GM Service Manual Online
For 1990-2009 cars only

Object Number: 613966  Size: MF
Handling ESD Sensitive Parts Notice
Lighting Systems Components
Headlamp Relay, BCM and Headlamp Switch

Circuit Description

The Body Control Module (BCM) uses the ambient light sensor signal to control the automatic headlamp control system. The ambient light sensor is connected directly to the BCM through the control ambient light signal circuit and the headlamp auto control ambient light ground reference circuit.

Conditions for Running the DTC

The DRL feature or the automatic headlamp control feature must be enabled.

Conditions for Setting the DTC

    • The BCM detects a short to ground in the headlamp auto control ambient light ground reference circuit.
    • The condition above must be present for more than 0.3 seconds.

Action Taken When the DTC Sets

    • The BCM stores DTC B2647 in memory.
    • The SERVICE VEHICLE SOON telltale will illuminate
    • The headlamps, park lamps, tail lamps and marker lamps will illuminate to night intensity.

Conditions for Clearing the DTC

    • The BCM no longer detects a short to ground in the ambient light sensor signal circuit.
    • A history DTC B2647 will clear after 100 consecutive ignition cycles if the condition for the malfunction is no longer present.
    • Use a scan tool in order to clear the DTC.

Diagnostic Aids

    • If the DTC B2647 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections .
    • The following conditions may cause an intermittent malfunction to occur:
       - An intermittent short to ground in the ambient light signal circuit
       - The ambient light sensor is intermittently shorted to ground internally
       - The BCM is intermittently shorted to ground internally

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. Tests for the proper operation of the circuit in the high voltage range.

  2. Tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.

Step

Action

Value(s)

Yes

No

1

Did you perform the Lighting System 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, observe the ambient light sensor parameter in the Body Control Module (BCM) data list.

Does the scan tool indicate that the ambient light sensor input parameter is within the specified range?

0.25 -2.5 Volts

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the ambient light sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the ambient light sensor parameter in the Body Control Module (BCM) data display - data list.

Does the scan tool indicate that the ambient light sensor input parameter is greater than the specified value?

2.5 Volts

Go to Step 4

Go to Step 5

4

  1. Turn OFF the ignition.
  2. Connect a 3 amp fused jumper wire between the signal circuit of the ambient light sensor and the ground circuit of the ambient light sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the ambient light sensor input parameter.

Does the scan tool indicate that the ambient light sensor input parameter is less than the specified value?

0.25 Volts

Go to Step 8

Go to Step 6

5

Test the signal circuit of the ambient light sensor 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 12

Go to Step 9

6

Test the signal circuit of the ambient light sensor for a short to voltage, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 7

7

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

Did you find and correct the condition?

--

Go to Step 12

Go to Step 9

8

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 in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

9

Inspect for poor connections at the harness connector of the Body Control Module. 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 12

Go to Step 11

10

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

Did you complete the replacement?

--

Go to Step 12

--

11

Important: Perform the set up procedure for the body control module.

Replace the BCM. Refer to Body Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12

  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