Diagnosing Trailer Light Issues

My favorite thing! Hoping you guys can help.

So last weekend on the way up to Mille Lacs I noticed my running lights weren’t working on my trailer. Got it home and using one of those trailer light plug-in testers determined it was an issue on the truck end (I didn’t get anything from the 4 pin connector, or the 4/7 pin adapter), changed out the fuse the next day, and they worked when I tested them. Then last night I ran up to the Croix. They were working until we unplugged them to load the boat, then they weren’t working on the way home either. Swapped out fuses and they worked for a few minutes, then went out again. I thought something was blowing the fuses, but they started flickering on and off through the remainder of the drive home. Once I got home I checked the wires, which were very hot to the touch.

I don’t know much at all about electrical stuff, so any and all advice would be helpful. I’m guessing something is bad with the plug on the truck, or with the wiring leading up to it? I’m going to go check for loose/corroded wires in a bit, but I always feel out of my element dealing with electrical stuff. Thanks for any advice or information guys!

First off, give yourself some credit. You’re doing a good job of troubleshooting this so far!

Which wires were getting hot, the trailer wires or the truck side wires? Either way, I’d guess there’s a bad spot in the insulation and that’s leading to an intermittent short.

You might be able to find the bad spot, or it might be “inside” someplace. You might end up pulling new wires.

S.R.

Totally agree with SR, you have a short somewhere on the trailer side. So basically we have it down to 3 wires.

Big clue as to WHERE the short is (which wire) may be this: What fuse are you replacing on the truck side? You say you replaced “the fuse”. Which one? Hopefully, the trailer lights aren’t all on one fuse. If you can narrow it down to which wire is shorting, it may help you.

I was chasing the exact same issue with my dad’s boat last year. Turn on headlights and pop goes the running light fuse 3 minutes later.

Now here I have to tell you that IME it is very difficult to run down a short in installed trailer wire. I have found one by pulling all the wire out of the trailer, but hunting a short in installed wire is maddening. Even if you find where the short is, the question is can you get at it to fix it without pulling all the wire? And then, of course,s you might just have found one short out of X number of shorts.

To be honest, nowadays I just buy a good harness and redo EVERYTHING. That fixes the short and minimizes future issues. Wire harnesses are not that expensive and to me it’s spending a little money to save lots of time and frustration trying to troubleshoot.

Grouse

Thanks very much for the responses guys. My running lights are the issue, and I’ve decided to rewire the thing like you mentioned Grouse. The wiring is 17 years old and it gives me an excuse to upgrade to LED lights as well.

Couple questions: have you guys ever used liquid electrical tap on the ground wires/bolt on the trailer or the connectors to the marker lights? Just thinking that might provide some added protection. Two, any advice for splicing those marker lights in? So far I’m thinking splicing the marker light positive wires into the running light wire at a couple points with waterpoof butt splices? And running one or two (I might try to splice the ground wires together) ground wires down to the ground points by the taillights. Any thoughts on that? Or general advice on trailer wiring of any sort? Thanks guys!

You have a positive wire that has the insulation wore off somewhere on your trailer. Look around points where the wire enters the frame first. could be spot it has rubbed on the frame or critter chewed the wire, could also be inside your actual light housing too. But I would start examining trailer wire.

As far as splicing marker lights, I say dont do it. Every cut you make is a potential for failure in the future. If you feel you MUST have them run a dedicated pair wire from an end of the wire rather than splice mid run.

Second point of advice is dont use the frame as a ground. Use a dedicated ground wire this will prevent 99% of any future problems from even occurring. I use 3 wire extension cords for trailer wire. double insulated and have a wire to use as ground. You just need to add a junction box to the tongue and tie your vehicle plug into the correct wires. I also recommend soldering connections when possible.

x2 on the dedicated ground wires, I think I got that tip here a while back. I upgraded to LEDs and kept blowing fuses, rewired all and haven’t had any issues since.

Had a new trailer many, many years ago before sealed lights, and the fuses blew so much at night, that I used to carry packs of fuses with me. Had a hard time figuring it out, and finally one mechanic said to ckeck the tab inside the light socket to see if the bulbs were loose in their socket. Turns out the bulbs were rattling around, and would apear to be fine till you drove down the road and they arked on and off. Just had to bend out the tab a bit.

Solder, heat shrink, and tape all connections. No butt connections or Scotch locks anywhere!