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.

### DTC Descriptor

DTC B2647: Ambient Light Sensor Circuit Low
DTC B2648: Ambient Light Sensor Circuit High

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Ambient Light Sensor Signal | B2647 00 | B2648 00 | B2648 00 | --
Ambient Light Sensor Low Reference | -- | 2 | 2 | --
Daytime Running Lamp Relay Control | B2602 00 | B2602 00 | B2603 00 | --
Daytime Running Lamp - Left Voltage Supply | 2 | 2 | 2 | --
Daytime Running Lamp - Right Voltage Supply | 2 | 2 | 2 | --
Headlamp - Left Front Low Beam Supply Voltage | 1 | 1 | 1 | --
Headlamp - Right Front Low Beam Supply Voltage | 1 | 1 | 1 | --
Daytime Running Lamp - Left Ground | -- | 2 | -- | --
Daytime Running Lamp - Right Ground | -- | 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 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) or the headlamps for auto headlamp operation. Any function or condition that turns ON the headlights will cancel the DRL operation. With the headlight switch in the AUTO 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 ambient light sensor signals the BCM for DRL operation in daylight conditions, the BCM illuminates the low beam headlamps at a reduced intensity utilizing the DRL relay. The DRL will operate when the ignition switch is in the RUN position, the headlamp and panel dimmer switch is in the AUTO 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.

### Conditions for Running the DTC

The system voltage must be between 9-16 volts.

### Conditions for Setting the DTC

#### B2647

When the ambient light sensor signal voltage is less than 1.75 volts.

#### B2648

When the ambient light sensor signal voltage is greater than 4.9 volts.

### Action Taken When the DTC Sets

The ALC will remain always ON or always OFF in any ambient light state.

### Conditions for Clearing the DTC

• | This DTC will clear immediately after the condition for setting the fault is corrected.

• | A history DTC will clear after 100 consecutive ignition cycles without a fault present.

### 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 Testing

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, test for less than 5.0 ohms 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. Ignition ON, verify the scan tool ambient light sensor parameter is greater than 4.9 volts.

⇒ | If less than the specified range, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal A and the low reference circuit terminal B. Verify the scan tool ambient light sensor parameter is less than 0.5 volts.

⇒ | 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.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Ambient Light Sensor Replacement

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