Hello everyone. I tried searching but did not find anything.

I have 2004 FX4 with the 5.4L and keep blowing the backup light fuse. this circut also runs the trailer running lights. The lights work fine on the trailer and two other trucks connect and pull without blowing fuses.

the truck has not been modified in any way. Everything is factory. I have been pulling this trailer for 4 years with no problems. Then 2 weeks ago started blowing fuses (4 fuses).

Any help would be great.

Also from Michigan and pulling the trailer through a lot of snow lately

 

First I have a couple of questions. Have you pulled this trailer with another truck within the last two weeks? Also does the fuse blow when you aren't pulling the trailer?

 

I have the same problem with my '05 F150. I can pull the same trailer with my wifes '01 Expy with no troubles but the 150 pops running light fuses. I beginning to think the problem lies in the factory trailer harness.
Watching this string for an answer.

 

If you hook up to the trailer, turn on the lights and just sit there do fuses pop? Or is it when you are driving?

The reason i ask is because the snow and water on the road splashing up could be causing a short.

If it doesn't blow sitting at a stand still, then the problem might be with the trailer.

If it does only blow when you're driving, do a visual inspect on your trailer wires and check your grounds.

 

Deisele75 has the right idea. The problem needs to be isolated to one area before you know where to start looking. In 2fords1jeep's case it sounds like he needs to crawl under his 150 and start looking for frayed or bare wire.

 

OK. Have tried a couple things.
1) Hooked up trailer to other trucks and everything works.
2) Hooked up my enclosed trailer and fuse blows.
3) Hooked up enclosed trailer to other trucks and everything works.

Based on this I believe the problem is with my truck.

1) Fuse blows with or without trailer.

I noticed a lot of corrosion in the factory 4-pin connector. I cleaned it out and replaced the fuse. This lasted approx. 2 days before the fuse blew again. I looked at the 4-pin and it was covered again (greenish-blue flakey stuff).

I cleaned out the 4-pin and sprayed with WD40 but with 2 days the corrosion has returned.

Any suggestions?

2fords1jeep's is your 4-pin connector have corrosion on it to?

 

Try one of these repair connectors; Quick FiTM) 4 Pole Flat Car and Trailer Ends 38195 : Trailer hitch bike rack and trailer hitches - etrailer.com

 

did you take your plug apart and look at the back side where the wires connect. or is it a flat connector. if it is the round 4-7 pin connector the major corrosion is in side the connector? If you install a new connector solder the bare wire ends before installing and then coat ever thing with dieletic grease, same stuff used on spark plug boots.

 

I use the 7 pin round connector with an adapter to 5-pin flat for my trailer. Corrosion doesn't seem to be an issue and the trailer lighting behaves when connected to our 2001 Expedition with the same adapter.
Can I replace just the trailer wiring harness?

 

From your description the problem is in your truck. These two pieces will replace your existing receptacles;
Plug-N-Tow (TM) Vehicle Wiring Harness 30185 : Trailer hitch bike rack and trailer hitches - etrailer.com

Adapter 4 Pole to 7 Pole and 4 Pole 37185 : Trailer hitch bike rack and trailer hitches - etrailer.com

 

...i agree with shorebird, seems as though you need a replacement adapter if you cant clean up the existing one on the truck. Use a bit of contact cleaner instead of WD.... and pack that with white lith grease after youve taken a brush to the corrosion. And then before you try hooking up, make sure the ground wire to the frame from the harness is still attached and isnt caked in road salt...you had said you were in michigan in the snow....

 

One thing I have noticed with my truck. The fuse will hold until I apply the brakes(trailer connected), then it blows. Funny thing is that the fuse controls the running lights to the trailer, not the brake lights...they still work after the fuse blows.

 

2fords1jeep, basic 4 wire trailer connector has left turn, right turn, tail (running) lights and ground. a seven pin connector has the addition of battery charging, and trailer breaks and a 7th pin for future use I believe. What is the fifth wire for on your adapter? Also, when you apply the breaks and the running light fuse blows are the running lights turned on?

 

I believe the fifth wire is for the reverse surge-brake lockout for the trailer brakes. Yes, the running lights were on when the fuse blows (after applying the brakes).
Once the fuse blows I have no running lights, but have both turn and brake lights.

 

I would unplug the trailer connector(s) from the truck wiring harness, then take an ohm meter and look for continuity across any of the 7 pins. There shouldn't be any. If there is, I don't know if you can take that apart or not, but you will either have to fix it or replace it like Shorebird suggests.