GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2008 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Driveline/Axle 🢒 Rear Drive Axle 🢒 Differential Lock On at All Times

Circuit Description

The differential lock is an electric and air-operated system. This is used to lock the right and left side axle shafts in order to prevent an inter-axle differential occurrence, and the torque is transmitted equally to both wheels. This circuit will only diagnose the electrical portion of the system and assumes that the air-operated portion is operating normal, mechanically. With the differential lock switch turned on, voltage is applied to the differential lock control valve. The differential lock control valve then opens, supplying air pressure in order to engage the lockout.

Test Description

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

  1. This step tests for an intermittent condition.

    2.

  2. This step isolates the differential lock switch for the circuit.

    3.

  3. This step isolates the differential lock solenoid for the circuit.

Step

Action

Yes

No

Schematic Reference: Differential Lock Schematics

1

Did you review the Differential Lock Description and Operation and perform the necessary inspections?

Go to Step 2

Go to Symptoms - Locking/Limited Slip Rear Axle

2

Verify the fault is present.

Does the system operate normally?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn the ON the ignition, with the engine OFF.
  2. Disconnect the differential lock switch.

Is the differential lock still engaged?

Go to Step 4

Go to Step 5

4

Disconnect the differential lock solenoid.

Is the differential lock still engaged?

Go to Step 7

Go to Step 6

5

Replace the differential lock switch. Refer to Differential Lock Switch Replacement .

Did you complete the replacement?

Go to Step 8

--

6

Repair the short to the voltage in the differential lock solenoid valve feed circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 8

--

7

Replace the differential lock solenoid. Refer to Inter-Axle Differential Lock Control Valve Replacement .

Did you complete the replacement?

Go to Step 8

--

8

Operate the vehicle within the condition for causing the symptom as specified in the supporting text.

Does the system operate normally?

System OK

Go to Step 2