GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The body control module (BCM) controls the headlamp relay by grounding the control circuit of the headlamp.

Conditions for Running the DTC

Voltage supplied to the module is in the normal operating voltage range (approximately 9-16 volts).

Conditions for Setting the DTC

    • The BCM detects a short to ground or an open in the control circuit of the headlamp relay coil or the low beam headlamp control circuit relay when the BCM is not energizing the headlamp relay.
    • The condition is present for more than 3 ms.

Action Taken When the DTC Sets

    • The BCM stores DTC B2577 in memory.
    • The SERVICE VEHICLE SOON telltale illuminates.
    • With the ignition in the RUN position, the BCM deactivates the daytime running lamps and turns on the park lamps.

Conditions for Clearing the DTC

    • A current DTC B2577 clears if the BCM no longer detects a short to ground or an open in the control circuit of the headlamp relay control circuit of the low beam relay headlamp control circuit.
    • A history DTC B2577 clears after 100 consecutive ignition cycles.
    • The BCM receives the Clear Code commend from the scan tool.

Diagnostic Aids

    • If the DTC B2577 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • An intermittent short to ground or an open in the following locations may cause an intermittent malfunction to occur:
       - The low beam headlamp relay control circuit
       - The headlamp relay control circuit
       - The headlamp relay
       - The BCM

Step

Action

Yes

No

Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Lighting Systems

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the HDLP low beam relay ON and OFF.

Does the HDLP low-beam relay turn ON and OFF with each command?

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the HDLP low beam relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the coil side supply circuit of the HDLP Low-beam relay with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 4

Go to Step 10

4

  1. Connect a test lamp between the control circuit of the HDLP low beam relay and the coil side supply circuit of the HDLP low beam relay.
  2. With a scan tool, command the HDLP low-beam relay ON and OFF.

Does the test lamp turn ON and OFF with each command?

Go to Step 8

Go to Step 5

5

Does the test lamp remain illuminated with each command?

Go to Step 7

Go to Step 6

6

Test the control circuit of the HDLP low beam relay and the headlamp control circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

7

Test the control circuit of the HDLP low beam relay and the headlamp relay control circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

8

Inspect for a poor connection at the HDLP low-beam relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

9

Inspect for a high resistance at the harness connector of the body control module (BCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Replace the underhood fuse block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.

Did you complete the repair?

Go to Step 13

--

11

Replace the HDLP low beam relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 13

--

12

Important: Perform the set up procedure for the body control module (BCM). Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control Systems.

Replace the BCM. Refer to Body Control Module Replacement in Body Control System.

Did you complete the replacement?

Go to Step 13

--

13

  1. Use the scan tool in order to clear the DTCs .
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK