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jsprad · 08-20-2011, 08:33 AM

The last time I pulled my 2005 International, the clearance lights didn't work. I traded it for a 2011 International (28 Serenity....love it!) and the first time I towed it the clearance lights didn't work. A few days later, my 2007 GMC 2500 HD diesel gave me an error message in the system, telling me to check the trailer wiring (it has an internal trailer brake control system). I took it to my dealer and they said they could find nothing wrong and that GM had put out a technical bulletin on this and what it referred to with "excessing clearance lights." I was told it was a grounding issue between the trailer and truck. I was told to do things like make sure the hitch ball was properly greased, which I normally do and have so for years. All this sounds a little strange and just plain incorrect to me. Has anybody else had this issue? Thanks.

Nuvite-F · 08-20-2011, 08:45 AM

The error message sounds strange to me, too, but you have to remember, high on the list of computer truisms is, "Error messages almost never indicate the actual problem."

Late model GM products factory wired for towing have fuses for all the trailer circuits in the main fuse box under the hood. The first thing I would do is check that fuse.

If that isn't it, you need to check for voltage at the trailer running lights pin of the tow vehicle end of the trailer cable. Years ago I built a "break out box" with a 7 pin plug on one end and a 7 pin socket on the other so I could get at all those circuits. Has saved an awful lot of time over the years.
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mimiandrews · 08-20-2011, 09:02 AM

Don't know about your truck, but in our Dodge, there is a relay in the tail light circuit that provides power to the trailer running and tail lights. The power comes through a different fuse at (or near) the main panel. The theory behind this is that a lot of running lights, etc. on a trailer would pull too much current through the truck's headlight switch and burn out the switch.

In any case, when the parking lights come on in the truck, power from that circuit closes the relay. That provides power to the trailer plug for its running and tail light circuit.

Therefore, with one bad fuse or one inoperative relay, there will be tail and parking lights on the truck, but nothing at all on the trailer.

Suggest you check your truck's trailer connector to see if there's power to the correct pin when the parking lights are on.

These people have great wiring diagrams and drawings of which pin in the connector goes to what.

Trailer Wiring Diagrams | etrailer.com