I have a 1990 4X4 F150 and I recently have had a problem with the fuse blowing out on the circuit that powers the tail lights and the lights on the instrument panel. What happens is the fuse will last anywhere from 10 to 45 minutes before blowing out. Does anyone who has run into this sort of problem before have any hints as to the most likely cause? I figured it could be the one of the bulbs, but they are all lit. Maybe old wires that only short occasionally as I drive?

 

Easy way to get a better idea - leave the affected lights on and let it sit still for an hour (have batter charger handy if need be, but just parking lights for an hour should not drain it too badly if it's any good).

If it doesn't blow, it's very likely some sort of intermittent short that's happening as the truck bounces. If it does blow while sitting still - well, I don't know. Finding the affected wire tends to be rather annoying. Look for places where the wires pass through metal and are either out of the plastic overjacket (loom) or where the loom is cracked going though metal.

One quick place to look - does this truck have a trailer harness (or the remains of a trailer harness)? Those often get abused, or are put in with less care to keep them from chafing than the original wiring.

 

Let me tell you a little story that once happened to me. Several years ago, a customer came to me with a truck, and the complaint was that the cruise control wouldn't work at night. I went through everything, and couldn't find any faults. I thought about it, and the only thing different at night was the headlights would be on, so I tried driving the truck around with the lights on, but the cruise control always worked. Finally, I asked the customer to stop by around sunset, and take me for a drive in the truck, so I could see the problem occur. We climbed in the truck, and drove around, and it worked fine. Then, as it was getting dark, he turned on his headlights, and flipped a switch on the dash turning on his fog lights, and from that point on, the cruise control no longer worked. Fortunately, I had a pretty good idea of what the problem was.

As it turned out, the owner had installed a set of fog lights, and he ran the power feed to the brake light fuse. In this particular truck, the cruise control measured the amps in the brake light circuit, so that anytime the brakes were applied, the cruise control would turn off. When this guy turned on his fog lights, it told the cruise control that there were amps flowing through the brake light circuit, and caused it to remain off until the fog lights were turned off.

That was the long way of my asking if you've done anything at all to the electrical system in your truck recently?

 

I have not done anything to alter the electrical system. This truck has 290,000 miles on it so I'm guessing it is probably worn wires. I do have a trailer hitch so I'll start by looking at that to see if I can identify a short there.

 

I had the same problem in my '93 model. It would blow a fuse every couple of days or so, then it stopped. Then a couple of years later it started again. I never did figure it out..

 

I'd bet a dollar on the headlight switch being melted internally.

Adrianspeeder

 

That's probably a good guess. I know the Mustangs had problems with their headlight switch. I've not experienced a similar issue with the switch in the trucks, but logic does suggest it could be an issue. I'd check the trailer wiring first, and then have a look at the switch.

 

I inspected the wire for the trailer electric connection and it had a wire worn through and exposed. I protected the exposed wire, put a new fuse in and thought I was set. I drove a mile, and went over my first bump and the fuse blew. I think I need to inspect all of the wires I can see underneath. Arghh

 

even if fixing the exposed wiring didn't fix your imediate problem, you probably prevented one.

BTW, you haven't replaced the bulbs lately, have you? I had a e-150 and tried to put those LED tail lights in, it did the same thing, it would blow fuses at random. Regular bulbs worked fine so I put them back in and never had a problem again. Didn't make any sense, but...

I just thought I'd throw that in even though it was asked earlierr if you'd done anything to it--- I wouldn't consider replacing a bulb to be electrical work.

 

Just a thought but is it the factory radio or replacement? Because you know there's a wire that runs to the radio plug for powering the display on the radio when the lights are on and i have seen them be taped off not very well and actually touch something and blow the fuse. Just a though good luck!


Daniel