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Trailer lights issue with 2006 Cayenne

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Wife and I went to go get our first ever travel trailer, only to be disappointed.

We have the factory tow package installed with my only having to buy the 7 pin adapter.

We did our walk through, signed papers, bought a whole bunch of stuff and were happy until we were ready to go home with the trailer. Hooked it up and turned on the parking lights. Those worked... Tried the turn signal and just sort of got a dimming of ALL the lights on the trailer, all the vehicle lights still worked though. Tried the same with the brake lights. Technicians came out and checked all the trailer lights and wiring. That was all good. Went to our vehicle and they were getting the constant dimming sort of flash of the lights on ALL pins of the harness in the back of the vehicle. So NONE of the brakes or turns work. No they are not LED lights...

Luckily the Porsche dealer was literally right next door, so I went over there and a tech was available to look at the car. She said it was usually coding issues. Well, she checked the coding and it was US spec. She troubleshot for about an hour and couldn't find anything. She said there were no codes and everything looked "good" through her computer but it doesn't check the outputs of the towing controller.

When I got home, I ripped apart the walls of the cargo area where the towing controller is located and ended up finding fuse 1 on the passenger side of the vehicle was toast. It was a 15A fuse labeled "Towing Attachment." GREAT! Found my issue?! No I didn't... Same thing... Fuses are all good now and still having the same problem. Which again, the lights come ON for the parking, but no brake or turn. Only a steady fluctuating dimming of the parking lights.

I ordered a new trailer control unit (factory one) and will go from there. But I was wondering if anyone knew of a wiring diagram or the path all of the signals take. Does the input for the control unit come straight from the vehicle lights or does it run through another computer before it goes to the trailer control unit. Anyone have a pinout of the control unit?! I have found two wires are constant hot, one that is hot with ignition on and the brake signal. So, I had a solid 12V going in on the brakes but couldn't find the turns. If I had good brake lights then the controller should have recognized that and put it out to the trailer. I didn't find a signal from the turn signals at all at the control unit but have a feeling that is why you need the control unit is because they may be can-bus signals and not measureable with a test light or DVM.

I really hope this is the control box and not something else before that but I want to know if anyone has any sort of help with this since the dealer couldn't diagnose it with the limited time that we had. (They want me to drop it off for another 3 hours of diagnostics.)

Thanks a ton guys!

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My guess....your wires chaffed where it goes into the body and you have a ground and power wire touching. Since they aren't LED, power can backfeed through the ground and still work since standard bulbs are not diodes.

disconnect the harness from both ends and do a resistance check between the pins....every pin should be isolated from the other.....if you get a reading on your meter, then you have a cut somewhere on the harness.

My guess will be it is either where the harness goes through the back of the car (passenger side in the lower corner where the control box is located.....there SHOULD be a grommet that stops this, but it could have been a crappy install.

The other location would be where the plug goes into the connector down by the hitch....I doubt it was inside the bumper simply because the harness runs along the bumper frame with quite a few attachment clips....but if you were rear ended it could have damaged the harness.

Bottom line. I doubt it's the control unit.....

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  • 3 years later...
  • 1 month later...
  • 3 weeks later...

I am new to having a Porsche, don't yet have mine wired for a trailer.


Typically, when a single circuit works, but multiple circuits on at the same time don't.  It is often a bad ground connection.

As was said, for non-led lighting, the power can backfeed through the common ground to the other light circuits.  That is not enough for them all to light because the power is split to all the other lights as well.  So, only the intended lights actually light (often not as bright as normal).


One method to test this is with the trailer connected, multiple circuits on.  Is to bounce on the hitch, to make the suspension move.  If the lights get brighter, dimmer while doing that.  That confirms that the hitch is not making a good electrical connection back through the hitch/ball.  Proper trailer wiring includes a ground wire connection as part of the connector plug.   Since you can not rely on a good electrical connection through the hitch ball and receiver.


Another method is:  Connect a battery jumper cable from a good clean place on the tralier/hitch to a good clean place (no paint) on the car.  work the cables a bit to make sure they bite through any corrosion/dirt.  If the lights work correctly.  That confirms a bad ground in the wiring system.


If it is a bad ground.  You'll have to trace the problem to either the trailer wiring (often the ground connection is made near the hitch) or vehicle.



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  • 4 weeks later...

Going to ask a similar question, my "trailer light check" randomly comes on, thing is, I have no trailer hooked up and never had. The trailer hitch is a factory fitted electric swing out version (European car 2003 CTT).

Would this be an wiring issue or a faulty module? One other thing the alarm also goes off randomly!! Could this be an indication of a failing battery, it was only replaced 3 years ago.

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The trailer wiring harness is prone to fraying against the metal bumper support on the passenger's side near where the wiring enters the passenger compartment. Clarksongli suggested a few other places to check. It might be worth taking the rear bumper cover off and having a look. Your alarm issue could be related. If a trailer is sensed when the vehicle is locked then the alarm will be armed and go off if the trailer is removed. If the wiring is frayed and making intermittent contact then it could cause the behavior you described.



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