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, AUTOMATIC LIGHT, PARKING LIGHT, and LOW BEAM. The default headlamp switch position is AUTOMATIC LIGHT, 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 headlamp 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 the headlamps. The Park position of the headlamp switch will only illuminate the vehicle parking lamps. The LOW BEAM position of the headlamp switch will illuminate both the parking lamps, as well has the headlamps. With the headlamp switch in the LOW BEAM 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.

With High Intensity Discharge (HID) headlamps RPO TT2/TT6

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 headlamp low beam relay, causing the relay switch to close. This then applies B+ to both low beam high intensity discharge (HID) headlamps through the left low beam and right low 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 high beam relay, causing the relay switch to close. This then applies B+ to both low beam HID headlamps through the left high beam and right high beam fuses, illuminating the high beam headlamps.

Without High Intensity Discharge (HID) headlamps RPO TT4

The BCM controls the headlamps based on the inputs explained above. When a low beam request is received, the BCM supplies an B+ through the headlamp low beam relay for the left headlamp and through daytime running lamp relay - right for the right headlamp. This then applies B+ to the low beam headlamps, 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 headlamp high beam relay, causing the relay switch to close. This then applies B+ to both high beam headlamps through the left high beam and right high 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 Automatic Headlamp Disable Switch parameter while cycling the headlamp switch between the AUTOMATIC LIGHT 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 Headlamp ON Switch parameter while cycling the headlamp switch between the AUTOMATIC LIGHT and LOW BEAM ON position. The parameter should cycle between Active and Inactive.

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

3. With the low beams ON, observe the scan tool Headlamps Flash Switch parameter while pulling and releasing the turn signal/multifunction switch. The parameter should cycle between Active and Inactive.

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

4. With the low beams 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 cycle between Active and Inactive.

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

5. Command the Low Beams ON and OFF with a scan tool. Verify the low beam headlamps turn ON and OFF.

⇒	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.

⇒	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 S30 Headlamp Switch.
2. Verify that a test lamp illuminates between the ground circuit terminal 6 and B+.

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

3. Ignition ON, test for B+ between the signal circuit terminals listed below and ground.

•	Headlamp switch low beams ON signal terminal 4

•	Park lamp switch ON signal terminal 3

•	Headlamp switch OFF signal terminal 5

⇒	If less than the specified value, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

4. Install a 3 A fused jumper wire between the park lamp ON signal circuit terminal 3 and ground. Verify the scan tool Parklamp Switch parameter changes from Inactive to Active.

⇒	If always Inactive, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If always active, test the signal circuit for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

5. Install a 3 A fused jumper wire between the low beams ON signal circuit terminal 4 and ground. Verify the scan tool Headlamp Switch parameter changes from Inactive to Active.

⇒	If always Inactive, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If always active, test the signal circuit for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

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

⇒	If always Inactive, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If always active, test the signal circuit for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

7. If all circuits test normal, replace the S30 Headlamp Switch.

Turn Signal/Multifunction Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the turn S78 Turn Signal/Multifunction Switch.
2. Ignition OFF, test for less than 5.0 Ω between the ground circuit terminal 3 and ground.

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

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

⇒	If not the specified value, test the signal circuit terminal 4 for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

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

⇒	If not the specified value, test the signal circuit terminal 2 for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

5. Install a 3 A fused jumper wire between the signal circuit terminal 4 and ground. Verify the scan tool Headlamps Flash 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 K9 Body Control Module (BCM).

6. Install a 3 A fused jumper wire between the signal circuit terminal 2 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 K9 Body Control Module (BCM).

7. If all circuits test normal, test or replace the S78 Turn Signal/Multifunction Switch.

Low Beam Headlamp Malfunction - with HID headlamps RPO TT2/TT6

1. Ignition OFF, disconnect the KR49 Headlamp Low Beam Relay.
2. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

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

⇒

If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay coil B+ circuit fuse is open, test the relay controlled output circuit for a short to ground. If the circuit tests normal, test or replace the KR49 Headlamp Low Beam Relay.

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

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

5. Connect a test lamp between the relay coil B+ circuit terminal 85 and the relay coil control circuit terminal 86.
6. 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 relay coil control circuit for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If the test lamp is always OFF, test the relay coil control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

7. Connect a 20 A fused jumper wire between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87.

⇒	If the low beam headlamp illuminates, test or a replace the KR49 Headlamp Low Beam Relay.

