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Interior Backlighting Does Not Dim

WARNING: This page is about a different car, the 2007 Cadillac STS. However, it is still accessible from the selected car via links, so may be relevant.
Interior Backlighting Does Not Dim

Step Action 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? Go to Step 2  Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2 Are all of the interior backlighting lamps always ON? Go to Step 3  Go to Step 8 
3
  1. Install a scan tool.
  2. With a scan tool, observe the Dimming Pot A/D parameter in the IPM inputs data list.
  3. Move the instrument panel (I/P) dimmer switch from minimum to maximum.
Does the scan tool indicate that the Dimming Pot A/D parameter is changing when the I/P dimmer switch is moved?
Go to Step 4  Go to Step 6 
4 With a scan tool, observe the Park Lamp Switch status in the IPM inputs data list.
Does the scan tool display ON?
Go to Step 5  Go to Step 7 
5 With a scan tool, observe the Ambient Light Status parameter in the lighting data list.
Does the scan tool display DARK?
Go to Step 10  Go to Twilight Sentinel Inoperative 
6 Test the I/P dimmer switch low reference circuit for an open, high resistance, or 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 9 
7 Test the park lamp relay control circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 14  Go to Step 10 
8 Inspect for poor connections at the harness connector of the appropriate control module of the inoperative backlight lamp. 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 11 
9 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 14  Go to Step 12 
10 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 14  Go to Step 13 
11 Replace the appropriate control module for the inoperative backlight lamp. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.
Did you complete the replacement?
Go to Step 14  -
12 Replace the I/P lamps dimmer switch. Refer to Instrument Panel Dimmer Switch Replacement .
Did you complete the replacement?
Go to Step 14  -
13 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 14  -
14 Operate the system in order to verify the repair.
Did you correct the condition?
System OK Go to Step 2