GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The body control module (BCM) monitors the ambient light sensor signal circuit in order to determine if the daytime running lights (DRL) or the auto headlights (ALC) should be turned ON in the AUTO mode. When the BCM senses the ambient light sensor voltage is between 1.75-4.9 volts, either the DRL will be commanded ON or the ALC will be commanded ON.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2645 Ambient Light Circuit

Conditions for Running the DTC

The system voltage must be between 9-16 volts.

Conditions for Setting the DTC

    • When the ambient light sensor signal voltage is less than 1.75 volts.
    • When the ambient light sensor signal voltage is more than 4.9 volts.
    • If there is a short to battery positive voltage or a short to ground on the 10-volt reference circuit of the brake pedal position sensor (BPPS).

Action Taken When the DTC Sets

The ALC will remain ON in any ambient light state.

Conditions for Clearing the MIL/DTC

    • This DTC will clear immediately after the condition for setting the fault is corrected.
    • A history DTC will clear after 100 consecutive ignition cycles without a fault present.
    • Use a scan tool in order to clear history and current DTCs.

Step

Action

Yes

No

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

Connector End View Reference: Lighting Systems Connector End Views or Data Communication Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Test for a short to battery positive voltage or a short to ground on the 10-volt reference circuit of the brake pedal position sensor (BPPS). Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 11

Go to Step 3

3

  1. Disconnect the body control module (BCM) connector C1.
  2. Using a test lamp connected to battery positive voltage, probe the ambient light sensor signal circuit.

Does the test lamp illuminate?

Go to Step 5

Go to Step 4

4

Test for a poor connection or an open in the ground circuit of the ambient light sensor. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 11

Go to Step 6

5

Test the ambient light sensor circuit of the ambient light sensor for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 8

6

Test the ambient light sensor signal circuit of the ambient light sensor for a short to battery positive voltage or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 11

Go to Step 7

7

Inspect for poor connections at the harness connector of the ambient light sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 11

Go to Step 9

8

Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 11

Go to Step 10

9

Replace the ambient light sensor. Refer to Daytime Running Lamp (DRL) Ambient Light Sensor Replacement or Sun Load Sensor Replacement .

Did you complete the replacement?

Go to Step 11

--

10

Replace the BCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 11

--

11

  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