The vehicle headlamps are controlled by the body control module (BCM) based on inputs from the headlamp switch and the headlamp dimmer switch. The headlamp switch has four positions: Off, Auto, Park, and Headlamps and is located in the turn signal/multifunction switch. 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 exterior lamps off input circuit, prompting the BCM to turn off the headlamps. 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 and 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 headlamp driver module (HDM), a solid state relay. With the HDM energized, B+ is applied to both low beam headlamps through the LT LO BEAM and RT LO BEAM fuses, illuminating the low beam headlamps. When a high beam request is received, the BCM grounds the headlamp high beam relay control circuit. This energizes the coil in the HI BEAM relay, causing the relay switch to close. This then applies B+ to both high beam headlamps through the LT HI BEAM and RT HI BEAM fuses, illuminating the high beam headlamps.

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 Headlamp Off Switch parameter while cycling the headlamp switch between the Auto and Off positions. The parameter should cycle between Active and Inactive.

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

2. Observe the scan tool Low Beam Switch parameter while cycling the headlamp switch between the Auto and Head position. The parameter should cycle between On and Off.

⇒	If the parameter does not cycle 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 cycle between Active and Inactive.

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

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

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

5. Command the Low Beam/DLR ON and OFF with a scan tool. Verify the low beam headlamps turn ON and OFF when changing between the commanded states.

⇒	If the low beam headlamps do not cycle between the commanded states, refer to Low Beam Headlamp Malfunction.

6. Command the High Beams ON and OFF with a scan tool. Verify the low beam headlamps turn ON and OFF when changing between the commanded states.

⇒	If the high beam headlamps do not cycle between the commanded states, refer to High Beam Headlamp Malfunction.

Headlamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal 17 and ground.

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

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

⇒	If not the specified value, test the signal circuit terminal 18 for a short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Low Beam Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 5 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 18 and ground. Verify the scan tool Headlamp Off 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 5 and ground. Verify the scan tool Low Beam 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 headlamp switch.

Headlamp Dimmer Switch Malfunction

1. Ignition OFF, disconnect the X1 harness connector at the multifunction turn signal lever.
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 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 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 multifunction turn signal lever.

Low Beam Headlamp Malfunction

1. Ignition OFF, disconnect the HDM.
2. Ignition OFF, test for less than 1 ohm between the ground circuit terminal B11 and ground.

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

3. Ignition ON, verify a test lamp illuminates between the B+ circuit terminal A12 and ground.

⇒	If the test lamp does not illuminate, replace the underhood fuse block.

4. Install a test lamp between the B+ circuit terminal A12 and the control circuit terminal C12.
5. Command the Low Beam/DLR 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.

6. Ignition OFF, connect the HDM. Disconnect the harness connector at the appropriate low beam headlamp bulb.
7. Ignition OFF, test for less than 5 ohm between the ground circuit terminal B and ground.

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

8. Connect a test lamp between the control circuit terminal A and ground.
9. Ignition ON, command the Low Beam/DLR 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 HDM.

⇒	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 HDM.

10. If all circuits test normal, test or replace the low beam headlamp bulb.

High Beam Headlamp Malfunction

1. Ignition OFF, disconnect the HI BEAM relay.
2. Ignition ON, verify a test lamp illuminates between the B+ circuit terminal G19 and ground.

⇒	If the test lamp does not illuminate, replace the underhood fuse block.

3. Verify a test lamp illuminates between the B+ circuit terminal G20 and ground.

⇒	If the test lamp does not illuminate, replace the underhood fuse block.

4. Install a test lamp between the B+ circuit terminal G19 and the control circuit terminal E20.
5. 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.

6. Ignition OFF, connect the HI BEAM relay. Disconnect the harness connector at the appropriate high beam headlamp bulb.
7. Ignition OFF, test for less than 5 ohm of resistance between the ground circuit terminal B and ground.

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

8. Connect a test lamp between the control circuit terminal A and ground.
9. 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 HI BEAM relay.

⇒	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 HI BEAM relay.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.