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 HDM and high beam relay receive battery positive voltage directly from the battery. The BCM supplies a 100% duty cycle PWM ground signal to the HDM for automatic headlight control and normal headlight operation. The BCM also supplies ground to the high beam relay coil for high beam headlight operation. When the driver places the headlamp switch in the HEADLAMP position and the dimmer switch is in the low beam position, the HDM switches to full output sending current flow through the left and right low beam headlamp fuses to both low beam headlamps. The dimmer switch sends a ground signal to the BCM in the high beam position and a momentary ground signal in the flash-to-Pass (FTP) position from G102 to deactivate the low beam headlights and activate the high beam headlights. With the headlights in the low beam position, the high beams may be momentarily turned on or flashed by activating the FTP portion of the switch.
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. With the headlamps ON, observe the scan tool high beam switch parameter while switching the headlamp dimmer switch between the low and high beam positions. The parameter should cycle between on and off.
^ If the parameter does not cycle between the specified values, refer to the Turn Signal/Multifunction Switch Circuit Test.
4. With the headlamps ON, observe the headlamps while switching the headlamp dimmer switch between the low and high beam positions. The high beam headlamps should turn on and off.
^ If 1 or both of the high beam headlamps do not turn on and off, refer to the High Beam Headlamp Circuit Test.
Circuit/System Testing
Headlamp Switch Circuit Test
1. Ignition OFF, disconnect the harness connector X1 at the headlamp switch.
2. Test for less than 1 ohm between the ground circuit terminal X1 C 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 headlamp on switch signal circuit terminal X1 D for a short to ground. If the circuit tests normal, replace the BCM.
4. Install a 3A fused jumper wire between the headlamp on switch signal circuit terminal X1 D and the ground circuit terminal X1 C. Verify the scan tool headlamp switch parameter is active.
^ If not the specified value, test the headlamp on switch 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.
Turn Signal/Multifunction 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 X2 A 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 switch parameter is off.
^ If not the specified value, test the high beam switch signal circuit terminal X2 C for a short to ground. If the circuit tests normal, replace the BCM.
4. Install a 3A fused jumper wire between the high beam switch signal circuit terminal X2 C and the ground circuit terminal X2 A. Verify the scan tool high beam switch parameter is on.
^ If not the specified value, test the high beam switch 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 turn signal/multifunction switch.
Low Beam Headlamp Circuit Test
1. Ignition OFF, disconnect the harness connector at the affected headlamp bulb.
2. Test for less than 1 ohm between the ground circuit terminal B 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 A and the ground circuit terminal B.
4. Turn the low beam headlamps on and off. 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 low beam headlamp relay.
^ 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 low beam headlamp relay.
5. If all circuits test normal, test or replace the affected headlamp bulb.
High Beam Headlamp Circuit Test
1. Ignition OFF, disconnect the harness connector at the affected headlamp bulb.
2. Test for less than 1 ohm between the ground circuit terminal B 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 A and the ground circuit terminal B.
4. Turn the high beam headlamps on and off. 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 high beam headlamp relay.
^ 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 high beam headlamp relay.
5. If all circuits test normal, test or replace the affected headlamp bulb.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests
* Headlamp Bulb Replacement (Envoy, Rainier) Headlamp Bulb Replacement (TrailBlazer)
* Headlamp Switch Replacement
* Turn Signal Multifunction Switch Replacement