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
--- | --- | --- | --- | ---
Headlamp Control Circuit in DRL Setting | B2575 | B2575 | B2575 | --
Headlamp Relay Control Circuit | 1 | 2 | 2 | --
Park Lamp Switch Signal Circuit | 1 | 2 | 2 | --
Headlamp Switch Supply Circuit | 2 | 2 | -- | --
Headlamp Switch Ground Circuit | -- | 2 | 2 | --
Relay Switch Signal Circuit | 1 | 2 | 2 | --
1. Headlamps Always ON High Beam 2. Headlamps Inoperative High Beam

### Circuit/System Description

The headlamp switch of the multifunction switch controls the ground for the body control module (BCM). The BCM monitors the park lamp switch signal. Once the BCM senses a voltage drop on the park lamp switch signal the BCM supplies a ground to the headlamp relay control circuit. The headlamp relay than supplies a ground to the headlamp circuit thus turning on the headlamps. Both the high and low beam headlamp relays are soldered to the underhood fuseblock.

### Diagnostic Aids

Replacement of either high or low beam relays also means replacing the underhood fuseblock due to the relays being soldered to the circuit board.

A short to voltage, short to ground in the following location may cause a malfunction to occur:

• | The high beam headlamp relay control circuit.

• | The high beam headlamp relay

• | The headlamp switch

### 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 Data Definitions

• | Scan Tool Data List

• | Scan Tool Output Controls

### Circuit/System Verification

Ignition ON, turn on the headlamps ON with the headlamp switch. Cycle the headlamp setting on the multifunction switch between high and low beams. Observes the headlamps, the headlamps should cycle between high and low beam as well.

### Circuit/System Testing

#### Headlamp High Beam Light Bulb Circuit Testing

1. Ignition OFF, disconnect the harness connector at the headlamp high beam light bulb terminal pin 3.
2. Ignition ON, verify that a test lamp illuminates between the headlamp high beam voltage supply circuit and ground.

⇒ | If the test lamp does not illuminate, repair the voltage supply circuit for a short to ground or an open/high resistance. If the voltage supply circuit fuse is open, test or replace the headlamp high beam light bulb.

3. Connect a test lamp between the control circuit and the voltage supply circuit.
4. Command the headlamps 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 remains ON all the time, test for a short to ground on the control circuit.

⇒ | If the test lamp remains OFF all the time, test for a short to voltage or an open/high resistance on the control circuit.

5. If all circuits test normal, test or replace the headlamp high beam light bulb.

#### Multifunction/Headlamp Switch Circuit Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch terminal C1 pins 4 and 2 .
2. Test for less than 1 ohm of resistance between the ground circuit and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

3. Verify the headlamps are OFF.

⇒ | If the headlamps are not OFF, test the signal circuit for a short to ground.

4. Install a 3-amp fused jumper wire between the signal circuit and ground. Verify the headlamps are ON.

⇒ | If the headlamps are not ON, test the signal circuit of the headlamp switch for a short to voltage or an open/high resistance.

5. If all circuits test normal, test or replace the headlamp switch, part of the multifunction switch.

### Component Testing

#### Headlamp High Beam Light Bulb Test

1. Ignition OFF, disconnect the harness connector at the headlamp high beam light bulb, pin 3.
2. Test for 0.5-3.5 ohms of resistance between the control terminal and the voltage supply terminal of the headlamp high beam light bulb.

⇒ | If the resistance is not within the specified range, replace the headlamp high beam light bulb.

#### Multifunction/Headlamp Switch Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch, terminal C1 pin 4.
2. Test for infinite resistance between the signal terminal and the voltage supply terminal with the switch in the open position.

⇒ | If less than infinite resistance, replace the headlamp switch.

3. Test for less than 1.5 ohms between the signal terminal and the high voltage supply terminal with the switch in the closed position.

⇒ | If greater than the specified range, replace the headlamp switch.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Headlamp Replacement

• | Headlamp Bulb Replacement

• | Headlamp Aiming

• | Turn Signal Multifunction Switch Replacement