This is most likely the result of the insane over-use of unnecessary electronics and computer controls where they aren't needed, coupled with, I suspect, the need for a trailer lighting adapter. There are two things that might be why the lights work intermittently. Chrysler developed a "positive temperature coefficient", (PTC) semiconductor device in the '90s to replace fuses. With a short circuit they simply stop current flow to protect the wiring, then they start working again when the short goes away or is removed. Other manufacturers use similar devices now, so there may not even be a fuse involved with this problem.
Second, Ford is the leader in replacing simple, reliable circuits with severely-overcomplicated computer controls. There are advantages afforded by them, but they come with a nightmare of frustration when it comes to diagnosing problems. Repairs are stunningly expensive too. In this case, an over-current situation is likely being detected and the computer is turning off the affected circuits. It may be necessary to turn the ignition switch off to reset the computer, once the short is removed, or it might reset on its own a short while after you unplug the trailer. Regardless, don't bother looking for a blown fuse. They are never intermittent. When one blows, you're done until the short is removed and the fuse is replaced. The fact your lights started working on their own is proof no fuse was involved.
The place to start is by looking at the trailer lights and the vehicle's lights. Most manufacturers today make hooking up trailers impossible without an expensive adapter. Trailers have two light bulbs built into one, in other words, two filaments in one bulb. One is less bright and is for the tail light. The brighter one is for the brake light and signal light. Cars had the same system since as long as they had lights, so it was real easy to add a trailer wiring harness.
Now, the insane engineers have simplified the turn signal switch by using two separate bulbs in the rear, one for the brake light and a different one for the signal, (especially when they use a yellow signal light on the rear that belongs on the front). How are you supposed to connect two separate circuits to a single one on the trailer? That's where the adapter harness comes in. What little they simplified in the signal switch they more than made up for in that harness, so they get to charge a pile for it. The exception is on a lot of trucks now they use multiple relays to feed the circuits in a "trailer tow" package. If you have that, you don't need a separate adapter harness.
Start by verifying that you indeed have two different bulbs on each side of the vehicle, one for the brake, and a different one for the signal, then look at the trailer connector. If you have the common four-wire connector, there is no way to run two signal lights, a common brake light, a single tail light, and a ground. Even the larger 7-pin connector isn't wired for three separate brake, left signal, and right signal circuits.
Next, determine if that harness came with the vehicle from the factory or if it was added on later. If it was added on, is it just the common four-wire harness or is it an adapter harness that combines the brake and signal light circuits for the trailer only and not for the truck?
Sunday, July 6th, 2014 AT 10:23 PM