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.

The battery voltage must be between 9-16 volts.

ALC will remain ON in any ambient light state.

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 OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, test for less than 5.0 ohms between the listed low reference circuit terminal and ground.

•	Ambient light sensor terminal B - manual heater controls

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

⇒	If greater than the specified range, 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 4.9 volts.

•	Ambient light sensor terminal A - manual heater controls

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

⇒	If less than the specified range, test the listed signal circuit terminal 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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.