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With RPO Code T4Q




DTC B2600 (With T4Q)

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

DTC Descriptor
DTC B2600 00
- Daytime Running Lamp Control Circuit

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 V depending on outside lighting conditions. The body control module (BCM) provides a 5 V 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.

Conditions for Running the DTC

* Battery voltage must be between 9-16 V.
* Headlamp switch in AUTO position.

Conditions for Setting the DTC

The BCM detects a short to voltage in the DRL resistor control circuit.

Action Taken When the DTC Sets

* The Service Headlamp Assembly indicator is commanded ON.
* The BCM will disable the DRL resistor control circuit.

Conditions for Clearing the DTC

* The condition responsible for setting the DTC no longer exists.
* A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

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 (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

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.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Headlamp Replacement (Cadillac Escalade) (Headlamp Replacement (Cadillac Escalade))Headlamp Replacement (GMC Yukon Two-mode Hybrid) (Headlamp Replacement (GMC Yukon Two-mode Hybrid))Headlamp Replacement (Yukon) (Headlamp Replacement (Yukon))Headlamp Replacement (Chevrolet Tahoe Two-mode Hybrid) (Headlamp Replacement (Chevrolet Tahoe Two-mode Hybrid))Headlamp Replacement (Tahoe, Avalanche) (Headlamp Replacement (Tahoe, Avalanche))Headlamp Replacement (Escalade PLATINUM) (Headlamp Replacement (Escalade Platinum))Headlamp Replacement (Non HID) (Headlamp Replacement (Non HID))
* Underhood Electrical Center or Junction Block Replacement (Underhood Electrical Center or Junction Block Replacement)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming

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