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 low beam headlamps are controlled by the BCM through the headlamp low/HID relay, and the high beams are controlled by the BCM through the high beam relay. Both relays are supplied battery voltage and are energized when the BCM grounds the low side drive control circuits. The headlamp on switch and high/low beam select switch signal circuits are referenced from battery voltage by the BCM. When a switch is activated the signal circuit is grounded, and the BCM determines the switch is active when the signal circuit voltage is low.
Circuit/System Verification
1. Ignition ON, observe the scan tool Headlamp Switch parameter while cycling the headlamp switch between the auto and headlamp position. The parameter should cycle between Active and Inactive.
^ If the parameter does not cycle between the specified values, refer to the Headlamp Switch Circuit Test.
2. Observe the headlamps while cycling the headlamp switch between the auto and headlamp position. The low beam headlamps should turn on and off.
^ If 1 or both of the low beam headlamps do not turn on and off, refer to the Low Beam Headlamp Circuit Test.
3. Low beam headlamps ON, observe the scan tool High Beam Select Switch parameter while switching the high/low beam select 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 the High/Low Beam Select Switch Circuit Test.
4. Connect a test lamp between the relay side of a high beam headlamp fuse and ground.
5. Verify that the test lamp turns on and off while switching the high/low beam select switch between the low and high beam positions.
^ If the test lamp does not turn on and off while changing between the commanded states, refer to the High Beam Relay Control Circuit Test
6. If 1 or both of the high beam headlamps do not turn on and off, refer to the High Beam Headlamp or Solenoid Circuit Test.
Circuit/System Testing
Headlamp Switch Circuit Test
1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Test for less than 1 ohm 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 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 the ground circuit terminal 8. 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.
5. If all circuits test normal, test or replace the headlamp switch.
High/Low Beam Select Switch Circuit Test
1. Ignition OFF, disconnect the harness connector X2 at the turn signal/multifunction switch.
2. Test for less than 1 ohm 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 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.
4. Install a 3A fused jumper wire between the signal circuit terminal E and the ground circuit terminal D. 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.
5. If all circuits test normal, test or replace the signal/multifunction switch.
Low Beam Headlamp Circuit Test
1. Ignition OFF, disconnect the headlamp low/HID relay.
2. Ignition ON, verify that a test lamp does not illuminate between the headlamp control circuit terminal 87 and ground.
^ If the test lamp illuminates, test the headlamp 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, replace the underhood fuse block.
4. Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.
^ If the test lamp does not illuminate, replace the underhood fuse block.
5. Disconnect the harness connector at the affected headlamp.
6. Test for less than 1 ohm between the listed headlamp ground circuit terminal and ground.
* Cadillac terminal 2
* Chevy terminal A
* GMC terminal B
^ If greater than the specified range, test the ground circuit for an open/high resistance.
7. Connect the harness connector at the headlamp.
8. Connect a 20A fused jumper wire between the B+ circuit terminal 30 and the headlamp control circuit terminal 87. Verify the low beam headlamps are on.
^ If the headlamps do not turn on, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the affected headlamp.
9. Connect a test lamp between the B+ circuit terminal 85 and the relay control circuit terminal 86.
10. 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 relay control circuit for a short to ground. If the circuit tests normal, replace the BCM.
^ If the test lamp is always OFF, test the relay 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 headlamp low/HID relay.
High Beam Relay Control Circuit Test
1. Ignition OFF, disconnect the harness connector X4 at the BCM.
2. Low beam headlamps ON, verify the high beam headlamps are not on.
^ If the high beam headlamps are on, test the high beam relay control circuit for a short to ground. If the circuit tests normal replace the underhood fuse block.
3. Install a 10A fused jumper wire between the relay control circuit terminal 16 and ground. Verify the high beam headlamps are on.
^ If the high beam headlamps are not on, test the high beam relay control circuit for an open/high resistance. If the circuit tests normal replace the underhood fuse block.
4. If all circuits test normal, replace the BCM.
High Beam Headlamp Circuit Test (Chevy, GMC)
1. Ignition OFF, disconnect the harness connector at the affected headlamp.
2. Test for less than 1 ohm between the listed ground circuit terminal and ground.
* Chevy terminal A
* GMC terminal B
^ If greater than the specified range, test the ground circuit for an open/high resistance.
3. Connect a test lamp between the listed control circuit terminal and ground circuit terminal.
* Chevy terminal B to terminal A
* GMC terminal C to terminal B
4. 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.
5. If all circuits test normal, test or replace the affected headlamp.
High Beam Solenoid Circuit Test (Cadillac)
1. Ignition OFF, disconnect the harness connector at the affected solenoid.
2. Test for less than 1 ohm between the ground circuit terminal 1 and ground.
^ If greater than the specified range, test the ground circuit for an open/high resistance.
3. Connect a test lamp between the control circuit terminal 2 and the ground circuit terminal 1.
4. 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.
5. If all circuits test normal, test or replace the affected solenoid.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests
* Headlamp Bulb Replacement (Yukon) Headlamp Bulb Replacement (Cadillac) Headlamp Bulb Replacement (Tahoe, Avalanche)
* Headlamp Switch Replacement (Cadillac) Headlamp Switch Replacement (Except Cadillac)
* Turn Signal Multifunction Switch Replacement