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Trailer Brake Lights Not Working when Connected to 2004 Porsche Cayenne S with Factory Tow Package


Hi there, I had a problem back in 2011 with no backup or tail lights. The trailer I had then had electric brakes and I couldnt go in reverse the brakes locked up because no power to backup lights. The local Austin Porsche dealer replaced part #955-618-040-10 control unit. Im assuming that everything worked then. Everythings fine on Porsche. The running lights, and brake lights work on trailer but no brake lights on trailer. I now have a trailer with drum hydraulic brakes and it works fine with my sons 97 Chev C1500. The Porsche has a 7 round and I use an adapter to my four flat which the Chevy has. I would start checking fuses if I knew which ones and their location. Any help would be appreciated.

Expert Reply:

The first place I recommend checking is the trailer connector on your 2004 Porsche Cayenne without the adapter or trailer plugged in. Use a circuit tester such as # 3808 to test the pins in the 3 o'clock and 9 o'clock positions while someone sits in the vehicle and presses the brake pedal.

If there is no power on those pins during this test then it could be a fuse or relay related to the tow package wiring. Your Cayenne has a fuse box under the hood and in the cab on each end of the dash. On the right side of the dash there are tow package fuses in locations 1, 3, 5, 19, and 41. there aren't any in the other fuse panels. Any relays associated with the tow package wiring would need to be handled by a Porsche dealer as specified by the owner's manual.

If there is power on the trailer connector pins then plug in the adapter you have and test the output side of the adapter. If there is no power on the output side then the issue is with the adapter and it will need to be replaced with a new one such as # TR20141.

If there is power on the output side of the adapter then the issue is on the trailer; likely a bad ground or corroded trailer connector. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by:
Michael H

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