Hello everyone.

Recently purchased a 2003 F350 dually and a gooseneck horse trailer. The horse trailer is 1994 and was wired as a 6-pin. Previous owner had someone “install” a newer 7-pin in the truck after someone had drove off the truck while it was still attached to the trailer.

I ended up in Kentucky purchasing this large steel horse trailer, hooked it up with an adapter from the 6-pin to the 7-pin and was fine and had lights until we stopped by a local Walmart to install a new taillight on the trailer to avoid getting pulled over on our 6 hour drive home. After installing the new taillight, we were testing the lights and nothing came on unless the plug to the truck was jiggled just right. After 4 hours fussing with this, we decided to just risk the ticket and attempted to drive away. As soon as the gas was pressed, the trailer brakes locked and we were unable to move. Another hour of fussing with no relief and we decided to un-plug the trailer and do our best with no lights and no trailer brakes.

We would stop for gas and try new things on our journey back. Ended tried adjusting the brake controller to see if that would fix the issue. No luck.

We make it home and park the trailer and haven’t moved it since. It needs a lot of work done to it to be horse-safe and I haven’t been particularly motivated.

It was recommended to me that the issue could be the trailer wiring was wrong and that something might not be translating correctly with the 6-pin to 7-pin adapter or that the 7-pin rigging could be an issue in the truck (it was a disaster of wires spliced and lots of duct tape). So I replaced the 7-pin rigging in the bed and back of the truck. It took some ingenuity as the part that connects to the wiring in the truck didn’t readily come on the rigging to run to the back and the bed of the truck as one would typically have to haul a gooseneck. But I get it done. Next step was re-wiring the horse trailer which is another beast in itself but I’ve almost completed that.

In the middle of re-wiring the horse trailer, I received a call from a friend who needed me to move her horse using her horse trailer. I figured sure, the rigging is brand new and her horse trailer works perfectly with all their vehicles, it shouldn’t be a problem.

I hook up to her trailer and the brakes don’t lock but are very sensitive. I chalked this up to being because I’d fussed with the brake controller the last time we had it hooked up and was trying my best to sort it out. At this time, said friend calls and informs me that none of my lights are working and asks me to stop in the driveway so she could check that it was plugged in all the way. She checks and it is. We say screw it because we’re only moving him down the road and she would be following in her vehicle. We don’t make it far and I use my left turn signal and with every flash of the signal, it hits the trailer brakes and makes them lock.

So now we do believe the issue is the truck and not my horse trailer (at least it’ll have nice new wiring regardless). From what research I’ve done, it sounds like the easiest solution to this would be a grounding issue. I do not see a grounding wire in the back under the truck that would ground into the frame, as I’ve read there should be. I have had no luck researching to find out what or where I should be looking. I also understand that this could be just the tip of the ice burg in trying to find a solution.

Also being a female means when I take my vehicles in to a shop to even get my oil changed I get harassed about unnecessary services that my vehicle “needs” simply because I’m a woman and they think they can pull one over on me, which makes me hesitant to trust anyone I could take my vehicle to locally, especially if it’s something I can do on my own with the right guidance.

Any help and/or insight is greatly appreciated.

 

First of all just take a jumper cable and attach one end to your horse trailer frame and the other to the frame of your truck. That takes care of any grounding issue. Now the way I approach this type of problem is to forget the trailer and start with the truck. Then I start rewiring the seven pin one pin at a time. For example turn on the right blinker and make sure it is feeding the pin on the seven pin for the right blinker. Proceed in that way. I have had to rework a lot of miswired seven pins and for the most part it stimply takes time rather than skill. You do need a wire for each function you are using so if you don't something is wrong.

So who can wire a seven pin on your truck? just about anyone with a test light and a basic understanding if what they are attempting. Plenty of wiring diagrams online. Being a female in my book is not a limitation in siturations like this as it is not a question of muscle to lift a heavy weight. I knew a number of female techs who were very good. You might try a place that sells trailers and see if someone there can help, but even a friend who works on their own rig could show you how it is done. Good luck!

 

Like he said ^

You've already started (or mostly finished) rewiring the trailer. That's a good thing. Make it correct and you will KNOW that it's good.

As far as the truck, it's the same thing. If everything on the truck works, (lights). Then all you have to do is make sure that the correct wires are feeding into the correct pins on the 7 pin plug. (Hint: only 5 wires are used on most trailers.)



If you'd feel more comfortable taking it somewhere, UHAUL is a great place for things like this. (A real Uhaul center, not a gas station with Uhaul trucks). Their "hitch pro" guys are trained in exactly this type of thing. (I was one of them, when I was in college)

 

I agree with your intuition that poor grounding on the truck is a likely cause. The most definitive way to diagnose this is with a volt/ohm meter and a long piece of decent-gauge wire. Attach the wire to the ground terminal of the truck's battery. Extend the wire outside the truck to the truck's trailer socket. Now you can measure ohms resistance between the ground pin of the socket and your new known-good reference ground at the battery itself. If the ground is poor or intermittent you can now get under and in the truck and inspect/wiggle the socket to chassis wire as well as the battery ground to chassis heavy wires.
Another very useful aid is a trailer emulator. These can had for about $25. Plug it in to the truck to verify the truck's wiring via a set of lights on the emulator. Search for "7 way trailer emulator" at eTrailer.com.
Finally, I suspect you already have this, but I'll put it in anyway. A link to a good "pin out" of the 7-pin trailer connector.
https://www.ajtnt.com/Info/7-Way-Diagram

 

I agree that grounding causes a lot of trouble with truck and trailer light issues but it will never apply the trailer brakes. If when you turn on your idicators the trailer brakes lock up you could have a few issues. As the indicator is on loo kat you rbrake controller. If it is flashing on and off then you know that you've wired into the wrong wire to feed it. On my 93 when the 4 ways are on it feeds the brake controller power and sets the brakes on and off.
More likely though if your brakes lock up everytime the turn signals are lit then it's because the pins aren't wired in the correct spot. Or you could have a short between them. if in the back of the plug some of the copper strands are loose and floating around they can touch the other pins.
I would start simple though. make sure your pins are all wired in the correct spot and then check that your brake controller doesn't turn on with the indicators.