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C0388




DTC C0388

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

DTC Descriptors
DTC C0388 01
- Rear Axle Differential Lock Circuit - Short to Battery

DTC C0388 02
- Rear Axle Differential Lock Circuit - Short to Ground

DTC C0388 04
- Rear Axle Differential Lock Circuit - Open

Diagnostic Fault Information





Circuit/System Description

The rear differential control circuit consists of an actuator which engages and disengages the rear axle differential lock. The rear differential actuator consists of a coil assembly and a locking assembly. The locking assembly is comprised of springs and pins which lock the rear differential.

The rear axle differential lock actuator consists of the following circuits:

* The rear differential lock high control circuit, supplies voltage to the differential lock coil assembly from the transfer case shift control module (TCCM).
* The rear differential low reference, provides ground through the TCCM.

The rear differential lock high control circuit provides a constant 1-4 V diagnostic voltage. When the rear differential lock is requested, a current limiting driver increases voltage to battery voltage. In order for the request to be accepted the transfer case must be in 4LO and vehicle speed must be below 5km/h (3 mph). The rear differential lock automatically disengages above 32 km/h (20 mph).

Conditions for Running the DTC

* Ignition ON.
* System voltage 9-18 V.

Conditions for Setting the DTC

The TCCM monitors the rear differential lock control and the rear differential low reference circuit for high and low voltage. 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

* All rear differential lock requests will be disabled. Transfer case mode shifts will not be affected.
* SERVICE 4WD indicator will be illuminated for the remainder of the current ignition cycle.
* The Rear Axle Differential Lock Indicator shall flash for 15 seconds and then turn off.
* The TCCM shall store Freeze Frame Data with corresponding DTC number.

Conditions for Clearing the DTC

* The TCCM will clear the DTC once the condition for setting the DTC is no longer present.
* A history DTC will clear after 33 consecutive ignition cycles without a fault present.
* History DTCs can be cleared using a scan tool.

Diagnostic Aids

The transfer case drain plug provides a good ground point.

Reference Information
Schematic Reference

* Transfer Case Control Schematics (Electrical Diagrams)
* Differential Lock Schematics (Electrical Diagrams)

Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

* Locking Differential Description and Operation (Locking Differential Description and Operation)
* Rear Drive Axle Description and Operation (Rear Drive Axle Description and Operation)

Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning)
Special Tools

J 35616 GM-Approved Terminal Test Kit

Circuit/System Verification

1. Key ON, Engine OFF. Parking Brake applied. Transmission in Neutral.
2. Install a scan tool. Using the Data Display function of the scan tool, monitor the Rear Axle Lock Request parameter while activating and deactivating the rear axle lock using the Multifunction Switch on the instrument panel. The Rear Axle Lock Request parameter should display ON when activated and OFF when deactivated.

Circuit/System Testing

1. Key OFF. Disconnect the X1 connector at the rear differential lock actuator (RDLA).
2. Using the GY probe J 35616-2A, test for 0.10-0.15 V between the Rear Differential Lock High Control circuit, RDLA X1 pin B, 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.
• If the voltage is greater than the specified value, test the affected circuit for a short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.

3. Key ON (Ignition C), engine OFF. Install a scan tool.

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 terminal test kit. Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.


4. Using the GY probe J 35616-2A, test for 0.4-4.6 V between the Rear Differential Lock High Control circuit, RDLA X1 pin B, 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.
• If the voltage is greater than the specified value, test the affected circuit for a short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.

5. Using the GY probes J 35616-2A, test for 0.4-4.6 V between the Rear Differential Lock High Control circuit, RDLA X1 pin B, and the rear differential lock Low Reference circuit RDLA X1 pin A.

• If the voltage is less than the specified value, test the rear differential lock low reference circuit for an open/high resistance, short to ground or short together. If all circuits and connections test normal, test the TCCM for proper operation.
• If the voltage is greater than the specified value, test the rear differential lock low reference circuit for a short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.

6. Using the GY probes J 35616-3, test the locking differential coil assembly for 2.2-4.3 ohms between pin A and pin B.

• If the resistance is less than the specified value, test the rear differential coil assembly harness for a short together condition. If all circuits and connections test normal, test the locking differential coil assembly for proper operation.
• If the resistance is greater than the specified value, test the rear differential coil assembly harness for an open condition. If all circuits and connections test normal, test the locking differential coil assembly for proper operation.

Component Testing

Locking Differential Coil Assembly
1. Key OFF. Disconnect the X1 connector at the RDLA.
2. Using the GY probes J 35616-3, test the locking differential coil assembly for 2.2-4.3 ohms between pin A and pin B.

• If the resistance is less than the specified value, test the rear differential coil assembly pigtail for a short together condition. If all circuits and connections test normal, replace the locking differential coil assembly.
• If the resistance is greater than the specified value, test the rear differential coil assembly pigtail for an open condition. If all circuits and connections test normal, replace the locking differential coil assembly.

3. Using the GY probes J 35616-3, test for greater than 10K ohms between the locking differential coil assembly pin A and pin B and a good ground.

• If the resistance is less than the specified value, replace the locking differential coil assembly.

4. Perform the Locking Differential Diagnosis (Locking Differential Diagnosis).

TCCM - Current Limiting Driver
1. Key OFF. Disconnect the X1 connector at the RDLA.
2. Key ON (Ignition C), engine OFF. Install a scan tool.

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 terminal test kit. Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.


3. Go to the Differential Lock Test in Special Functions on the scan tool. Using the GY probes J 35616-2A, monitor the voltage change between the Rear Differential Lock Actuator Control circuit, RDLA X1 pin B, and the rear differential lock Low Reference circuit, RDLA X1 pin A. When Low is commanded, a 0.4-4.6 V diagnostic voltage should be seen. When High is commanded, B+ should be seen.

• If the voltage is different than the specified values, reprogram the TCCM. If the DTC continues to set replace the TCCM.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Transfer Case Shift Control Module Programming and Setup (Programming and Relearning)
* Transfer Case Shift Control Module Replacement (Service and Repair)
* Locking Differential Diagnosis (Locking Differential Diagnosis)
* Locking Differential Disassemble (Locking Differential Disassemble)
* Electronic Locking Differential Coil Lock Replacement (Rear Drive Axle)
* Locking Differential Assemble (Locking Differential Assemble)

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