License Plate Lights don't work (and other lights)

[ManMN]

Hello All,

 

The warning "!" indicated that my License Plate lights are out.  I checked - the bulbs are fine (I replaced with new ones just for the heck of it!). The fuse is OK because when I take the bulbs out and put them right back (like creating an open circuit) - start the car, the message is gone, I check the lights - it's OK (working) BUT as soon as I press on the break to put it in Drive, the message came back on and lights stop working!!! and I can repeat this step!

 

At the same time, when I start the car and drive, the dome light is on until I hit the dome light switch.  When I turn off the car, take key out then eventually the dome light turned off by itself.

 

I don't have any Porsche diagnostic tools except for a generic code reader.  The "Check Engine" light is not on.  Both batteries replaced by the dealer about 20 months ago.

 

My car is CayenneS "04 with 102K mikes on it.  Any suggestions are greatly appreciated.

 

ManN

 

 

 

[Loren]

The system is sensing high resistance. There is a ground point in each rear tail light - I would check those for corrosion. As well remove the license plate bulbs and make sure they and their sockets are clean of any corrosion.

[ManMN]

Thanks for the tip Loren. I checked all ground connections, adding additional ground wire on each side of the tail light (just to make sure) and sanded the contact points at the license plate lights, well... that didn't solve the problem! Is there such thing as "light control module" that could be bad?

My next try will be tab on a tail light's circuit to feed the license plate light externally, just for testing purpose. Will this do any harms? or may be I will get another warning indicate that the tail light is failing because it draws more current?

ManN

[Loren]

Well, there is a rear control unit - that controls those lights. I do not know why it would sense a high resistance (the warning light) like a light was out unless there were a damaged connector or wiring somewhere.

It would be very odd for a control unit to only have this problem -- not very likely but I guess possible. PIWIS would be the only sure test.