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 B3909 05: Low Beam Switch Circuit Open or High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Headlamp Switch Signal | 1 | B3909 05 | B3909 05 | --
Headlamp Switch Ground | -- | 1 | -- | --
1. Headlamps Malfunction

### Circuit/System Description

The headlamp switch is supplied with voltage from the instrument panel cluster (IPC) by two signal circuits, the headlamp signal circuit and the park lamp signal circuit. When the headlamp switch is placed in either the AUTO or HEAD position, the headlamps signal circuit is provided with ground through the switch contacts of the headlamp switch to a common headlamp switch ground back to the IPC. The IPC interprets the flow of voltage and responds by sending a serial data message to the underhood electrical center (UEC) requesting the headlamps be activated. The UEC responds by applying voltage to the left and right headlamp control circuits illuminating the headlamps. Ground for the left headlamp is supplied at G101 and the right headlamp receives ground at G102.

### Conditions for Running the DTC

• | Battery voltage must be between 9-16 volts.

• | AUTO headlamp ON.

### Conditions for Setting the DTC

The DTC will set when the IPC detects a short to voltage or an open/high resistance in the headlamp switch AUTO signal circuit.

### Actions Taken When the DTC Sets

When DTC B3909 05 is present, the AUTO headlamp signal circuit will be disabled.

### Conditions for Clearing the DTC

• | The condition responsible for setting the DTC no longer exists.

• | A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

### 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 headlamp switch.
2. Ignition OFF, verify a test lamp illuminates between B+ and the ground circuit terminal 11.

⇒ | If the test lamp does not illuminate, test the ground circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the IPC.

3. Ignition ON, verify the scan tool Auto Headlamp Drive parameter is OFF.

⇒ | If not the specified value, test the signal circuit terminal 5 for a short to ground. If the circuit tests normal, replace the IPC.

4. Install a 3A fused jumper wire between the signal circuit terminal 5 and ground. Verify the scan tool Auto Headlamp Drive parameter is ON.

⇒ | If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the IPC.

5. If all circuits test normal, replace the headlamp switch.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Headlamp Switch Replacement

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