With the headlamp switch in the ON position, the headlamp switch headlamp ON signal circuit is grounded through the turn signal/multifunction switch at G201. In response to this input, the body control module (BCM) provides ground to either the headlamp low beam relay control circuit or the headlamp high beam relay control circuit. The position of the headlamp dimmer switch determines which relay coil is supplied with ground. The BCM supplies ground to the relay coil circuits, if the headlamps are necessary. The underhood fuse block supplies battery voltage to the relay coil and switch circuits. The low beam headlamps operate when the BCM grounds the relay coil control circuit to the relay coil of the low beam headlamp relay. This energizes the low beam relay causing the switch contacts to close. With the switch contacts closed, battery voltage is applied to the LT LOW and RT LOW beam fuses through to the left and right low beam headlamps. When the headlamp high beam relay coil is energized, current flows through the LT HI BEAM and the RT HI BEAM fuses to the high beam headlamps. With the headlamp dimmer switch in the high beam position the BCM sends a message via serial data to the instrument panel cluster (IPC) requesting the IPC to illuminate the high beam indicator.

For automatic lamp control (ALC) the body control module (BCM) receives an input from the ambient light sensor with regards to outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The BCM provides a low reference ground and 5-volt reference signal to the ambient light sensor. The BCM monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights (DRL) or ALC when the headlamp switch is in the AUTO position. In daylight conditions the BCM will command the DRLs ON. During low light conditions the BCM will command the low beam headlamps ON. Any function or condition that turns on the headlamps will cancel DRL 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 Headlamp On Switch parameter while turning the headlamp switch ON and OFF. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the commanded states, refer to Headlamp Switch Malfunction.

2. Observe the scan tool Headlamp Off Switch parameter while turning the headlamp switch ON and OFF. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the commanded states, refer to Headlamp Switch Malfunction.

3. Observe the scan tool Flash to Pass Switch parameter while applying and releasing the flash to pass switch. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the commanded states, refer to Headlamp Switch Malfunction.

4. Observe the scan tool High Beam Select Switch parameter while applying and releasing the high beam select switch. The parameter should change between Active and Inactive.

⇒	If the parameter does not change between the commanded states, refer to Headlamp Switch Malfunction.

5. Ignition ON, observe the scan tool Low Beam Relay Command parameter while turning the headlamp switch to the LOW and HIGH beam positions. The parameter should change between On and Off.

⇒	If the parameter does not change between the commanded states, refer to Headlamp Switch Malfunction.

6. Ignition ON, command the Low Beam Headlamps ON and OFF with the headlamp switch. The low beam headlamps should turn ON and OFF when changing between the commanded states.

⇒	If the low beam headlamps do not turn ON and OFF when commanded, refer to Headlamps Malfunction - Low Beam.

7. Ignition ON, with a scan tool command the High Beams ON and OFF. The high beam headlamps should turn ON and OFF when changing between the commanded states.

⇒	If the high beam headlamps do not turn ON and OFF when commanded, refer to Headlamps Malfunction - High Beam.

Headlamp Switch Malfunction

1. Ignition OFF, disconnect the X1 and X2 harness connectors at the turn signal/multifunction switch.
2. Ignition OFF, verify that a test lamp illuminates between B+ and the ground circuit terminal 1.

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

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

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

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

⇒	If not the specified value, test the signal circuit terminal 2 for short to ground. If the circuit tests normal, replace the BCM.

5. Verify the scan tool Headlamp Off Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal 4 for short to ground. If the circuit tests normal, replace the BCM.

6. Verify the scan tool Flash to Pass Switch parameter is Inactive.

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

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

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

8. Verify the scan tool Low Beam Relay Command parameter is Off.

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

9. Install a 3A fused jumper wire between the signal circuit terminal 2 and ground. Verify the scan tool Headlamp On Switch parameter is Active.

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

10. Install a 3A fused jumper wire between the signal circuit terminal 4 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 an open/high resistance. If the circuit tests normal, replace the BCM.

11. Install a 3A fused jumper wire between the signal circuit terminal C 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 an open/high resistance. If the circuit tests normal, replace the BCM.

12. Install a 3A fused jumper wire between the signal circuit terminal D 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 an open/high resistance. If the circuit tests normal, replace the BCM.

13. Install a 10A fused jumper wire between the signal circuit terminal F and ground. Verify the scan tool Low Beam Relay Command parameter is On.

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

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

Headlamps Malfunction - Low Beam

1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal C2 and B+.
3. Command the low beam headlamps ON and OFF with the headlamp switch. 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 turn signal/multifunction switch.

⇒	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 turn signal/multifunction switch.

4. Ignition OFF, connect the X2 harness connector at the underhood fuse block.
5. Disconnect the harness connector at the inoperative low beam headlamp.
6. Ignition OFF, scan tool OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal B and ground.

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

7. Connect a test lamp between the control circuit terminal A and ground.
8. Command the low beam headlamps ON and OFF with the headlamp switch. 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 and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

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

9. If all circuits test normal, replace the inoperative low beam headlamp.

Headlamps Malfunction - High Beam

1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal A2 and B+.
3. 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. Ignition OFF, connect the X2 harness connector at the underhood fuse block.
5. Disconnect the harness connector at the inoperative high beam headlamp.
6. Ignition OFF, scan tool OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal B and ground.

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

7. Connect a test lamp between the control circuit terminal C and ground.
8. 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 and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

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

9. If all circuits test normal, replace the inoperative high beam headlamp.

Automatic Lamp Control (ALC) Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, scan tool OFF, doors CLOSED, test for less than 10 ohms between the low reference circuit terminal B and ground.

⇒	If greater than the specified range, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

3. If all circuits test normal, replace the ambient light sensor.

Ignition ON, with a scan tool perform the Display Test for the instrument panel cluster (IPC) and observe the high beam indicator.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.