The vehicle headlamps are controlled by the body control module (BCM) based on inputs from the headlamp switch and the turn signal/multifunction switch. The headlamp switch has four positions: Off, Auto, Park, and Headlamps. The default headlamp switch position is Auto, in which the BCM uses the ambient light sensor to determine when headlamps are needed. The Off portion of the headlamp switch is a momentary switch which overrides the automatic headlamps and will turn off the vehicle headlamps. When in the Off position, the headlamp switch grounds the headlamp switch headlamps off signal circuit, prompting the BCM to turn off all exterior lamps. The Park position of the headlamp switch will only illuminate the vehicle parking lamps. The Headlamp position of the headlamp switch will illuminate both the parking lamps, as well has the headlamps. With the headlamp switch in the Headlamp position, the headlamp switch headlamps on signal circuit is grounded, prompting the BCM to turn on the headlamps, regardless of other factors such as ambient light.

The headlamp high beams are controlled through the flash to pass switch and the headlamp dimmer switch, both located in the turn signal/multifunction switch. The flash to pass switch is a momentary type switch, designed to illuminate the high beams as long as the switch is held. With the switch closed, the flash to pass switch signal circuit is grounded, prompting the BCM to turn on the high beam lamps. The headlamp dimmer switch allows the operator to select between full time high or low beam operation. Unlike the flash to pass switch, it is not a momentary switch. When the headlamp dimmer switch is in the high beam position, the headlamp dimmer switch high beam signal circuit is grounded. This prompts the BCM to turn on the high beam lamps.

The BCM controls the headlamps based on the inputs explained above. When a low beam request is received, the BCM grounds the headlamp low beam relay control circuit. This energizes the coil in the LO BEAM PCB Relay, causing the relay switch to close. This then applies B+ through the LT LO BEAM and RT LO BEAM fuses, the headlamp supply voltage circuits to the headlamp ballasts, illuminating the headlamps. When a high beam request is received, the BCM grounds the headlamp high beam relay control circuit. This energizes the coil in the HIGH BEAM PCB Relay, causing the relay switch to close. Battery voltage is then applied through both HI BEAM fuses and the high beam headlamp supply voltage circuits to the headlamp high beam solenoids. The solenoids then open the low beam shields to fully expose the headlamp bulbs for high beam operation.

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

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the scan tool Auto Hdlp. Disable Sw. parameter while cycling the headlamp switch between the Auto and Off positions. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the specified values, refer to Headlamp Switch Malfunction.

2. Observe the scan tool Headlamp Switch parameter while cycling the headlamp switch between the Auto and Head position. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the specified values, refer to Headlamp Switch Malfunction.

3. With the headlamps ON, observe the scan tool Flash to Pass Switch parameter while pulling and releasing the flash to pass switch. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the specified values, refer to Turn Signal/Multifunction Malfunction.

4. With the headlamps ON, observe the scan tool High Beam Select Switch parameter while switching the headlamp dimmer switch between the low and high beam positions. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the specified values, refer to Turn Signal/Multifunction Malfunction.

5. Ignition ON, with a test lamp connected to ground, probe each of the test points on either LO BEAM Fuse while commanding the Low Beams ON and OFF with a scan tool. The test lamp should turn ON and OFF at one or both of the fuse test points when changing between commanded states.

⇒	If the test lamp is always OFF, or remains always ON, refer to Low Beam PCB Relay Malfunction.

⇒	If the test lamp turns ON and OFF, refer to Headlamps Malfunction.

6. Ignition ON, with a test lamp connected to ground, probe each of the test points on either HI BEAM Fuse while commanding the High Beams ON and OFF with a scan tool. The test lamp should turn ON and OFF at one or both of the fuse test points when changing between commanded states.

⇒	If the test lamp is always OFF, or remains always ON, refer to High Beam PCB Relay Circuit Test.

⇒	If the test lamp turns ON and OFF, refer to High Beam Malfunction.

Caution: The high intensity discharge system produces high voltage and current. To reduce the risk of severe shocks and burns:

Headlamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Ignition OFF, verify that a test lamp illuminates between the ground circuit terminal 8 and B+.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Headlamp Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 1 for a short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Auto Hdlp. Disable Sw. parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 3 for a short to ground. If the circuit tests normal, replace the BCM.

5. Install a 3A fused jumper wire between the signal circuit terminal 1 and ground. Verify the scan tool Headlamp Switch parameter is Active.

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

6. Install a 3A fused jumper wire between the signal circuit terminal 3 and ground. Verify the scan tool Auto Hdlp. Disable Sw. parameter is Active.

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

7. If all circuits test normal, test or replace the headlamp switch.

Turn Signal/Multifunction Switch Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the turn signal/multifunction switch.
2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal D and ground.

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

3. Ignition ON, verify the scan tool Flash to Pass Switch parameter is Inactive.

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

4. Verify the scan tool High Beam Select Switch parameter is Inactive.

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

5. Install a 3A fused jumper wire between the signal circuit terminal A and ground. Verify the scan tool Flash to Pass Switch parameter is Active.

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

6. Install a 3A fused jumper wire between the signal circuit terminal E and ground. Verify the scan tool High Beam Select Switch parameter is Active.

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

7. If all circuits test normal, test or replace the turn signal/multifunction switch.

Low Beam PCB Relay Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal 10 and B+.
3. Ignition ON, command the Low Beams 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 is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, replace the underhood fuse block.

Headlamps Malfunction

1. Disconnect the harness connector at the inoperative headlamp assembly.
2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal F and ground.

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

3. Connect a test lamp between the control circuit terminal E and ground.
4. Ignition ON, command the Low Beams 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 is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the underhood fuse block.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

5. If all circuits test normal, test or replace the inoperative headlamp.

High Beam PCB Relay Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Connect a test lamp between B+ and the control circuit terminal 19.
3. Ignition ON, command the High Beams 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 is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, replace the underhood fuse block.

High Beam Malfunction

1. Disconnect the harness connector at the inoperative headlamp assembly.
2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal F and ground.

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

3. Connect a test lamp between the control circuit terminal D and ground.
4. Ignition ON, command the High Beams 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 is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the underhood fuse block.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

5. If all circuits test normal, test or replace the inoperative headlamp assembly.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.