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 Low or High Beam 2. Headlamps Inoperative Low or High Beam

### Circuit/System Description

The headlamp may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with the headlamp switch placed in the OFF position activating automatic lamp control (ALC). The LH HDLP and RH HDLP fuse in the fuse block - underhood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation, when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determines if the conditions are present to illuminate the daytime running lamps or park lamps.

### Diagnostic Aids

Both the High Beam and Low Beam relays are soldered into the underhood fuseblock circuit board. The replacement of the underhood fuseblock is needed upon replacement of the High Beam and Low Beam relays.

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

• | The headlamp relay control circuit.

• | The headlamp relay

• | The BCM.

• | 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 the headlamp switch portion of the multifunction switch to ON with the headlamp switch. Cycle the headlamp setting on the multifunction switch between high and low beams. Observe the headlamps, the headlamps should cycle between high and low beam as well.

### Circuit/System Testing

#### 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, terminal C1 pin 1.

⇒ | 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. If the circuit tests normal, replace the BCM.

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. If circuit tests normal, replace the BCM.

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

#### Headlamp Relay Circuit Testing

1. Ignition OFF, disconnect the high and low beam headlamp relay terminals, C2 pin 18, C2 pin 8, C3 pin 9 and, C3 pin 21.
2. Ignition ON, verify the headlamps are deactivated.

⇒ | If the headlamps are still activated, repair the relay switch control circuit for a short to ground, terminals C3 pin 9 and, 21.

3. Connect a test lamp between the relay coil control circuit, terminals C2 pins 8 and 18 and the underhood fuseblock B+ supply circuit.
4. Turn the headlamps ON by the headlamp switch. The test lamp should turn ON.

⇒ | If the test lamp remains ON all the time, test for a short to ground on the relay coil control circuit. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the underhood fuseblock.

### Component Testing

#### 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 which is part of the turn signal multifunction switch.

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

⇒ | If greater than the specified range, replace the headlamp switch which is part of the turn signal multifunction switch.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Underhood Electrical Center or Junction Block Replacement

• | Turn Signal Multifunction Switch Replacement

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