GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Envoy - 4WD 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC B2600

Circuit Description

The BCM monitors the voltage on the headlamp dimmer switch high beam signal circuit in order to determine the amount of voltage present in the high beam circuit. The BCM will set DTC B2600 when either of the following conditions exists:

    • The headlamp dimmer switch high beam signal circuit is shorted to battery voltage.
    • The headlamp dimmer switch high beam signal circuit is shorted to ground and the daytime running lamps (DRL) are requested ON.

The BCM monitors the headlamp dimmer switch high beam signal circuit in order to determine if either of the following lamps are ON:

    • The DRL
    • The high beams

Conditions For Running the DTC
    • The system voltage must be between 9.0 and 16.0 volts.
    • The ignition switch must be in the RUN position.
    • The park brake must be released.
    • The headlamp switch must be in the OFF or AUTO position.
    • Daylight conditions must exist.

Conditions for Setting the DTC
    • The BCM commands the DRL relay ON under the following conditions:
       - An open exists in the headlamp dimmer switch high beam signal circuit.
       - The headlamp dimmer switch high beam signal circuit is shorted to ground.
       - The headlamp dimmer switch high beam signal circuit is shorted to battery voltage.
    • The system voltage is between 9.0-16.0 volts.
    • The above conditions exist for 0.5 seconds.

Action Taken When the DTC Sets

The daytime running lamps will not operate correctly.

Conditions for Clearing the MIL/DTC
    • A current DTC will clear after the next ignition cycle that does not contain a fault.
    • A history DTC will clear after 100 consecutive ignition cycles without a fault present.
    • History and current DTCs may be cleared using a scan tool.

Diagnostic Aids
    • Always diagnose the first DTC that is listed on the scan tool.
    • Verify that the scan tool displays DTC B2600 as a current code before you perform diagnostics.
    • The scan tool can command the DRL relay ON. This test is located under the Special Functions menu.
    • Compare the state of the DRL relay as indicated on the scan tool with the actual state of the DRL relay.
    • Test for an inoperative high beam headlamp.
    • Use an incandescent shop light or a flashlight. Direct the lighting toward the light sensor in order to force the light sensor into a LIGHT state.
    • Inspect for loose or poor connection at all of the 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 the headlamp dimmer switch high beam signal circuit for a short to ground.

    2.

  2. This step tests the headlamp dimmer switch high beam signal circuit for a short to battery voltage.

    3.

  3. This step tests the power feed circuit for the HDLP PWR relay.

    4.

  4. This step tests the HDLP PWR relay.

    5.

  5. This step tests the power feed circuit to the HDLP fuses.

    6.

  6. This step test the DRL relay.

    7.

  7. This step tests the headlamp dimmer switch high beam signal circuit for an open or short to voltage.

Step

Action

Value(s)

Yes

No

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

1

Did you perform the Lighting Systems Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - Lighting Systems

2

Use a scan tool in order to test for current DTCs.

Is DTC B2647 or DTC B2648 set as a current code?

--

Go to DTC B2647 or DTC B2648

Go to Step 3

3

Does DTC B2577 set as a current code?

--

Go to DTC B2577

Go to Step 4

4

  1. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  2. Turn the ignition switch to the RUN position.
  3. Verify that the headlamp switch is in the AUTO/OFF position.
  4. Place the beam selector switch in the LOW beam position.
  5. Use a digital multimeter (DMM) in order to measure the voltage at the BCM connector for the headlamp dimmer switch high beam signal circuit input. Refer to Circuit Testing in Wiring Systems.

Does the DMM display less than the specified percentage of battery voltage?

25 percent

Go to Step 6

Go to Step 5

5

  1. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  2. Turn the ignition switch to the RUN position.
  3. Verify that the headlamp switch is in the AUTO/OFF position.
  4. Place the beam selector switch in the LOW beam position.
  5. Use a DMM in order to measure the voltage from the BCM connector for the headlamp dimmer switch high beam signal circuit input. Refer to Circuit Testing in Wiring Systems.

