GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 4WD 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC B2620

Circuit Description

The body control module (BCM) directly sends a 5-volt reference voltage to the instrument panel (I/P) dimmer switch which is then adjusted based on the I/P dimmer switch position and returned as a dimming return and dimming input to the BCM. The BCM uses this signal to directly control the desired level of instrument panel illumination lamp intensity.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2620 Display Dimming Pulse Width Modulation (PWM) Input Circuit

Conditions for Running the DTC

The battery voltage must be between 9-16 volts.

Conditions for Setting the DTC
    • The BCM monitors a dimming return voltage of less than 4.5 volts.
    • The BCM monitors a 5-volt reference voltage of greater than 5.5 volts.
    • The BCM monitors dimming input voltage of equal or greater than 4.86 volts for more than 1 second.

Action Taken When the DTC Sets

The instrument panel illumination lamp dimming will not operate.

Conditions for Clearing the MIL/DTC
    • This DTC will clear on current status after the condition for setting the fault is corrected.
    • A history DTC will clear after 100 consecutive ignition cycles without a fault present.
    • History and current DTCs can be cleared using a scan tool.

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. This step tests for a high resistance or an open in the dimming return circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Interior Lights Dimming 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
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With the scan tool, observe the Dimming Input data parameter in the body control module (BCM) Data List.

Does the scan tool indicate that the Dimming Input data parameter is within the specified range?

4.5-5.5 V

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the headlamp switch.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the Dimming Input data parameter.

Does the scan tool indicate that the Dimming Input data parameter is less than the specified value?

4.5 V

Go to Step 4

Go to Step 10

4

  1. Turn OFF the ignition.
  2. Connect a 3-amp fused jumper wire between the 5-volt reference circuit of the instrument panel (I/P) dimmer/headlamp switch and the dimming input circuit of the I/P dimmer/headlamp switch.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the Dimming Input data parameter.

Does the scan tool indicate that the Dimming Input data parameter is greater than the specified value?

4.86 V

Go to Step 5

Go to Step 8

5

  1. Disconnect the fused jumper wire.
  2. Measure the voltage between the 5-volt reference circuit of the I/P dimmer/headlamp switch and the dimming return circuit of the I/P dimmer/headlamp switch.

Does the voltage measure less than the specified value?

5.15 V

Go to Step 6

Go to Step 7

6

  1. Turn OFF the ignition.
  2. Disconnect the negative battery cable.
  3. Measure the resistance from the dimming return circuit of the I/P dimmer/headlamp switch to a good ground.

Does the resistance measure less than the specified value?

5 ohms

Go to Step 12

Go to Step 11

7

Test the 5-volt reference circuit of the I/P dimmer/headlamp switch for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 13

8

Test the 5-volt reference circuit of the I/P dimmer/headlamp switch 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 16

Go to Step 9

9

Test the dimming input circuit of the I/P dimmer/headlamp switch 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 16

Go to Step 13

10

Test the dimming input circuit of the I/P dimmer/headlamp switch for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 13

11
  1. Disconnect the body control module.
  2. Test the dimming return circuit of the I/P dimmer/headlamp switch for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 13

12

Inspect for poor connections at the harness connector of the I/P dimmer/headlamp switch. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 14

13

Inspect for poor connections at the harness connector of the BCM. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 15

14

Replace the headlamp switch. Refer to Headlamp Switch Replacement .

Did you complete the replacement?

--

Go to Step 16

--

15

Replace the BCM. Refer to Control Module References .

Did you complete the replacement?

--

Go to Step 16

--

16
  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