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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 DeVille 🢒 Body 🢒 Lighting Systems 🢒 DTC B2556

Circuit Description

The instrument panel (I/P) dimmer switch is a potentiometer. The instrument panel module (IPM) determines the control setting based on the voltage from the I/P dimmer switch. The IPM sends out a class 2 message to the dash integration module (DIM). The intensity of the of the interior backlighting shall be adjustable when ever the park lamps are on. The intensity of the vacuum florescent displays shall only be adjustable when it is night and the park lamps are active.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2556 Passenger Compartment Lamp Control Circuit Range/Performance

Conditions for Running the DTC

The IPM is powered.

Conditions for Setting the DTC

The interior lamp rheostat voltage is less than 0.09 volt or greater than 4.80 volts.

Action Taken When the DTC Sets

Set dimming level to 100 percent, full brightness for all displays.

Conditions for Clearing the DTC

The IPM does not detect a voltage that is less than or greater than the predetermined value during the diagnostic self test.

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

Connector End View Reference: Master Electrical Component List in Wiring 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 Interior Dim Pot parameter in the instrument panel module (IPM) Open/Short data list.
  4. Move the instrument panel (I/P) dimmer switch from the minimum position to the maximum position.

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

0.09-4.80 V

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 I/P 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 greater than the specified value?

4.80 V

Go to Step 4

Go to Step 8

4

  1. Turn OFF the ignition.
  2. Connect a 3-amp fused jumper wire between the instrument panel lamps dimmer switch signal circuit of the I/P dimmer switch and the low reference circuit of the I/P 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?

0.09 V

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 I/P dimmer switch and the instrument panel lamps dimmer switch signal circuit of the I/P 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?

4.80 V

Go to Step 7

Go to Step 6

6

Test the 5-volt reference circuit of the I/P 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 I/P 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 instrument panel lamps dimmer switch signal circuit of the I/P 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 instrument panel lamps dimmer switch signal circuit of the I/P 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 low reference circuit of the I/P 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 I/P dimmer 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 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 I/P dimmer switch. Refer to Headlamp Switch Replacement .

Did you complete the replacement?

--

Go to Step 15

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14

Replace the IPM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

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