The ambient light sensor is a light sensitive transistor that varies the voltage signal to the body control module (BCM) based on outside lighting conditions. When the ambient light sensor indicates daytime conditions with engine running, the body control module (BCM) grounds the DRL relay control circuit, energizing the DRL relay. With the DRL relay energized, voltage is applied through the relay switch contacts, the DRL 1 fuse, and the DRL 2 fuse to the left and right low beam headlamps. Any function or condition that turns the headlamps ON will cancel DRL operation.

Schematic Reference

Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

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. The voltage reading should be approximately 2-3.5 volts.

1. Ignition OFF, disconnect the DRL Relay from the rear fuse block.
2. Ignition ON, headlamps OFF, verify that a test lamp does not illuminate between the supply voltage circuit terminal 94 and ground.

⇒	If the test lamp illuminates, test the low beam supply voltage circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 90 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 between the B+ circuit terminal 93 and ground.

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

5. Connect a test lamp between the B+ circuit terminal 90 and the control circuit terminal 92.
6. 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.

7. Headlamps OFF, connect a 3A fused jumper wire between the B+ circuit terminal 93 and the supply voltage circuit terminal 92. The low beam headlamps should illuminate.

⇒	If the headlamps do not illuminate, test the supply voltage circuit for an open/high resistance. If the circuit tests normal, test or replace the DRL fuse or the rear fuse block.

8. If all circuits test normal, test or replace the DRL relay.

Relay Test

1. Ignition OFF, disconnect the DRL relay.
2. Test for 70-100 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 85

•

30 and 86

•

30 and 87

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 10A fused jumper wire between relay terminal 85 and B+. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.