For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Ambient Light Sensor Signal | B2645 02 | B2645 05 | B2645 05 | --
DRL Relay Control | 1 | 1 | 1 | --
Headlamp - Low Beam Control - Right | 2 | 2 | 2 | --
Headlamp - Low Beam Control - Left | 2 | 2 | 2 | --
Ambient Light Sensor Low Reference | -- | 1 | 1 | --
Headlamp - Low Beam Ground - Right | -- | 2 | -- | --
Headlamp - Low Beam Ground - Left | -- | 2 | -- | --
1. Daytime Running Lamps Malfunction 2. Headlamps Malfunction

### Circuit/System Description

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, the body control module (BCM) grounds the DRL relay control circuit causing voltage to flow from the DRL fuse which energizes the DRL relay. With the DRL relay energized, the switch contacts close and voltage is applied to the left and right low beam headlamps. Any function or condition that turns on the headlamps will cancel DRL operation.

### Reference Information

#### 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

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Ignition ON, with a scan tool command the Headlamps 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 Inactive.

⇒ | If not the specified value, refer to Symptoms - Hydraulic Brakes.

### Circuit/System Testing

1. Ignition OFF, disconnect the DRL relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 30 and ground.

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

3. Verify that a test lamp illuminates between the B+ circuit terminal 86 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 control circuit terminal 30 for a short to ground. If the circuit tests normal, test or replace the DRL relay.

4. Verify that a test lamp illuminates between the B+ circuit terminal 87 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 86 and the control circuit terminal 85.
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. If all circuits test normal, test or replace the DRL relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Relay Replacement

• | Control Module References for BCM replacement, setup, and programming