GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 HUMMER 🢒 2007 🢒 H2 🢒 Driveline/Axle 🢒 Rear Drive Axle 🢒 DTC C0388

Circuit 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 with springs and pins to lock the rear differential, all within the rear differential assembly.

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

    • The rear differential lock control circuit, which is connected to the transfer case shift control module and supplies battery voltage.
    • The rear differential lock return ground circuit, which is also connected to the transfer case shift control module.

Whenever the rear differential lock has been requested the transfer case shift control module provides battery voltage through the rear differential lock control circuit, and provides ground through the rear differential lock return circuit through a current limiting driver.

Whenever a rear differential lock is requested the transfer case must be in 4 LO and the vehicle speed must be below 5 km/h (3 mph) for the request to be accepted. The rear differential lock will remain locked up to 32 km/h (20 mph) after which the rear differential lock will disengage.

This DTC detects a short to ground, short-to-voltage, or an open circuit.

Conditions for Running the DTC
    • The ignition is ON.
    • The system voltage is 9-18 volts.

Conditions for Setting the DTC
    • The transfer case shift control module monitors the state of the rear axle differential lock actuator. The transfer case shift control module reads the rear differential lock control, and the rear differential lock return circuits for a high and low voltage. The DTC will set if these circuits are high when a low voltage is expected or when the circuits are low when a high voltage is expected.
    • The DTC will set if this circuit is high when a low voltage is present or when the circuit is low when a high voltage is present.

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 latched on for the remainder of the current ignition cycle.

Conditions for Clearing the DTC
    • The Transfer case shift control module will clear the DTC if the condition for setting the DTC is not currently present.
    • A history DTC will clear after 33 consecutive ignition cycles without a fault present.
    • History DTCs can be cleared using a scan tool.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines of the coil has excessive resistance or if the windings are shorted together.

    2.

  2. This step determines if the coil or pigtail is shorted to ground.

    3.

  3. This step determines if either the differential lock control or differential lock return circuits are shorted to ground.

    4.

  4. This step determines if the differential lock control and differential lock return circuits are shorted together.

    5.

  5. This step tests for an open or high resistance in both the return and control circuits.

    6.

  6. This step refers to transfer case shift control module replacement.

    7.

  7. This step refers to encoder motor replacement.

Step

Action

Value (s)

Yes

No

Schematic Reference: Transfer Case Control Schematics

Connector End View Reference: Rear Axle Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

 

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Disconnect the rear differential lock harness from the rear differential lock coil pigtail.
  2. With a DMM, measure the resistance of the rear differential lock coil and pigtail by probing the rear differential lock return and the rear differential lock control circuits.

Was the resistance reading within the specified values?

2.2-4.3 ohms

Go to Step 3

Go to Step 8

3

With the connector still disconnected, with a DMM, probe the return then the control circuits and check for resistance to ground.

Did the DMM indicate a short to ground?

--

Go to Step 8

Go to Step 4

4

  1. Disconnect connector C1 from the transfer case shift control module.
  2. Disconnect the rear differential lock harness from the rear differential coil pigtail.
  3. With a DMM, check for a short to power or short to ground on the differential lock control and differential lock return circuits. Refer to Circuit Testing and Wiring Repairs .

Was the condition found and corrected?

--

Go to Step 9

Go to Step 5

5

Important: This circuit contains a reversed bias diode. The red lead of the DMM must be placed on the rear differential lock high control circuit, and the black lead on the rear differential lock low reference circuit. This is done to avoid showing the circuits falsely shorted together.

Check the differential lock control and the differential lock return circuits for being shorted together. Refer to Circuit Testing and Wiring Repairs .

Was the condition found and corrected?

--

Go to Step 9

Go to Step 6

6

Check the suspect circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs .

Was the condition found and corrected?

--

Go to Step 9

Go to Step 7

7

Replace the transfer case shift control module. Refer to Control Module References for replacement, setup, and programming.

Was the repair completed?

--

Go to Step 9

--

8

Replace the rear differential lock coil. Refer to Electronic Locking Differential Coil Lock Replacement .

Was the repair completed?

--

Go to Step 9

--

9
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 1

System OK