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
--- | --- | --- | --- | ---
DRL Relay Control | 1 | 1 | 1 | --
DRL Ambient Light Sensor Signal | B2645 02 | B2645 05 | B2645 05 | --
DRL Ambient Light Sensor Low Reference | -- | 1 | 1 | --
Headlamp - Low Beam Voltage Supply - Right | 1, 2 | 1, 2 | 2 | --
Headlamp - Low Beam Voltage Supply - Left | 1, 2 | 1, 2 | 2 | --
Headlamp - Low Beam Ground - Right | -- | 2 | 2 | --
Headlamp - Low Beam ground - Left | -- | 2 | 2 | --
1. Daytime Running Lamps (DRL) Inoperative 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

Lighting Systems Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Output Controls

• | Scan Tool Data List

• | Scan Tool Data Definitions

### Circuit/System Verification

Ignition ON, 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 relay switch B+ output circuit and ground.

⇒ | If the test lamp illuminates, test the relay switch B+ output circuit for a short to voltage.

3. Verify that a test lamp illuminates between the relay coil B+ circuit and ground.

⇒ | If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay coil B+ circuit fuse is open, test the relay switch B+ output circuit 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 relay switch B+ circuit and ground.

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

5. Connect a test lamp between the relay coil B+ circuit and the relay coil control circuit.
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 relay coil control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒ | If the test lamp is always OFF, test the relay coil 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, replace the DRL relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Underhood Electrical Center or Junction Block Replacement

• | Relay Replacement

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