Does the DMM display more than the specified percentage of battery voltage?

75 percent

Go to Step 20

Go to Step 17

6

  1. Turn the ignition switch to the OFF position.
  2. Remove the HDLP PWR relay.
  3. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  4. Place the headlamp switch in the HEADLAMP position.
  5. Turn the ignition switch to the RUN position.
  6. Place the beam selector switch in the LOW beam position.
  7. Using the DMM, test for an open to the underhood fuse block in the battery positive voltage or headlamp relay coil supply voltage circuits. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Does the voltage equal the specified value on both circuits?

0 V

Go to Step 18

Go to Step 7

7

  1. Ensure that the HDLP PWR relay is removed.
  2. Use a fused jumper in order to energize the HDLP PWR relay.
  3. Connect the coil battery positive voltage supply terminal of the HDLP PWR relay to battery voltage.
  4. Connect the coil ground terminal of the HDLP PWR relay to ground.
  5. Use a DMM in order to measure between the HDLP PWR relay battery positive voltage terminal and headlamp supply voltage terminal of the relay. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-5 ohms

Go to Step 8

Go to Step 25

8

  1. Turn the ignition switch to the OFF position.
  2. Install the HDLP PWR relay.
  3. Use the scan tool in order to ensure that the vehicle is in the LIGHT state.
  4. Place the headlamp switch in the HEADLAMP position.
  5. Turn the ignition switch to the RUN position.
  6. Place the beam selector switch in the LOW beam position.
  7. Remove both of the headlamp fuses.
  8. Use a DMM in order to measure the supply voltage from the LT HDLP fuse in the underhood fuse block to ground.
  9. Use a DMM in order to measure the supply voltage from the RT HDLP fuse in the underhood fuse block to ground. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Does the DMM display battery voltage on both circuits?

B+

Go to Step 9

Go to Step 23

9
  1. Turn the ignition switch to the OFF position.
  2. Reconnect all of the connectors and the wiring.
  3. Use the scan tool in order to ensure that the vehicle is in the LIGHT state.
  4. Disconnect the DRL relay.
  5. Turn the ignition switch to the RUN position.
  6. Place the headlamp switch in the HEADLAMP position.
  7. Place the beam selector switch in the LOW beam position.
  8. Use a DMM in order to measure the voltage from the underhood fuse block cavity for the DRL relay left headlamp low beam supply voltage circuit to ground (cavities 85, 30 and 87A). Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Does the DMM display battery voltage on these circuits?

B+

Go to Step 10

Go to Step 23

10

  1. Ensure that the DRL relay is removed.
  2. Use a fused jumper wire in order to energize the DRL relay.
  3. Connect terminal 86 of the DRL relay to the battery voltage.
  4. Connect terminal 85 of the DRL relay to ground.
  5. Use a DMM in order to measure between DRL relay terminal 30 and terminal 87. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-5 ohms

Go to Step 11

Go to Step 26

11
  1. Verify that the DRL relay is still removed.
  2. Use a DMM in order to measure between the underhood fuse block cavities for the DRL relay left headlamp high beam supply voltage circuit and the underhood fuse block connector C4 terminal A2. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-5 ohms

Go to Step 12

Go to Step 23

12
  1. Remove the HDLP GND relay.
  2. Use a DMM in order to measure between the underhood fuse block cavities for the DRL relay control circuit and the body relay block HDLP GND relay headlamp dimmer switch high beam signal circuit. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-5 ohms

Go to Step 13

Go to Step 21

13
  1. Disconnect the BCM connector C1.
  2. Connect a test lamp to a good power source.
  3. Backprobe the underhood fuse block cavity for the DRL relay headlamp dimmer switch high beam signal circuit. Refer to Electrical Center Identification Views and Circuit Testing in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 22

Go to Step 14

14
  1. Verify that the circuits for the LT HDLP fuse and the RT HDLP fuse are not shorted to ground.
  2. Repair the circuits as needed.
  3. Replace the LT HDLP fuse and the RT HDLP fuse in the underhood fuse block as necessary. Ensure that the fuse is a 10A fuse.
  4. Repair as needed. Refer to Wiring Repairs in Wiring Systems.

