Daytime Running Lamps Malfunction (With T4Q)
Daytime Running Lamps Malfunction (With T4Q)
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (with HP2) (Initial Inspection and Diagnostic Overview)Diagnostic System Check - Vehicle (without HP2) (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview)provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
The ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The body control module (BCM) provides a 5-volt reference signal to the ambient light sensor. The body control module (BCM) monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights (DRL) or automatic lamp control (ALC) when the headlamp switch is in the AUTO position. In daylight conditions, the BCM will command the DRLs ON by applying ground to the DRL resistor via the DRL resistor control circuit. When the BCM applies ground to the DRL resistor control circuit, the DRL signal circuits to the right and left headlamp assemblies are pulled low. The DRL signal circuits are direct inputs to the right and left headlamp control modules. When the DRL signal circuits are provided with ground, the right and left headlamp control modules respond by turning OFF the low beam headlamps and activating the DRLs. Any function or condition that turns the headlamps ON will cancel DRL operation.
Diagnostic Aids
Important: This diagnostic procedure assumes that the low beam headlamps are operating within specifications. If the low beam headlamps are inoperative, diagnose the low beam headlamp malfunction before proceeding with the following diagnostic.
Reference Information
Schematic Reference
Headlights/Daytime Running Lights (DRL) Schematics (Headlights/Daytime Running Lights (DRL) Schematics)
Connector End View Reference
Component Connector End Views (Connector Views)
Description and Operation
Exterior Lighting Systems Description and Operation (Exterior Lighting Systems Description and Operation)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
Scan Tool Reference
Control Module References (Programming and Relearning)for scan tool information
Circuit/System Verification
Ignition ON, verify with a scan tool the park brake switch parameter displays Released.
If not as specified, refer to Park Brake System Diagnosis (Park Brake System Diagnosis)
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the DRL resistor.
2. Connect a test lamp between the control circuit terminal C and B+.
3. Command the DRL 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.
4. Ignition OFF, connect the harness connector at the DRL resistor.
5. Disconnect the harness connector at the inoperative headlamp assembly.
6. Connect a test lamp between the signal circuit terminal D and B+.
7. Command the DRL 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 signal circuit for a short to ground. If the circuit tests normal, replace the DRL resistor.
• If the test lamp is always OFF, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the DRL resistor.
8. Connect a test lamp between the signal circuit terminal B and B+.
9. Command the DRL 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 signal circuit for a short to ground. If the circuit tests normal, replace the DRL resistor.
• If the test lamp is always OFF, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the DRL resistor.
10. If all circuits test normal, replace the appropriate headlamp control module.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Headlamp Bulb Replacement (Cadillac) (Service and Repair)Headlamp Bulb Replacement (Yukon) (Service and Repair)Headlamp Bulb Replacement (Tahoe, Avalanche) (Service and Repair)
* Control Module References (Programming and Relearning)for BCM or headlamp control module replacement, setup, and programming