I'm searching like crazy but I'm in a bind for time. My girlfriend's 2000 F150 keeps blowing the position 13 fuse (20 amp) in the under-dash fuse panel. There's a 'no-name' controller connected to a 7-way harness. She tows a twin axle horse trailer with electric brakes. Everything works fine until she hooks up the trailer, then the fuse goes within 10 minutes of towing. I've replaced all of the connections in the wiring at the rear of the truck but the problem still exists. Before tearing into the wiring of a new trailer, is there something I should be looking at on the truck itself? I ran across a thread that stated the 20 amp fuse (#13) may not be enough for a 3 to 4 axle trailer. I'm not certain where her controller is getting power (wired by dealer years ago), so I'm off to check it now. Can/should I run a different power supply wire to the controller that's fused at 30 amps? Just throwing out options at this point....Any ideas? Thanks!

 

It blows the fuse without hitting the brakes?

 

No, she has to be driving it. Just sitting in the yard and repeatedly hitting the brakes while not moving won't do it. It's a Brake-Force controller, BTW, that's plugged into the factory wiring harness under the dash.

 

Sounds like a short in the trailer.

It is not uncommon. In fact some trailer manufacturer's are known for this exact problem.

The Ford has a factory wiring harness and is fused and relayed. The power supply for the brake controller is the same power source for the aux power supply for the trailer. You do NOT need over 20 amp fuse for only 2 axle trailer brakes.

By wiring in a larger fuse than what came with it you are just asking for an electrical fire on the truck or trailer.

Have a buddy help you with tracing down the short. Check 1st on the truck and with a good fuse installed and trailer connected and while the brake controller activated shake the wire harnesses behind the brake controller and near the 7 pin connector in hte rear. If the fuse doesn't blow during this exercise it is time to move on to the trailer.

The next step is doing the same type procedure on the trailer, starting at the plug and working back following the main wiring harness.
Check inside the trailer plug itself on the trailer. Many a time I have seen a stray wire inside the plug causing a direct short. Where the wiring harness passes through any opening check for abrasion or cuts in the wir insulation. This also included where the brake wires come out to go to each wheel. There is also a hole in the axle tube where the wires pass from the drivers side to the passengers. This is another common area for shorts.

My point is there are lots of places that a short can occur. Trailers are not made to automobile specifications for QC and workmenship. I would suspect the trailer before the truck for the short.

If the trailer is new like you stated, will the trailer dealer stand behind his product and check it out for you?...they should at no charge if the trailer was purchases in the last month or so.

 

Thanks! That's what I was suspecting myself but needed to cover my bases. I'll run through your suggestions and let you know what I find. Thanks again.

 

Unplug the brake pressure switch on the master cylinder under the hood and see if the problem persists. That's where I'd put my money, especially if the switch hasn't yet been replaced as a part of the recall which *should* affect her truck.

Odds are she's using the brakes a lot heavier with the trailer connected, even with the controller hooked up. That's probably what's causing the fuse to blow. It's the most likely candidate.

-Joe

 

Also another way to see which side the problem is on would be to hook up to another trailer/truck and see if the problem is still there.

 

Well, we can now rule out the trailer. I'm going to look into the pressure switch (thanks GIJoeCam). Can anyone tell me about the recall of this switch? Also, what exactly does the #13 fuse protect? Thanks again for the help.

 

Fuse 13 feeds the brake pedal position switch, speed control deactivation switch, and the flasher relay. The BPP switch then feeds a number of other things as well, including the trailer tow stop lamp relay, center high mounted stop lamp, and the multifunction switch for the brake light input.

Search this site for "speed control deactivation switch" or "cruise control switch" and you'll find all the info you could want on it. Either way, do what I instructed above ASAP to prevent the truck from turning into a crispy critter.

-Joe

 

The vehicle doesn't have cruise control, so would this still be an issue? There appear to be two switches attached to the master cylinder. Also, any ideas how to chase this problem down??