8. With the jumper wire still connected, disconnect the harness connector at the appropriate inoperative low beam headlamp.
9. Verify that the test lamp illuminates between the appropriate inoperative low beam headlamp terminal 3 and ground.

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

10. Verify that a test lamp illuminates between the low beam headlamp ground terminal 1 and B+:

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

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

Low Beam Headlamp Malfunction - without HID headlamps RPO TT4

1. Ignition OFF, disconnect the KR42L Daytime Running Lamp Relay - Left or KR42R Daytime Running Lamp Relay - Right.
2. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit terminal 87A and ground.

⇒	If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

3. Connect a test lamp between the relay switch circuit terminal 30 and ground.
4. 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 relay switch circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If the test lamp is always OFF, test the relay coil control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

5. Connect a 10 A fused jumper wire between B+ circuit and the relay controlled output circuit terminal 87A.

⇒	If the low beam headlamp illuminates, test or a replace the appropriate inoperative relay.

6. With the jumper wire still connected, disconnect the harness connector at the appropriate inoperative low beam headlamp.
7. Verify that a test lamp illuminates between the low beam control circuit terminal 3 and ground:

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

8. Verify that a test lamp illuminates between the low beam headlamp ground circuit terminal 1 and B+.

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

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

High Beam Headlamp Malfunction - with and without HID headlamps RPO TT2/TT4/TT6

1. Ignition OFF, disconnect the KR48 Headlamp High Beam Relay.
2. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

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

⇒

If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay coil B+ circuit fuse is open, test the relay controlled output circuit for a short to ground. If the circuit tests normal, test or replace the KR48 Headlamp High Beam Relay.

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

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

5. Connect a test lamp between the relay coil B+ circuit terminal 85 and the relay coil control circuit terminal 86.
6. Command the high 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 relay coil control circuit for a short to ground. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If the test lamp is always OFF, test the relay coil control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

7. Connect a 20 A fused jumper wire between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87.

⇒	If the high beam headlamps illuminates, test or replace KR48 Headlamp High Beam Relay.

8. With the jumper wire still connected, disconnect the harness connector at the appropriate headlamp high beam solenoid module.
9. Verify that a test lamp illuminates between the high beam headlamp control circuit terminal 2 and ground.

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

10. Verify that a test lamp illuminates between the headlamp high beam solenoid module ground circuit terminal 1 and B+.

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

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

Static Test

Headlamp Switch

1. Ignition OFF, disconnect the harness connector at the S30 Headlamp Switch.
2. Test for infinite resistance between the signal terminal 5 and the ground terminal 6 with the switch in the AUTOMATIC LIGHT position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

3. Test for infinite resistance between the signal terminal 4 and the ground terminal 6 with the switch in the AUTOMATIC LIGHT position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

4. Test for infinite resistance between the signal terminal 3 and the ground terminal 6 with the switch in the PARK LIGHT position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

5. Test for less than 2.0 Ω of resistance between the signal terminal 5 and the ground terminal 6 with the switch in the OFF position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

6. Test for less than 2.0 Ω of resistance between the signal terminal 4 and the ground terminal 6 with the switch in the LOW BEAM position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

7. Test for less than 2.0 Ω of resistance between the signal terminal 3 and the ground terminal 6 with the switch in the PARK LIGHT position.

⇒	If not the specified value, replace the S30 Headlamp Switch.

Turn Signal/Multifunction Switch

1. Ignition OFF, disconnect the harness at the S78 Turn Signal/Multifunction Switch.
2. Test for infinite resistance between the signal terminal 4 and the ground terminal 3 with the flash to pass switch in the open position.

⇒	If not the specified value, replace the S78 Turn Signal/Multifunction Switch.

3. Test for infinite resistance between the signal terminal 2 and the ground terminal 3 with the headlamp dimmer switch in the LOW BEAM position.

⇒	If not the specified value, replace the S78 Turn Signal/Multifunction Switch.

4. Test for less than 2.0 Ω of resistance between the signal terminal 4 and the ground terminal 3 with the flash to pass switch in the closed position.

⇒	If not the specified value, replace the turn S78 Turn Signal/Multifunction Switch.

5. Test for less than 2.0 Ω of resistance between the signal terminal 2 and the ground terminal 3 with the headlamp dimmer switch in the HIGH BEAM position.

⇒	If not the specified value, replace the S78 Turn Signal/Multifunction Switch.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.