Last night I hooked up to my BIL's flatbed, and had no turn signals or brake lights on it. Truck had them, trailer didn't. Electric brakes were fine and so were running lights. Thought it was the trailer until he hooked it up to his truck and all lights worked fine. Tried my horse trailer and again, no turns or brake lights, but good electric brakes and running lights. So I'm figuring a blown fuse on the truck, especially since we found an exposed wire that had been rubbing on the trailer's license plate.

Now, according to the manual, there are five possible fuses in the two different panels that it could be - but only one that affects both sides. The other four appear to be specific to the right or left side. So my question is, could it be that the wire blew two fuses at once, or should I start with the one (isn't really a fuse, forget what it's called) that appears to be for both? Where would you start?

 

If you have access to a multimeter using the diagram on your fuse box check for continuity between posts on each fuse. That is a easy way to see if the fuse is blown before you pull it. I blew what sounds like the same fuse when I was converting to a semi style plug below my rear bumper. I had trouble finding a 25 amp fuse anywhere but the dealer. The fuse I am referring to was a new type being a square configuration 25amp . The fuse is in the engine compartment box. Good luck.

 

Thanks! Yeah, that's the one I'm guessing it is. I got one from Autozone but won't know if it's the right one until I pull the old one out - I had a choice between short, medium and long and went with the middle. If I'd had my ladder with me I'd have pulled it while at the parts house.

And now that I have read my owner's manual again and am not going on memory, there are three fuses and two relays - not five fuses. The two 15 amp seem to be specific to a side and it sounds like if they went the rear blinkers and brake lights would have gone out on the truck as well. So I'm figuring it's that 25 amp. Maybe I'll have to go visit my DEF fans at the local dealership again if it's not!

 

I am pretty sure that the trailer fuses are under the hood. Also, the relays should be fine, just the fuses are the things to look at.

These trucks use the new mini short fuse I believe but the med and tall ones work too. It should be easy to spot blown fuses as long as you have a step ladder and a flashlight.

Did you truck not tell you "trailer wiring fault" in the message screen (LCD screen)? Mine always yells at me when there is faulty or intermittent shorts/grounding on a trailer.

And, at Northern Tool or someplace similar you can find a tester that goes into your 7-pin connector. It looks like the male end of a trailer connector but has lights on it, tells you if you have good signal on all circuits so you can be sure that your truck is not the problem.

Also, I always carry a can of dielectric grease and keep the trailer plugs coated with them. I don't know if it is really all that important but it keeps the trailer connections coated so that they do not get corroded.

Tracking down unknown shorts in trailer wiring can be a nightmare and I don't wish it on anybody, but if your fuses blew then there is a reason and it will happen again unless the frayed wire or corroded bulb socket or something is found.

If you're lucky the connections in the 7-pin plug on your trailer is the culprit. They can get tugged on or stressed without you knowing it while driving/backing. It is also good to take it apart when testing circuits, so might as well check it out.

 

rushejj, that post should be a sticky! Thanks for all the good advice. I hope to tackle it this evening, will let y'all know what I find.

 

Well, thanks for the compliment, but I'm just trying to think of everything that could be the trouble. I tow a lot of stuff and deal with this kind of thing all the time. The 12V hot circuit is the most common problem with my trailers.

Here is the tester, nice to have.

7 Way Trailer Connector Vehicle Tester Tow Ready Tools TR20117

 

Well holy crap!! That is way cool! I gotta have one. Thanks for the link!

 

I bought a little tester from Farm and Fleet that plugs directly into the 7-Pin receptacle on the truck. It has LEDs on it that show when current is coming through for the different functions such as turn signals, lights, brakes, etc. This little plug has saved me a lot of diagnostic and troubleshooting on the truck side. I had to do this a lot on my 6.0 with an aftermarket brake controller. I haven't had any issues yet on my 6.7.

 

That sounds like the one I ordered from the link rushejj posted, can't wait for it to get it here. In the meantime, I pulled the 25 amp fuse last night and sure enough, it's busted. I can see it clearly. Unfortunately, the fuse pack I bought to replace has everything BUT 25 amps. So off to the dealership I go this morning, will pick up several for just in case.

 

Good, glad you spotted a bad fuse. Also, I carry a bag in my toolbox full of fuses, all the types the truck uses. It's expensive but you might want to get a bunch of them and keep them with your new tester.

 

Most of the parts house should have that short Maxi fuse. I've bought mine at Autozone and Checker Auto. They don't seem to stock very many. but seem to keep ONE on the shelf. Every time I go in to buy one and spare, They never have a spare. And why are they about $5 a piece?

 

Yes, I feel your pain. I probably have $50 in fuses in my tool box. Spares for the truck, air compressor, audio system, strobe lights. It is expensive.

 

It has been a year since I blew my 25 amp fuse and then all I could find was a 20 AMP at Autozone that I used for about 6 months with no problem until I went to the dealer and even then they had to order in a supply of 25 AMP in at that time. Evidently no one else had blown that fuse. It seems like I paid more than $5.00 at the dealer. I kept my 20 AMP as a spare. I keep wondering why the auto manufacturers changed the design of the fuses in the newer cars. I need to buy an assortment of the new type fuses for spares. 2 horses I am glad that you found your problem fuse.

 

WASNT THERE A POST A WEEK OR SO BACK ABOUT BROKEN WIRES IN THE WHEEL WELL AND TAIL LIGHTS

 

2horses, I had similar symptons last week. Right turn/brake trailer out. I checked the 2 fuses and they were good. Turned out to be the relay in the box under the hood was bad. Went to the dealer and they showed non in stock at any dealer in the Northeast so they had to get it from Ford distribution. Anyway, when I was changing it, I noticed corrosion (green stuff) all in that fuse box. So, next week it goes to the dealer for them to go through all the relays and clean the box up.