OEM Trailer Wiring Troubleshooting Help

cjett · #1 07-07-2010, 11:11 AM

I installed the BMW oem trailer kit about 4 years ago in my 2003 X5 4.4. No issues until this past weekend. No lights at all on trailer. I also tried connecting another trailer and the same... no lights. Of course I'm 500 miles away from my full set of tools (including my volt meter), so, my testing has been limited to making up a bulb pigtail and checking for 12v on any of the pins and swapping the fuse in the trunk area (fuse looks fine, but, swapped with another 20 amp to be 100%). I also followed the wiring through the body into the spare tire area to make sure it wasn't cut or chaffed anywhere (everything looks good). The pdc is not disenganging either... so, the x5 doesn't seem to be "seeing" the trailer at all. Oh, and everything else in the car seems to be working fine (lights, etc)

Does anyone have a schematic, or any good troubleshooting tips? Thanks in advance.

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JCL · #2 07-07-2010, 06:23 PM

The wiring schematic is included in the installation instructions for the hitch. I have attached a copy of the one on this site; it is from 2000, but the trailer module itself didn't change (the wire colours may have). The instructions show the Euro hitch ball, so the wiring may be slightly different for that reason, but the basics should be the same.

http://www.bmwmotorsports.org/pdf/e5...iler_hitch.pdf

There is a legend right before the schematic. I see a couple of fuse holders in the schematic. One fuse would just be too easy.

Following the principle that what has been added on is the most likely thing to have failed, I would remove the fuses, then disconnect and reseat the plug in the trailer control module, and the one in the wiring to the hitch. I would also buy a test light at an autoparts store for $3.

Hope you're not reading the PDF on a cell phone.

Good luck.

cjett · #3 07-07-2010, 08:37 PM

Thanks Jcl. Of course my cell is all I have with me at the moment. But, I'll see if I can make do... At least to determine if it's something easy like a fuse. I seem to recall an inline fuse or 2 when I installed it.
Thanks again, much appreciated.

cjett · #4 07-07-2010, 08:51 PM

So, the 20 amp fuse shown in the schematic is in the rear fuse bank above the subwoofer. Jcl ( or anyone else) do you know where the 50 amp fuse is located? Is it inline or in one of the other fuse boxes?

JCL · #5 07-07-2010, 10:16 PM

If you have disconnected and reconnected all the plugs, in case there is a bad connection there, I would move on to checking the grounds. X494 and X498 are both ground connections, and X1986 is reportedly a body ground post with a fuse.

I would also check the fuses in the rear quarter panel, and the second fuse block in the glove compartment (working from memory here, but I think that is where it is, at least it is in the right front of the passenger compartment, near the glove box if not inside it).

If the fuses are all good, I would consider the trailer module as a potential culprit (since the vehicle lights are all working). I think it is more likely to be a connection, though.

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