Was a repair needed?

--

Go to Step 27

Go to Step 15

15

  1. Reconnect all of the components and the wiring.
  2. Place the headlamp switch in the AUTO/OFF position.
  3. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  4. Place the transmission in NEUTRAL (or any position except PARK).
  5. Turn the ignition switch from the OFF position to the RUN position.
  6. Ensure that the beam selector switch is in the LOW position.
  7. Use a DMM in order to test the headlamp dimmer switch high beam signal circuit for an open or short to voltage from the underhood fuse block connector C4 terminal B2 to both high beam lamps (including an open high beam lamp). Refer to Circuit Testing and Electrical Center Identification Views in Wiring Systems.
  8. If the percentage of battery voltage is not within the specified range, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair needed?

25-75 percent

Go to Step 27

Go to Step 16

16
  1. Place the headlamp switch in the AUTO/OFF position.
  2. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  3. Turn the ignition switch from the OFF position to the RUN position.
  4. Verify that the beam selector switch is in the LOW position.
  5. Use a DMM in order to test the headlamp dimmer switch high beam signal circuit for an open or a short to voltage from the underhood fuse block connector C1 terminal A 3 to the splice. Refer to Circuit Testing and Electrical Center Identification Views in Wiring Systems.
  6. Use the DMM in order to test the headlamp dimmer switch high beam signal circuit for an open or short to voltage from the splice to connector C211 terminal E11, including the HIGH/LOW beam switch. Refer to Circuit Testing in Wiring Systems.
  7. Use the DMM in order to test the headlamp dimmer switch high beam signal circuit for an open or short to voltage from the splice to the BCM connector C2 terminal B5. Refer to Circuit Testing in Wiring Systems.
  8. Repair as needed.

Was a repair needed?

--

Go to Step 27

Go to Step 24

17
  1. Use the scan tool in order to verify that the vehicle is in the LIGHT state.
  2. Turn the ignition switch to the OFF position.
  3. Disconnect the BCM connector C2.
  4. Remove the headlamp dimmer switch high beam signal circuit from the BCM connector C2.
  5. Use a DMM in order to measure the voltage from the BCM harness on the headlamp dimmer switch high beam signal circuit to ground. Refer to Circuit Testing in Wiring Systems.

Is the percentage of battery voltage within the specified range?

25-75 percent

Go to Step 24

Go to Step 20

18

Repair the open the circuits to the underhood fuse block for the HDLP PWR relay battery positive voltage or headlamp relay coil supply voltage circuits, including the headlamp switch. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 27

--

19

Repair the open or the short to voltage in the headlamp dimmer switch high beam signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 27

--

20

Repair the short to voltage in the headlamp dimmer switch high beam signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 27

--

21

Repair the open in the headlamp dimmer switch high beam signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 27

--

22

Repair the short to ground in the headlamp dimmer switch high beam signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 27

--

23

Replace the underhood fuse block.

Is the repair complete?

--

Go to Step 27

--

24
  1. Reconnect all of the connectors and the wiring.
  2. Replace the BCM. Refer to Body Control Module Replacement in Body Control System.
  3. Program the correct calibrations into the BCM. Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control System.
  4. Perform the learn procedure.

Is the repair complete?

--

Go to Step 27

--

25

Replace the HDLP PWR relay.

Is the repair complete?

--

Go to Step 27

--

26

Replace the DRL relay.

Is the repair complete?

--

Go to Step 27

--

27
  1. Reconnect all of the connectors and the wiring.
  2. Clear the DTCs.

Is the repair complete?

--

Go to Diagnostic System Check - Lighting Systems

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