GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Blazer - 2WD 🢒 Restraints 🢒 SIR 🢒 DTC B2648

Circuit Description

The body control module (BCM) monitors the courtesy lamp relay coil control 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 1.7-4.7 volts, the DRL will be commanded ON. When the BCM senses the ambient light sensor voltage is between 0.2-1.0 volts, the ALC will be commanded ON.

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.

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 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 immediately clear history and current DTCs.

Diagnostic Aids

Perform a visual inspection for loose or poor connections at all related components. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests for the proper operation of the circuit in the low voltage range.

    2.

  2. This step tests for the proper operation of the circuit in the high voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to ground.

    3.

  3. This step tests for a short to voltage in the 5-volt reference circuit.

    4.

  4. After replacement of the BCM you must calibrate the new module for proper operation.

Step

Action

Value(s)

Yes

No

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

Connector End View Reference: Lighting Systems Connector End Views or Computer/Integrating Systems Connector End Views in Computer/Integrating Systems

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

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 5-volt 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 5-volt 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 5-volt reference circuit of the ambient light sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

7

Test the 5-volt 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 in Wiring Systems.

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 in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

9

Test the signal circuit of the ambient light sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

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 in Wiring Systems.

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 in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 13

12

Replace the ambient light sensor. Refer to Daytime Running Lamp (DRL) 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 in Computer/Integrating Systems 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