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
--- | --- | --- | --- | ---
Park Lamp Switch Signal Circuit | 1 | 2 | -- | --
Headlamp Switch Supply Circuit | 1 | 2 | -- | --
Headlamp Switch Ground Circuit | -- | 2 | 2 | --
1. Headlamps Always ON Low Beam 2. Headlamps Inoperative Low 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 Left High Beam, Right High Beam, Left Low Beam, and Right Low Beam fuses 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 low and high beam relays located in the underhood fuseblock are soldered to the circuit board. A replacement of the relays would also mean replacing the underhood fuseblock.

• | A short to voltage, short to ground in the headlamp switch may cause a malfunction to occur.

### 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

#### 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 terminal C1 pin 1 and ground.

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

3. 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.

4. If all circuits test normal, test or replace the headlamp switch, replace the whole multifunction switch.

### Component Testing

#### Multifunction/Headlamp Switch Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch, terminal C1 pins 4 and 2.
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, replace the whole multifunction 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, replace the whole multifunction 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