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Truck fuses

So what is the truck fuse that controls the auxillary lights in a trailer? Not the fuses that control the turn signals and brakes.

I have a bumper pull trailer with an electric jack that doesn’t work. I hauled it off to somebody that understands trailer electrical issues and we confirmed that the jack works fine. Besides the jack I have lights in the dressing room that do not work either. So it is either a blown fuse ( which I have had in this crap truck) or the 7 point plug which I have replaced after I bought the truck.

I do not have an owner’s manual and I have been trying to determine which fuse this is in the diagrams. All the turn signals, brakes, lights work fine and I know there is another fuse for the auxiliary electrical systems in the trailer like the dressing room lights and the jack which plugs into the 7 point prong. I am going to try to replace the fuse if I can figure out which one it is and eliminate that issue before I take my truck back to him for more diagnostics. It would be nice if I could eliminate this or even better - find and fix it myself and not have to cart the truck off again when he has more time to look at it.

do an internet search for your specific truck for fuse box locations, our F150 has multiple fuse boxes

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If you are willing to post the year, make, and model of your truck I’d be happy to look this up for you.

You could just check each fuse until you find the culprit, but there is also a possibility that the fuse is fine and the problem is a bad power or ground connection in the wiring.

Or it may be corrosion or oxidation of pin 4 in your trailer plug. That can be sometimes easily corrected by using an electrical contact cleaner spray in both the plug and the truck,s outlet, or just doing some gentle scraping with a knife or similar tool. If it were my trailer, the plug and outlet are where I’d start before moving on to fuses, and finally to the wires.

I did. I don’t know if it is called the Trailer Tow Park Lamps. Trailer Tow Battery Charger, etc. Nothing is called a Trailer Tow Auxiliary Power source. I know it is there - I just don’t know what they call it.

Ford F250 2014. I wonder if it is the #7 prong/pin back up lights? Or could it be pin #4? I had a bad prong/plug when I bought the truck and had all that replaced.

Can I check the /pin plug with a 2-range voltage tester to see if the prongs are getting power? Or do I need something else?

The jack has a separate plug. But we tested that with his truck and that plug works. It is a truck side problem.

You’re looking for the fuse that says “recharge” line. Sometimes the manufacturers do not put a fuse in there from the factory. I think GM doesn’t have them. And with Ford you have the key on in order for the interior lights to work.

OK I will look for that. This jack used to work with the truck. Before and after the plug thing on the truck was replaced. And yes - the truck has to be running for the jack to work. And then one day - it wouldn’t work anymore.

So instead of thinking this through I ordered a new jack. After it came I realized that the motor was not bad on the jack and I probably did not need a new jack. Which we confirmed today. It is the truck that is the problem.

There is nothing on the Ford diagram that says “recharge”. But there are two different fuses that are labeled “trailer tow battery charge”. Could that be the fuse I am after?

Yes, that’s it. It will be fuse under the hood. Jacks do require 100% good batteries so if you didn’t have a fuse, you had no jack.

I am pretty sure I had a good fuse for a year or two when everything worked. Now I have an issue which may or may not be the fuse. But this gives me a starting point to figure out the problem. A few years ago I had a bad fuse with the regular trailer lights and when the Ford place replaced the bad fuse my trailer lights worked. So it could very well be this fuse that is bad. I will check this fuse out.

Check fuse 90 and relay 102 in the engine compartment fuse box. Power is supplied through these from the truck to pin 4 on the truck’s trailer plug receptacle. If fuse 90 or relay 102 is bad there will be no power from the truck to the trailer auxiliary lights. Also there will be no charging power from the truck to the trailer breakaway battery. At least this is how I read the on line wiring diagram for a 2014 Ford F250.



Those are the two fuses that I had earmarked after reading Tobruk’s post so this makes me think that I may be on the right track. I may not be able to fix this problem by myself if it is not a fuse but at least this is narrowing down where the problem is and will make it easier to fix.

Thanks everybody!

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Nice job LCDR :clap:

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Before changing fuses, at least try the contact spray. I had that problem a couple of weeks ago. Spraying and cleaning both sides of the contact had me fully back in business.

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You can look at a fuse to see if it’s blown.

A fuse multi pack typically comes with a fuse tester if you’re still unsure.

It could also be the plug, as ATR said. I can’t get the one in my truck bed to work for left turn signals no matter how much I clean it. The bumper one works fine, and the bed one is pigtailed off of it. All that to say, you may need to replace either the male or female end of that to fully troubleshoot.

Do you know anyone with a vehicle with a plug you can invite over for a beer so you can plug into their vehicle as a troubleshooting step?