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) provides a 5-volt reference signal to the ambient light sensor. 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 1 and DRL 2 fuses to the left and right LO HDLP fuses to the low beam headlamps. The DRL control circuit that supplies the low beam headlamp fuses with voltage contains a loop of wire that acts as resistance. This resistance is responsible for the low beam headlamps illuminating at a reduced intensity. 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, with a scan tool command the Low Beams ON and OFF. The low beam headlamps should turn ON and OFF when changing between the commanded states.

⇒	If the low beam headlamps do not turn ON and OFF when commanded, refer to Headlamps Malfunction .

2. 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 .

3. Ignition ON, with a scan tool command the DRL ON and OFF. The low beam headlamps should turn ON and OFF when changing between the commanded states.

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

⇒	If the low beam headlamps turn ON and OFF when commanded, refer to Ambient Light Sensor Malfunction.

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. Remove the DRL 2 fuse from the underhood fuse block.
6. Connect a test lamp between the relay control circuit of the DRL 2 fuse and ground.
7. 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 terminal A3 X1 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 terminal A3 X1 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 underhood fuse block.

Ambient Light Sensor Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, scan tool OFF, doors CLOSED, test for less than 5.0 ohms between the appropriate ground circuit terminal listed below and ground.

•	Terminal B - with C67/C42

•	Terminal C - with CJ2

⇒	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 appropriate signal circuit terminal listed below for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

•	Terminal A - with C67/C42

•	Terminal D - with CJ2

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

•	Terminal A - with C67/C42

•	Terminal D - with CJ2

⇒	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 sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.