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

Daytime Running Lamp Relay B+

1

1

--

--

Headlamp LO/HID Relay B+

1

1

--

--

High Beam Relay B+

1

1

--

--

Ambient Light Sensor Signal Circuit

B2645 03

B2645 07

B2645 07

--

Headlamp Flash to Pass Signal

4

4

4

--

Headlamp Dimmer Switch Signal

4

4

4

--

Daytime Running Lamp Relay Control

3

3

B2600 00

--

Headlamp Low Beam Control

1

1

1

--

Headlamp Low Beam Relay Control

1

1

B2575 00

--

Headlamp Switch Off Control

B257A 00

2

2

--

Headlamp Switch On Control

B257A 00

1

1

--

Headlamp High Beam Control

1

1

1

--

Headlamp High Beam Relay Control

1

1

B2580 00

--

Park Lamp Switch On Control

B257A 00

B257A 00

2

--

Ambient Light Sensor Ground Circuit

--

B2645 07

--

--

Headlamp Switch Ground

--

2

--

--

Turn Signal/Multifunction Switch Malfunction

--

4

--

--

  1. Headlamp Malfunction
  2. Headlamp Switch Malfunction
  3. Daytime Running Lamps Malfunction
  4. Turn Signal/Multifunction Switch Malfunction

Circuit/System Description

Daytime Running Lamps without HID

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 HVAC control module provides a low reference ground and 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 low beam headlamps ON. Any function or condition that turns on the headlamps will cancel DRL operation.

Daytime Running Lamps with HID

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 HVAC control module provides a low reference ground and 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 relay control circuits. When the BCM applies ground to the relay control circuit, the DRL relay coil energizes causing the switch contacts to close. With the DRL relay switch contacts closed, battery voltage flows through the DRL fuse to the left and right DRL lamps. Any function or condition that turns on the headlamps 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

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

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

Scan Tool Reference

Control Module References 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

Circuit/System Testing

Daytime Running Lamps Malfunction -- Z75/X88

  1. Ignition OFF, disconnect the harness connector X4 at the underhood fuse block.
  2. Connect a test lamp between the daytime running lamps relay control circuit terminal H4 and B+.
  3. Ignition ON, command the DRL ON and OFF with a scan tool. The test lamp should turn ON and OFF between the commanded states.
  4. 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.
  5. Ignition OFF, reconnect the harness connector X4 at the underhood fuse block.
  6. Disconnect the harness connector at the appropriate daytime running lamp.
  7. Test for less then 5 ohms between the gound circuit listed below and ground.
  8. • ground circuit terminal B -- Z75
    • ground circuit terminal C -- X88
         ⇒If less than the specified range, test the ground circuit for an open/high resistance.
  9. Ignition OFF, connect a test lamp between the control circuit terminal A and ground.
  10. Ignition ON, command the DRL ON and OFF with a scan tool. The test lamp should turn ON and OFF.
  11. If the test lamp is always ON, test the contol circuit for a short to voltage. If the control circuit tests normal, replace the underhood fuse block.
    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, and the DRL fuse is not open, replace the underhood fuse block.
  12. If all circuits test normal, replace the inoperative daytime running lamp.

Daytime Running Lamps Malfunction -- Z88

  1. Ignition OFF, disconnect the harness connector X4 at the underhood fuse block.
  2. Connect a test lamp between the daytime running lamps relay control circuit terminal H4 and B+.
  3. Ignition ON, command the DRL ON and OFF with a scan tool. The test lamp should turn ON and OFF between the commanded states.
  4. 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.
  5. Ignition OFF, reconnect the harness connector X4 at the underhood fuse block.
  6. Disconnect the harness connector at the appropriate headlamp.
  7. Test for less then 5 ohms between the gound circuit terminal B and ground.
  8. If more than the specified range, test the ground circuit for an open/high resistance.
  9. Ignition OFF, connect a test lamp between the control circuit terminal A and ground.
  10. Ignition ON, command the DRL ON and OFF with a scan tool. The test lamp should turn ON and OFF.
  11. If the test lamp is always ON, test the contol circuit for a short to voltage. If the control circuit tests normal, replace the underhood fuse block.
    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, and the DRL fuses are not open, replace the underhood fuse block.
  12. If all circuits test normal, replace the inoperative headlamp.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Ambient Light Sensor Replacement
    •  Daytime Running Lamp Bulb Replacement
    •  Daytime Running Lamp Bulb Replacement
    •  Headlamp Bulb Replacement
    •  Underhood Electrical Center or Junction Block Replacement
    •  Control Module References for BCM replacement, programming, and setup