The ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The body control module (BCM) monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights (DRL) or automatic lamp control (ALC) when the headlamp switch is in the AUTO position. In daylight conditions, the BCM will command the DRLs ON by applying ground to the DRL relay via the DRL relay control circuit. When the BCM applies ground to the DRL relay control circuit, the DRL relay coil energizes causing the relay switch contacts to close. With the DRL relay switch contacts closed, battery voltage flows through the DRL fuse to the left and right DRLs. Any function or condition that turns on the headlamps will cancel DRL operation.

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, park brake released, verify with a scan tool the Park Brake Switch parameter displays Released.

⇒	If not the specified value, refer to Brake Warning Indicator Malfunction.

2. Ignition ON, with a scan tool command the Daytime Running Lamps ON and OFF. The DRLs should turn ON and OFF when changing between the commanded states.

⇒	If the DRLs do not turn ON and OFF when commanded, refer to Daytime Running Lamps Malfunction.

⇒	If DRLs turn ON and OFF when commanded, refer to Ambient Light/Sunload Sensor Malfunction.

Ambient Light/Sunload Sensor Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient light/sunload sensor.
2. Ignition OFF, scan tool OFF, doors CLOSED, test for less than 5 ohms between the ground circuit terminal C and ground.

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

3. Ignition ON, verify the scan tool Ambient Light Sensor parameter is greater than 4.5 volts.

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

4. Install a 3A fused jumper wire between the appropriate signal circuit terminal D and ground. Verify the scan tool Ambient Light Sensor parameter is less than 0.196 volts.

⇒	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, replace the ambient light/sunload sensor.

Daytime Running Lamps Malfunction

1. Ignition OFF, disconnect the X4 harness connector at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal H4 and B+.
3. Command the DRL ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

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

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

4. Ignition OFF, connect the X4 harness connector at the underhood fuse block.
5. Ignition OFF, remove the inoperative DRL.
6. Test for less than 5 ohms of resistance between the ground circuit terminal 1 and ground.

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

7. Connect a test lamp between the control circuit terminal 3 and ground. Command the Daytime Running Lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF.

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

⇒	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 underhood fuse block.

8. If all circuits test normal, replace the inoperative DRL.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.