GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The body control module (BCM) uses the ambient light sensor signal to control the daytime running lamps (DRL). The ambient light sensor is connected directly to the BCM.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2648 Ambient Light Sensor Circuit High

Conditions for Running the DTC

    • The system voltage must be between 9-16 volts.
    • The ignition switch must be in the RUN position.
    • The park brake must be released.
    • The headlamp switch must be in the AUTO position.
    • The DRL feature must be enabled.

Conditions for Setting the DTC

    • When the ambient light sensor input voltage is greater than 4.78 volts
    • The courtesy lamp relay coil control circuit is shorted to the battery.
    • The above conditions must be met for 0.5 seconds.

Action Taken When the DTC Sets

    • The DRL will remain ON.
    • The BCM stores DTC B2648 in memory.
    • Exterior lamps illuminate to low light intensity.

Conditions for Clearing the MIL/DTC

    • A current DTC B2648 clears when the BCM no longer detects the fault in the signal circuit or the low reference circuit of the DRL ambient light sensor.
    • A history DTC B2648 clears after 100 consecutive ignition cycles.
    • The BCM receives the Clear Code command from the scan tool.

Diagnostic Aids

    • If the DTC B2648 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections .
    • An intermittent short to voltage or an open in the following locations may cause an intermittent malfunction to occur:
       - The low reference circuit of the DRL ambient light sensor
       - The sensor circuit of the DRL ambient light sensor
       - The ambient light sensor
       - The BCM

Step

Action

Value(s)

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 - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

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 parameter is within the specified range?

0.2-4.7 V

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.

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

0.02 V

Go to Step 4

Go to Step 9

4

  1. Turn OFF the ignition.
  2. Connect a 3-amp fused jumper wire between the low reference circuit of the ambient light sensor and the signal 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 parameter.

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

4.7 V

Go to Step 5

Go to Step 7

5

  1. Disconnect the fused jumper wire.
  2. Measure the voltage between the low reference circuit of the ambient light sensor and the signal circuit of the ambient light sensor.

Does the voltage measure less than the specified value?

5.25 V

Go to Step 10

Go to Step 6

6

Test the low reference circuit of the ambient light sensor for a short to battery positive voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

7

Test the low reference circuit of the ambient light sensor for a short to ground, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 14

Go to Step 8

8

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

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

9

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

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

10

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 14

Go to Step 12

11

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 14

Go to Step 13

12

Replace the ambient light sensor. Refer to Ambient Light Sensor Replacement .

Did you complete the replacement?

--

Go to Step 14

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13

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

Did you complete the replacement?

--

Go to Step 14

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14

  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