltage, and high resistance. This can also result from a electrically faulted
actuator.
1. With the ignition on check which DTC is set. If DTC C0379 is set with failure type byte 08
then do the following:
Shift the vehicle into 4 high and look at the scan tool data list to see if the front axle
actuator is engaging. If the axle actuator can be heard engaging but the scan tool still
indicates disengaged then check the axle switch signal circuit for an open or high
resistance. Shift the vehicle to 2 high, if the scan tool indicates the axle actuator is
engaged then check the axle switch signal for a short to voltage. If everything checks
out go to the next step.
2. Remove the front axle actuator and try to shifting from 4WD to 2WD and see if the states
change.
If the states change then inspect the front axle for mechanical conditions. Refer to
Four-Wheel Drive Does Not Engage or Four-Wheel Drive Does Not Disengage .
If this checks out then go to the next step.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
3. Disconnect the connector at the front axle actuator and check for battery voltage on the
ignition 1 circuit. Also check the front axle actuator ground for proper connections.
If these circuit test bad repair as necessary checking inline connectors X110 and X109.
If these circuits test good go to the next step.
4. Check the axle actuator control circuit for an open, short to ground, or short to voltage.
If the actuator control circuit checks good, then short the actuator control circuit to
ground at the transfer case control module connector X1 pin 15. This should cause the
actuator to engage, remove the ground and then reinstate the ground and the actuator
should disengage. If the front axle actuator engages and disengages then replace the
transfer case control module, program, and complete the motor learn procedure. If it
does not fully engage or disengage then 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
DTC C0387
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.
DTC Descriptor
DTC C0387 59
Unable to Complete Shift Circuit/Component Protection Timeout
Diagnostic Fault Information
Short to
Open/High
Short to Circuit/Component
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Transfer Case - MP 1625/1626-NQF - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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