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Discussion Starter #1 (Edited)
Im hoping someone out there can help me... Im stumped

Recently bought a 2008 Cayman S. Discovered a week later my rear license plate is not illuminated.
Im getting VERY little voltage to the lights (hence they wont light up). With car switched "ON" and headlights "ON" I get 0.2V at the rear license harness (grey wire). 0.0V with lights turned "off". I removed the rear bumper and checked at the connector there as well... same thing 0.2V

What other possibilities are there to look at?
Resetting the rear control module?

-All fuses are good (paid special attention to row A, #6,7,&8
-0.2V @ rear harness
-Ground under rear carpet looks good
-Dash lights work fine (seemed to go hand in hand with a bad headlight switch)
-All other lighting works fine
-Cleaned contacts... no change

I appreciate any assistance... right now im limited to driving during daylight hours only.


*also hello to planet 9.... hate to make my first post a "help me" thread... but i searched all over with no resolution in sight

SOLVED: After some trial and error investigating, the rear control modules are smarter than I anticipated. It will not supply 12V to the lights if it doesn't sense any resistance. Therefore, only 0.2V was detected when using a multimeter on the harness. Once bulbs are plugged in and providing resistance, the rear control module sends the full 12V required.
 

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Discussion Starter #3
So was the root of the no license plate lights a bad headlight switch?
Nope. The switch was fine. The wiring harness will not supply 12V unless there is a load. So if you just try and stick a multimeter into the harness... you won't see any voltage. Once the system detects a load (the light bulb), it supplies the 12V just fine.
 

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Nope. The switch was fine. The wiring harness will not supply 12V unless there is a load. So if you just try and stick a multimeter into the harness... you won't see any voltage. Once the system detects a load (the light bulb), it supplies the 12V just fine.
Maybe I'm dense, but I'm not following.

1. License plate lights not working.
2. Connections / wiring /contacts good.
3. Fuses good.
4. Multi-meter testing 0.0v and 0.2v.
5. Discovered you needed resistance for the rear control module to send the necessary 12v.
6. Lights working.

What happened between 5 and 6? I have to assume you had bulbs installed that were not illuminating before you started investigating. Did you have to reset the rear control module? New bulbs? Something else?
 
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