With the headlamp switch in the ON position, the headlamp switch headlamp ON signal circuit is grounded through the turn signal/multifunction switch at G200. 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 positive 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 LO and RT LO 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 GMLAN serial data to the instrument panel cluster (IPC) requesting the IPC to illuminate the high beam indicator.

Schematic Reference

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 Switch parameter while turning the headlamp switch ON and OFF. The reading should toggle between ACTIVE and INACTIVE.

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

2. Observe the scan tool Flash to Pass Switch parameter while applying and releasing the flash to pass switch. The reading should toggle between ACTIVE and INACTIVE.

⇒	If the reading does not change between the commanded states, refer to Dimmer Switch Malfunction.

3. Observe the scan tool High Beam Select Switch parameter while applying and releasing the high beam select switch. The reading should toggle between ACTIVE and INACTIVE.

⇒	If the reading does not change between the commanded states, refer to Turn Signal/Multifunction Switch Malfunction.

4. Command the Low Beams ON and OFF with a scan tool. The Low Beam Relay Command should turn ON and OFF when commanded.

⇒	If the reading does not change between the commanded states, refer to Headlamps Malfunction - Low Beam.

5. Command the High Beams ON and OFF with a scan tool. The High Beam Relay Command should turn ON and OFF when commanded.

⇒	If the reading does not change between the commanded states, refer to Headlamps Malfunction - High Beam.

6. Command the High Beams ON and OFF with a scan tool. The High Beam Indicators should turn ON and OFF.

⇒	If the High Beam Indicators do not turn ON and OFF, refer to High Beam Indicators Malfunction.

Headlamp Switch Malfunction

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

⇒	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 an 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 and Panel Dimmer Switch.

Headlamps Malfunction - Low Beam

1. Ignition OFF, disconnect the HDLP LO/HID Relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

4. Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.

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

5. Disconnect the harness connector at the inoperative low beam headlamp.
6. Ignition OFF, scan tool OFF, exterior lamps OFF, test for less than 5.0 ohms between the listed ground circuit terminal below and ground.

•	low beam headlamp terminal A -- X88

•	low beam headlamp terminal B -- Z88

•	low beam headlamp terminal 2 -- Z75

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

7. Connect the harness connector at the inoperative low beam headlamp.
8. Connect a 20A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the low beam headlamps are activated.

⇒	If the low beam headlamps do not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the inoperative low beam headlamp.

9. Connect a test lamp between the B+ circuit terminal 85 and the control circuit terminal 86.
10. Ignition ON, command the Low Beam headlamps 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.

11. If all circuits test normal, test or replace the HDLP LO/HID Relay.

Headlamps Malfunction - High Beam

1. Ignition OFF, disconnect the X4 connector at the underhood fuse block.
2. Connect a test lamp between terminal C3 and B+.
3. Ignition ON, toggle the high beams ON and OFF with a scan tool. The test 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.
5. Disconnect the harness connector at the inoperative high beam headlamp.
6. Ignition OFF, scan tool OFF, exterior lamps OFF, test for less than 5.0 ohms between the listed ground circuit terminal below and ground.

•	high beam headlamp terminal A -- X88

•	high beam headlamp terminal B -- Z88

•	high beam headlamp terminal 1 -- Z75

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

7. Connect a test lamp between the listed control circuit terminal and ground.

•	high beam headlamp terminal B -- X88

•	high beam headlamp terminal C -- Z88

•	high beam headlamp terminal 2 -- Z75

8. Ignition ON, command the high beam headlamps ON and OFF with a scan tool. The test lamp should turn ON and OFF.

⇒	If the test lamp is always ON, test the control circuit for a short 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.

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

Dimmer Switch Malfunction

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

⇒	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 an 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 an 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.

High Beam Indicator Malfunction

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.