C0379
DTC C0379
Diagnostic Instructions
- Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
- Review Strategy Based Diagnosis for an overview of the diagnostic approach. Initial Inspection and Diagnostic Overview
- Diagnostic Procedure Instructions provides an overview of each diagnostic category. Initial Inspection and Diagnostic Overview
DTC Descriptor
DTC C0379: Front Axle System
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, all within the actuator assembly.
The front axle actuator consists of the following circuits:
- An Ignition 3 voltage circuit
- An axle switch signal circuit
- An axle actuator control circuit
- A ground circuit
When a shift to AUTO 4WD, 4HI, or 4LO is requested, the transfer case shift control module (TCCM) engages the front axle by grounding the front axle control circuit through a current limiting driver.
Conditions for Running the DTC
- Ignition ON.
- System voltage 9-18 V.
Conditions for Setting the DTC
The TCCM monitors the front axle switch signal circuit and the front axle actuator control circuit for unexpected voltages. The DTC sets if the TCCM detects a high voltage when a low voltage is expected, or a low voltage when a high voltage is expected.
Action Taken When the DTC Sets
- The front axle remains in its current state at all times.
- The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.
Conditions for Clearing the DTC
- The TCCM clears the DTC once the conditions for setting the DTC no longer exist.
- A history DTC clears after 100 consecutive ignition cycle without a fault present.
- History DTCs can be cleared using a scan tool.
Diagnostic Aids
- A short to ground on the Ignition 3 circuit will open the 4WD fuse, rear underseat fuse block fuse 48, and may set B0790.
- The engine oil drain plug provides a good ground point.
Reference Information
Schematic Reference
Transfer Case Control Schematics NVG 226-NP8 - Transfer Case
Connector End View Reference
Transfer Case Control Connector End Views Diagrams
Description and Operation
Transfer Case Description and Operation NVG 226-NP8 - Transfer Case
Electrical Information Reference
• Circuit Testing Circuit Testing
• Connector Repairs Connector Repairs
• Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs Wiring Repairs
DTC Type Reference
DTC Symptom Description DTC Symptom Description
Scan Tool Reference
Control Module References for scan tool information. Control Module References
Special Tools
J 35616 GM-Approved Terminal Test Kit
Circuit/System Verification
1. Key ON, engine OFF. Parking Brake applied. Transmission in Neutral. Install a scan tool.
2. While monitoring the Front Axle Switch parameter of the Data Display function on the scan tool, shift the vehicle into 4HI. The Front Axle Actuator parameter should display Locked. You should be able to hear the axle actuator engaging after the encoder motor turns.
3. Shift the vehicle into 2HI. The Front Axle Actuator parameter should display Unlocked. You should be able to hear the axle actuator disengaging after the encoder motor turns.
Circuit/System Testing
Important: When test probing harness terminals, it is imperative to use the proper test probe from the most current version of the J 35616 GM-Approved Terminal Test Kit . Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.
1. Verify that the 4WD fuse, rear underseat fuse block, fuse 48, is NOT open.
- If the 4WD fuse is open, test the Ignition 3 voltage circuit, front drive axle actuator harness X1 pin D, for a short to ground.
2. Wheels blocked. Parking brake engaged. Install a scan tool.
3. Key ON, engine OFF. Transmission in Neutral.
4. Disconnect the X1 harness connector at the front drive axle actuator (FDAA).
5. Using the GY probe J 35616-2A, test for B+ (+0.0/-1.0 V) between the Ignition 3 voltage circuit, FDAA X1 harness pin D, and a good ground.
- If the voltage is less than the specified value, test the affected circuit for an open/high resistance or short to ground. If all circuits and connections test normal, test the TCCM for proper operation.
6. Using the GY probe J 35616-2A, test for B+ (+0.0/-1.0 V) between the Ignition 3 voltage circuit, FDAA X1 harness pin D, and the FDAA ground circuit, FDAA X1 harness pin B.
- If the voltage is less than the specified value, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
7. Using the GY probes J 35616-2A, test for a diagnostic B+ (+0.0/-1.0 V) between the Ignition 3 voltage circuit, FDAA X1 harness pin D and the front axle switch signal circuit, FDAA X1 harness pin C.
- If the voltage is less than the specified value, test the front axle switch signal circuit for an open/high resistance, short to ground or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
8. Using the GY probes J 35616-2A, test for a diagnostic B+ (+0.0/-1.0 V) between the Ignition 3 voltage circuit, FDAA X1 harness pin D and the front axle actuator control circuit, FDAA X1 harness pin A.
- If the voltage is less than the specified value, test the front differential lock low reference circuit for an open/high resistance, short to ground or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
9. Go to Front Drive Axle Actuator (FDAA) in Component Testing.
Component Testing
Front Drive Axle Actuator (FDAA)
1. Key OFF. Disconnect the X1 connector at the FDAA.
2. Remove the FDAA. Refer to Front Drive Axle Actuator Replacement. Front Drive Axle Actuator Replacement
3. Connect the FDAA X1 connector. Position the FDAA so that the FDAA plunger is visible from the side of the vehicle.
4. Using the transfer case shift control switch, select all modes while watching the FDAA plunger. When commanded to 2WD, the plunger on the FDAA should retract. When commanded to any other mode, the plunger should extend.
- If the FDAA does not perform as described, replace the FDAA.
5. Refer to Front Axle Will Not Engage or Front Axle Will Not Disengage in Diagnostic Information and Procedures.
Front Axle Will Not Engage Front Axle Will Not Engage
Front Axle Will Not Disengage Front Axle Will Not Disengage
TCCM
Important: When test probing harness terminals, it is imperative to use the proper test probe from the most current version of the J 35616 GM-Approved Terminal Test Kit . Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.
1. Key OFF. Disconnect the X1 connector at the front drive axle actuator (FDAA).
2. Key ON, engine OFF. Install a scan tool.
3. Using the GY probes J 35616-2A, connect a test light between the Ignition 3 voltage circuit, FDAA X1 harness pin D and the front axle actuator control circuit, FDAA X1 harness pin A.
4. Using the Engage Front Axle function of the Special Function feature of the scan tool, command the front axle ON and OFF. When commanded ON, the test light should illuminate. When commanded OFF, the test light should turn OFF.
- If the TCCM does not perform as described, replace the TCCM.
5. Go to Circuit/System Verification.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
- Transfer Case Shift Control Module Replacement NVG 226-NP8 - Transfer Case
- Front Axle Will Not Engage Front Axle Will Not Engage
- Front Axle Will Not Disengage Front Axle Will Not Disengage
- Front Drive Axle Actuator Replacement Front Drive Axle Actuator Replacement