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

4

4

--

--

Headlamp High Beam Relay Control

B2580 00, 3

B2580 00, 3

B2580 00, 3

--

Headlamp Low Beam Relay Control

B2575 00, 2

B2575 00, 2

B2575 00, 2

--

Headlamp Dimmer Switch High Beam Signal

3

3

--

--

Headlamp Switch Headlamps ON Signal

B257A 00, 1

B257A 00, 1

B257A 00, 1

--

Headlamp Switch Headlamps OFF Signal

B257A 00, 1

B257A 00, 1

B257A 00, 1

--

LF High Beam Headlamp Voltage Supply

3

3

3

--

LF Low Beam Headlamp Voltage Supply

2

2

2

--

RF High Beam Headlamp Voltage Supply

3

3

3

--

RF Low Beam Headlamp Voltage Supply

2

2

2

--

LF Headlamp Ground

--

2, 3

2, 3

--

RF Headlamp Ground

--

2, 3

2, 3

--

Turn Signal/Multifunction Switch Ground

--

3, 4

--

--

  1. Headlamp Switch Malfunction
  2. Headlamps Malfunction -- Low Beam
  3. Headlamps Malfunction -- High Beam
  4. Headlamps Flash to Pass Malfunction

Circuit/System Description

Ground is provided to the headlamp switch through the switch ground circuit and G202. With the headlamp switch in the ON position, the headlamps ON signal circuit from the body control module (BCM) is pulled low indicating the headlamps ON request. In response to this signal, the BCM will energize either the high or low beam relay illuminating the headlamps. At the same time when the high beam headlamps are ON, the BCM sends a message via GMLAN serial data to the instrument panel cluster (IPC) requesting the IPC to illuminate the high beam indicator. Anytime the headlamps are ON and the ignition is OFF or the driver door is opened, the BCM will also request the IPC to turn on the LIGHTS ON INDICATOR located in the driver information center (DIC).

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 Verification

Ignition ON, turn the headlamp switch to the headlamps ON position and observe the headlamps and the IPC headlamps On indicator. The headlamps and the headlamps On indicator should illuminate.

Circuit/System Testing

Ignition ON, with a scan tool perform the Display Test for the instrument panel cluster (IPC) and observe the lights On indicator.

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

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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