For 1990-2009 cars

### Circuit Description

The differential lock indicator lamp is located in the instrument cluster. Voltage is supplied from the IGN SW circuit breaker to the differential lock indicator through the ignition 3 voltage circuit. When the ignition switch is in the START position, current will travel through the bulb test signal circuit to the ignition switch grounding internally causing the differential lock indicator to illuminate. This lets the driver know that the indicator is functioning properly.

When the differential lock control valve is placed in the ON position this allows air pressure to build up in the system. The air pressure closes the differential lock switch providing ground for the differential lock switch signal circuit. This completes the indicator circuit illuminating the differential lock indicator.

### Test Description

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

1. This step tests for an intermittent condition.

2. This step checks for a short to voltage.

3. This steps isolates the bulb test signal circuit and the differential lock signal circuit.

4. This step tests the differential lock switch signal for proper voltage.

5. This step tests the differential lock ground circuit for continuity.

6. This step tests the ignition 1 voltage circuit for proper voltage.

Step | Action | Value(s) | 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 Locking Differential Description and Operation
2 | Verify the fault is present. Does the system operate normally? | -- | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 3
3 | Does the differential lock indicator stay on at all times with the differential lock disengaged? | -- | Go to Step 8 | Go to Step 4
4 | Does the differential lock indicator illuminate with the ignition switch in the start position? | -- | Go to Step 5 | Go to Step 7
5 |

1. Disconnect the differential lock switch.
2. Turn ON the ignition, with the engine OFF.
3. Test the differential lock switch signal circuit for proper voltage.

Does the voltage measure within the specified value?

9-14 V

Go to Step 6

Go to Step 9

6 | Connect a test lamp from the differential lock switch signal circuit to the ground circuit at the differential lock switch. Does the test lamp illuminate? | -- | Go to Step 10 | Go to Step 11
7 | Test the ignition 3 voltage circuit at the instrument panel cluster for proper voltage. Does the voltage measure within the specified value? | 9-14 V | Go to Step 12 | Go to Step 13
8 |

1. Test for a short to ground in the bulb test signal circuit or the differential lock switch signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
2. If OK, replace the differential lock switch. Refer to Inter-Axle Differential Lock Switch Replacement .

Did you complete the repair?

--

Go to Step 14

--

9 | Repair the open or high resistance in the differential lock switch signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | --
10 | Replace the differential lock switch. Refer to Inter-Axle Differential Lock Switch Replacement . Did you complete the replacement? | -- | Go to Step 14 | --
11 | Repair the open or high resistance in the ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | --
12 | Replace the instrument panel cluster. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | -- | Go to Step 14 | --
13 | Repair an open or high resistance in the ignition 3 voltage circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | --
14 | Operate the vehicle within the conditions for causing the symptom as specified in the supporting text. Does the symptom re-occur? | -- | Go to Step 3 | System OK