GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • 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

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Flash to Pass Switch Signal

1

1

1

--

Headlamp High Beam Relay Control

B2580 06

B2580 06

B2580 01

--

Headlamp Low Beam Relay Control

1

1

1

--

Headlamp Switch High Beam Signal

1

1

1

--

Headlamp Switch Headlamps On Signal

1

1

1

--

Headlamp Switch Headlamps Off Signal

1

1

1

--

LF High Beam Headlamp Voltage Supply

1

1

1

--

LF Low Beam Headlamp Voltage Supply

1

1

1

--

RF High Beam Headlamp Voltage Supply

1

1

1

--

RF Low Beam Headlamp Voltage Supply

1

1

1

--

LF Headlamp Ground

--

1

1

--

RF Headlamp Ground

--

1

1

--

Turn Signal/Multifunction Switch Ground

--

1

1

--

1. Headlamps Malfunction

Circuit/System Description

The ambient light sensor is a light sensitive transistor that varies the voltage signal to the HVAC control module. The HVAC control module sends a signal to the body control module (BCM) via class 2 serial data commanding the BCM to apply ground to the headlamp low beam relay control circuit. This energizes the low beam relay, closing the switched side and applies battery voltage to the LEFT and RIGHT LOW BEAM fuses. Battery voltage is applied from the low beam fuses, through the low beam voltage supply circuits to the ballast in the headlamp assembly illuminating the headlamps. The BCM also sends a message via class 2 serial data to the instrument panel cluster (IPC) requesting the lights on indicator be illuminated. For more detailed information concerning headlamp operation refer to Exterior Lighting Systems Description and Operation .

Diagnostic Aids

This diagnostic procedure assumes that the headlamps operate as described in Exterior Lighting Systems Description and Operation .

Reference Information

Schematic Reference

Instrument Cluster Schematics

Connector End View Reference

Displays and Gages Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Output Controls
    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Testing

Ignition ON, with a scan tool perform the display test for the IPC and observe the lights on indicator.

If the lights on indicator illuminates during the test, replace the BCM.
If the lights on indicator does not illuminate during the test or remains illuminated at all times, replace the IPC.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for BCM or IPC replacement, setup, and programming