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 B+ | 2 | 2 | 2 | --
Ambient Light Sensor Signal | B2645 02 | B2645 05 | B2645 05 | --
Daytime Running Lamps Relay Control | 2 | 2 | 2 | --
Daytime Running Lamps Control | 2 | 2 | 2 | --
Headlamp - Left Front Low Beam Control | 1 | 1 | 1 | --
Headlamp - Right Front Low Beam Control | 1 | 1 | 1 | --
Ambient Light Sensor Low Reference | -- | 2 | 2 | --
Headlamp - Left Front Ground | -- | 1 | -- | --
Headlamp - Right Front Ground | -- | 1 | -- | --
1. Headlamps Malfunction 2. Daytime Running Lamps Malfunction

### Circuit/System Description

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 low reference ground and 5-volt reference signal to the ambient light sensor. The 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 apply ground to the DRL relay control circuit causing the DRL relay coil to energize. With the DRL relay energized, the switch contacts close allowing battery voltage to flow through the DRL fuse via the DRL control circuit illuminating the low beam headlamps at reduced intensity. 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.

### 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.

3. Ignition ON, with a scan tool command the DRLs 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.

### Circuit/System Testing

#### Daytime Running Lamps Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal E5 and B+.
3. Command the DRLs 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 X2 harness connector at the underhood fuse block.
5. Disconnect the X3 harness connector at the underhood fuse block.
6. Connect a test lamp between the control circuit terminal F4 and ground.
7. Command the DRLs 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 voltage. If the circuit tests normal, replace the underhood fuse block.

⇒ | If the test lamp is always OFF and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

⇒ | If the test lamp is always OFF and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests 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 10 hms between the low reference circuit terminal B and ground.

⇒ | If greater than the specified range, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

3. If all circuits test normal, replace the ambient light sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Ambient Light Sensor Replacement

• | Underhood Electrical Center or Junction Block Bracket Replacement

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