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) through the HDM using the reduced output or the headlamps at full intensity for auto headlamp operation. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. The daytime running lamps (DRL) are the low beam headlights at a reduced intensity. With the headlight switch in the OFF 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 BCM signals the HDM for DRL operation in daylight conditions, the HDM illuminates the low beam headlights at a reduced intensity. The DRL will operate when the ignition switch is in the RUN 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 Ambient Light Sensor parameter voltage while carefully supplying sufficient light to simulate daylight conditions to the ambient light sensor, then covering to simulate darkness. The voltage reading should change from approximately 1.0-4.8 volts.

⇒	If the reading does not change between light and dark, refer to the Ambient Light Sensor Malfunction.

2. Ignition ON, command the Daytime Running Lamps ON and OFF with a scan tool. The daytime running lamps should turn ON and OFF.

⇒	If the daytime running lamps do not turn ON and OFF, refer to the Daytime Running Lamps Malfunction.

Ambient Light Sensor Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, verify that a test lamp illuminated between the listed low reference circuit terminal and B+.

•	Ambient light sensor terminal B - manual heater controls

•	Ambient light/sunload sensor terminal C - automatic heater controls

⇒	If the test lamp does not illuminate, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the BCM.

3. Ignition ON, verify the scan tool light sensor parameter is greater than 4.5 volts.

⇒	If less than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the listed signal circuit terminal and the low reference circuit terminal. Verify the scan tool light sensor parameter is less than 1.0 volt.

•	Ambient light sensor terminal A to B - manual heater controls

•	Ambient light/sunload sensor terminal D to C - automatic heater controls

⇒	If greater than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the ambient light sensor.

Daytime Running Lamps Malfunction

1. Ignition OFF, disconnect the HDM relay.
2. Verify that a test lamp illuminates between the ground circuit terminal 6 and B+.

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

3. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 5 and ground.

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

4. Verify that a test lamp illuminates between the B+ circuit terminal 2 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal, and the B+ circuit fuse is open, test the low beam headlamp control circuit terminal 4 for a short to ground.

5. Disconnect the harness connector at the inoperative low beam headlamp.
6. Test for less than 5.0 ohms between the low beam headlamp ground circuit terminal B and ground.

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

7. Connect the harness connector at the low beam headlamp.
8. Connect a 15A fused jumper wire between the B+ circuit terminal 2 and the low beam headlamp control circuit terminal 4. Verify the low beam headlamps are illuminated at full intensity.

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

⇒	If only one low beam headlamp illuminates, test the appropriate control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the inoperative low beam headlamp.

9. Connect a test lamp between the ground circuit terminal 6 and the relay control circuit terminal 5.
10. Command the Daytime Running Lamps ON and OFF with a scan tool. The test should turn ON and OFF between the commanded states.

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

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

11. If all circuits test normal, test or replace the HDM relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.