GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2002 🢒 DeVille 🢒 Body and Accessories 🢒 Lighting Systems 🢒 DTC B2556

Circuit Description

The interior lamp rheostat is a potentiometer. The Instrument Panel Integration Module (IPM) determines the control setting based on the voltage from the interior lamp rheostat. The IPM monitors the voltage in the instrument panel lamps dimmer switch signal circuit every 150 milliseconds.

Conditions for Running the DTC

The ignition switch is in the ON position.

Conditions for Setting the DTC

The interior lamp rheostat is less than 12 Counts or greater than 244 Counts on 3 consecutive tests.

Action Taken When the DTC Sets

Set dimming level to full brightness for all displays.

Conditions for Clearing the DTC

The IPM detects that the interior lamp rheostat is between 12 and 244 Counts.

Test Description

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

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

    2.

  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.

    3.

  3. Tests for a short to ground in the 5 volt reference circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Interior Lights Dimming Schematics

1

Did you perform the Lighting Systems Diagnostic System Check?

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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. Position the IP dimmer switch in approximately the center position.
  4. With a scan tool, observe the Interior Dim Pot data parameter in the Instrument Panel Module data list.

Does the scan tool indicate that the Interior Dim Pot data parameter is within the specified range?

12-244 counts

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

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 Interior Dim Pot data parameter.

Does the scan tool indicate that the Interior Dim Pot data parameter is greater than the specified value?

244 counts

Go to Step 4

Go to Step 8

4

  1. Turn OFF the ignition.
  2. Connect a 3 amp fused jumper wire between the signal circuit of the IP dimmer switch and the sensors return circuit of the IP dimmer switch.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the Interior Dim Pot data parameter.

Does the scan tool indicate that the Interior Dim Pot data parameter is less than the specified value?

12 counts

Go to Step 5

Go to Step 9

5

  1. Turn OFF the ignition.
  2. Disconnect the fused jumper wire.
  3. Connect a 3 amp fused jumper wire between the 5 volt reference circuit of the IP dimmer switch and the signal circuit of the IP dimmer switch.
  4. Turn ON the ignition, with the engine OFF.
  5. With a scan tool, observe the Interior Dim Pot data parameter.

Does the scan tool indicate that the Interior Dim Pot data parameter is greater than the specified value?

244 counts

Go to Step 7

Go to Step 6

6

Test the 5 volt reference circuit of the IP dimmer switch 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 15

Go to Step 12

7

Test the 5 volt reference circuit of the IP dimmer switch 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 15

Go to Step 11

8

Test the signal circuit of the IP dimmer switch 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 15

Go to Step 12

9

Test the signal circuit of the IP dimmer switch 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 15

Go to Step 10

10

Test the sensors return circuit of the IP dimmer switch 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 15

Go to Step 12

11

Inspect for poor connections at the harness connector of the headlamp switch. 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 15

Go to Step 13

12

Inspect for poor connections at the harness connector of the instrument panel integration module (IPM). 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 15

Go to Step 14

13

Replace the headlamp switch. Refer to Headlamp Switch Replacement .

Did you complete the replacement?

--

Go to Step 15

--

14

Important:: Perform the setup procedure for the Instrument Panel Integration Module (IPM). Refer to Body Control Module (BCM) Programming/RPO Configuration .

Replace the IPM. Refer to Instrument Panel Module Replacement in Body Control System.

Did you complete the replacement?

--

Go to Step 15

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15
  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