Headlamps Malfunction
Headlamps Malfunction
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach
* Diagnostic Procedure Instructions provides an overview of each diagnostic category
Diagnostic Fault Information
Circuit/System Description
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 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, 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 LOW BEAM PCB relay, causing the relay switch to close. This then applies B+ 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 HIGH BEAM PCB relay, causing the relay switch to close. This then applies B+ to both low beam headlamps through the LT HI BEAM and RT HI BEAM fuses, illuminating the high beam headlamps.
Circuit/System Verification
1. Ignition ON, observe the scan tool BCM Auto Hdlp. Disable Sw. 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 BCM Headlamp Switch parameter while cycling the headlamp switch between the Auto and Head 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 headlamps ON, observe the scan tool BCM 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 Turn Signal/Multifunction Switch Malfunction.
4. With the headlamps ON, observe the scan tool BCM 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 low beam headlamps do not cycle between the commanded states, refer to High Beam Headlamp Malfunction.
Circuit/System Testing
Headlamp Switch Malfunction
1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit terminal 8 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify the scan tool BCM 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. Install a 3A fused jumper wire between the signal circuit terminal 1 and ground. Verify the scan tool BCM 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.
5. Install a 3A fused jumper wire between the signal circuit terminal 3 and ground. Verify the scan tool BCM Auto Hdlp. Disable Sw. parameter cycles between Active and Inactive.
• If the Auto Hdlp. Disable Sw. parameter does not cycle between the specified values, test the signal circuit for a short to ground, a short to voltage, or an open/high resistance. If the circuit tests normal, replace the BCM.
6. If all circuits test normal, test or replace the headlamp switch.
Turn Signal/Multifunction Switch Malfunction
1. Ignition OFF, disconnect the harness connector C2 at the turn signal/multifunction switch.
2. Ignition OFF, test for less than 1.0 ohm of resistance 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 BCM 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 BCM 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 BCM 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 BCM 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 Headlamp Malfunction
1. Ignition OFF, disconnect the harness connector C4 at the underhood fuse block.
2. Ignition ON, headlamps off, verify a test lamp does not illuminate between the control circuit terminal H2 and B+.
• If the test lamp illuminates, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.
3. Disconnect the harness connector C3 at the underhood fuse block.
4. Verify a test lamp does not illuminate between the control circuit terminal B2 and ground.
• If the test lamp illuminates, test the control circuit for a short to ground. If the circuit tests normal, replace the underhood fuse block.
5. Disconnect the harness connector C2 at the underhood fuse block.
6. Connect a test lamp between the control circuit terminal A5 and B+.
7. 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.
8. Ignition OFF, connect the harness connectors at the underhood fuse block. Disconnect the harness connector at the affected low beam headlamp bulb.
9. Ignition OFF, test for less than 1.0 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.
10. Connect a test lamp between the control circuit terminal A and ground.
11. Ignition ON, Command the Low Beams ON and OFF with a scan tool. The scan tool 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.
• If the test lamp is OFF and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, replace the affected low beam headlamp bulb.
12. If all circuits test normal, test or replace the affected low beam headlamp bulb.
High Beam Headlamp Malfunction
1. Ignition OFF, disconnect the harness connector C2 at the underhood fuse block.
2. Connect a test lamp between the control circuit terminal C6 and B+.
3. Ignition ON, Command the High Beams ON and OFF with a scan tool. The scan tool 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 harness connector at the underhood fuse block. Disconnect the harness connector at the affected high beam headlamp bulb.
5. Ignition OFF, test for less than 1.0 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.
6. Connect a test lamp between the control circuit terminal A ground ground.
7. 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.
• If the test lamp is OFF and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, replace the affected high beam headlamp bulb.
8. If all circuits test normal, test or replace the affected high beam headlamp bulb.
Component Testing
Headlamp Switch
1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Test for infinite resistance between the signal terminal 3 and the ground terminal 8 with the switch in the Auto position.
• If not the specified value, replace the headlamp switch.
3. Test for infinite resistance between the signal terminal 1 and the ground terminal 8 with the switch in the Auto position.
• If not the specified value, replace the headlamp switch.
4. Test for less than 2.0 ohms of resistance between the signal terminal 3 and the ground terminal 8 with the switch in the Off position.
• If not the specified value, replace the headlamp switch.
5. Test for less than 2.0 ohms of resistance between the signal terminal 1 and the ground terminal 8 with the switch in the Headlamp position.
• If not the specified value, replace the headlamp switch.
Turn Signal/Multifunction Switch
1. Ignition OFF, disconnect the harness connector C2 at the turn signal/multifunction switch.
2. Test for infinite resistance between the signal terminal A and the ground terminal D with the flash to pass switch in the open position.
• If not the specified value, replace the turn signal/multifunction switch.
3. Test for infinite resistance between the signal terminal E and the ground terminal D with the headlamp dimmer switch in the low beam position.
• If not the specified value, replace the turn signal/multifunction switch.
4. Test for less than 2.0 ohms of resistance between the signal terminal A and the ground terminal D with the flash to pass switch in the closed position.
• If not the specified value, replace the turn signal/multifunction switch.
5. Test for less than 2.0 ohms of resistance between the signal terminal E and the ground terminal D with the headlamp dimmer switch in the high beam position.
• If not the specified value, replace the turn signal/multifunction switch.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
* Headlamp Switch Replacement (Impala) Headlamp Switch Replacement (Monte Carlo)
* Headlamp Bulb Replacement
* Turn Signal Multifunction Switch Replacement
* Junction Block Replacement - Accessory Wiring
* Control Module References for BCM replacement, setup and programming. Programming and Relearning Verification Tests