The ambient light sensor is a light sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes to the outside (ambient) light level. When the BCM receives this signal, it will either turn ON the daytime running lamps (DRL) or the headlamps for auto headlamp operation. Any function or condition that turns ON the headlights will cancel the DRL operation. With the headlight switch in the AUTO position, the headlights will either be turned ON or OFF, after an approximate 8 second delay depending on whether daylight or low light conditions are sensed. When the ambient light sensor signals the BCM for DRL operation in daylight conditions, the BCM illuminates the low beam headlamps at a reduced intensity utilizing the DRL relay. The DRL will operate when the ignition switch is in the RUN position, the headlamp and panel dimmer switch is in the AUTO position, the gear selector is not in the PARK position, and the parking brake is released. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.

Schematic Reference

Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the scan tool Headlamp OFF Switch parameter while cycling the headlamp switch between the Auto and OFF positions. The parameter should cycle between Active while in the OFF position and Inactive when released.

⇒	If the parameter does not cycle between the specified values, refer to Headlamp Switch Malfunction.

2. Observe the scan tool Headlamp ON Switch parameter while cycling the headlamp switch between the Auto and Head position. The parameter should cycle between Active and Inactive.

⇒	If the parameter does not cycle between the specified values, refer to Headlamp Switch Malfunction.

3. With the headlamps ON, observe the scan tool High Beam Select Switch parameter while switching the headlamp dimmer switch between the low and high beam positions. The parameter should cycle between Active and Inactive.

⇒	If the parameter does not cycle between the specified values, refer to Headlamp Dimmer Malfunction.

4. Command the Headlamps ON and OFF with a scan tool. Verify the low beam headlamps turn ON and OFF.

⇒	If the low beam headlamps do not cycle between the commanded states, refer to Headlamp Malfunction -- Low Beam.

5. Command the High Beams ON and OFF with a scan tool. Verify the high beam headlamps turn ON and OFF.

⇒	If the high beam headlamps do not cycle between the commanded states, refer to Headlamp Malfunction -- High Beam.

6. Ignition ON, headlamps ON, observe the scan tool high beam select switch parameter while operating the flash to pass switch ON and OFF. The readings should change between Active and Inactive.

⇒	If the high beam parameter or high beam headlamps do not turn ON and OFF, refer to Headlamp Dimmer Switch Malfunction.

7. Ignition ON, Headlamps ON, cycle the high beam switch ON and OFF. The High Beam indicator should turn ON and OFF.

⇒	If the high beam indicator does not flash ON and OFF, refer to High Beam Indicator Malfunction.

Headlamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the headlamp and panel dimmer switch.
2. Verify that a test lamp illuminates between the ground circuit terminal 3 and B+.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Headlamp ON Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 4 for a short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Headlamp OFF Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 5 for a short to ground. If the circuit tests normal, replace the BCM.

5. Install a 3A fused jumper wire between the signal circuit terminal 4 and ground. Verify the scan tool Headlamp ON Switch parameter is Active.

⇒	If not the specified value, test the signal circuit for a short to voltage or and open/high resistance. If the circuit tests normal, replace the BCM.

6. Install a 3A fused jumper wire between the signal circuit terminal 5 and ground. Verify the scan tool Headlamp OFF Switch parameter is Active.

⇒	If not the specified value, test the signal circuit for a short to voltage or and open/high resistance. If the circuit tests normal, replace the BCM.

7. If all circuits test normal, test or replace the headlamp and panel dimmer switch.

Headlamp Dimmer Switch Malfunction

1. Ignition OFF, disconnect the inline harness connector X204.
2. Test for less than 5 ohms between the ground circuit terminal B10 and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, headlamps ON, verify the scan tool Headlamp ON switch parameter is Active.

⇒	If not the specified value, test the signal circuit terminal A10 for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. Verify the scan tool high beam select switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal A11 for a short to ground. If the circuit tests normal, replace the BCM.

5. Install a 3A fused jumper wire between the signal circuit terminal A10 and the ground circuit terminal B10. Verify the scan tool Headlamp ON switch parameter is active.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

6. Install a 3A fused jumper wire between the signal circuit terminal A11 and the ground circuit terminal B10. Verify the scan tool high beam select switch parameter is active.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

7. If all circuits test normal, test or replace the turn signal/multifunction switch.

Headlamp Malfunction -- Low Beam

1. Ignition OFF, disconnect the HDLP relay.
2. Verify that a test lamp does not illuminate between the control circuit terminal 30 and B+.

⇒	If the test lamp illuminates, test the relay control circuit for a short to voltage.

3. Verify that a test lamp illuminates at the B+ circuit terminal 86 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

4. Verify that a test lamp illuminates at the B+ circuit terminal 87 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

5. Install a 10A fused jumper wire between the B+ circuit terminal 87 and the control circuit terminal 30. The low beam headlamps should illuminate.

⇒	If the low beam headlamps do not illuminate, test the control circuit for a short to ground or an open/high resistance.

6. Connect a test lamp between the relay control circuit terminal 85 and ground. With a scan tool command the Headlamps ON and OFF. The test lamp should turn ON and OFF between the commanded states.

⇒	If the test lamp is always ON, test the relay control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the relay control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

7. Ignition OFF, disconnect the harness connector at the inoperative headlamp.
8. Test for less than 5.0 ohms of resistance between the ground circuit terminal B and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

9. Connect a test lamp between the control circuit terminal A and ground.
10. Ignition ON, command the headlamps ON and OFF with a scan tool. The test should turn ON and OFF.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.

11. If all circuits test normal, replace the inoperative headlamp.

Headlamp Malfunction -- High Beam

1. Ignition OFF, disconnect the BEAM SEL relay.
2. Verify that a test lamp illuminates between the control circuit terminal 87 and B+.

⇒	If the test lamp does not illuminate, test the control circuit for an open/high resistance.

3. Verify a test lamp illuminates between the ground circuit terminal 30 and B+.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

4. Verify a test lamp illuminates between the ground circuit terminal 85 and B+.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

5. Connect a test lamp between the relay control circuit terminal 86 and ground. Command the High Beams ON and OFF with a scan tool. The test lamp should turn ON and OFF between the commanded states.

⇒	If the test lamp is always ON, test the relay control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the relay control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

6. Ignition OFF, disconnect the harness connector at the inoperative high beam headlamp.
7. Connect a test lamp between the ground circuit terminal B and B+.
8. Ignition ON, high beams ON, the test lamp should illuminate.

⇒	If the test lamp does not illuminate, test the ground circuit for an open high resistance.

9. Igniton OFF, connect a test lamp between the control circuit terminal A and ground.
10. Ignition ON, command the Headlamps ON and OFF with a scan tool. The test should turn ON and OFF.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.

11. If all circuits test normal, replace the inoperative high beam headlamp.

High Beam Indicator Malfunction

Ignition ON, with a scan tool perform the Display Test for the instrument panel cluster (IPC) and observe the high beam indicator.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.