Posted by: Realist | September 20, 2008

Trailer Wiring Voodoo

When we had the floor in our Brenderup horse trailer replaced, the shop also fixed some wiring issues.  When it came time to fix wiring/light issues on my 4×8 utility trailer, I thought I’d fix it myself rather than pay a pro.  I mean, how hard could it be?  I have a degree in electrical engineering and do much of the house wiring.  Yeah, right, 2 weeks and 8+ hours later…

The first challenge was replacing a broken light assembly.  This is the second time I’ve done it, and the first time was an unbelievable pain.  The trailer lights have 2 bolts that don’t have a slot for a screwdriver or a head for a socket.  They’re smooth.  So when you try to unscrew the nut, the entire bolt just turns without unscrewing.

The first time I changed one of these a few years ago I used a hacksaw blade wrapped in a rag and held in my hand to slowly and manually saw through the bolts.  A hacksaw or other powered saw wouldn’t fit in the cramped space.  It took a few hours of painful labor to cut through those two bolts.  I swore I would never do that again, which is why the second broken light stayed broken for years.

Then I stumbled on this wonderful invention called an angle grinder.  It spins a cutting wheel which lets you cut right through rusted bolts, metal, anything.  It even throws out a shower of sparks making you feel quite manly.  Below is the cheapo angle grinder I got for around $15.

Angle Grinder

Angle Grinder

So with my brand new angle grinder, I was albe to cut those bolts off the broken light in about 10 minutes.  I figured the rest of the wiring/light overhaul would be a cake walk.

I attached the new light assembly and connected up the wires.  I then tested it by turning on my truck turn signals.  No lights.  Hmm… that’s odd, maybe I have a short in the wiring.  Off to get the volt meter.

Now one thing they teach you in Physics 101 is that all circuits have two wires, one that is hot and the other for ground.  When I saw the two wires coming out of the light assembly, I just automatically assumed these were the two wires.  Oh no.  I’ll save you an hour or two of fussing with a voltmeter and the wiring harness and explain that the trailer frame itself is used as a ground, while the two wires are both hot (one for turn, one for stop).  This becomes obvious if you open up the light assembly and trace the ground to the bolts which are attached to the frame.  Until I figured this out, I did a lot of head scratching with the voltmeter results.

Somewhere around hour 4 I decided I must have a break in the wiring I couldn’t detect, since I wasn’t getting any voltage at the light assemblies.  It was old wiring, after all.  A sample of the wiring with cracks and exposed wire is shown below.

Old utility trailer wiring

Old wiring on utility trailer

I bought a replacement wiring kit at Tractor Supply.  I also turned the trailer on its side so I could access all the wiring.

The new wire went in without a hitch.  I even hooked up the ground wire to the trailer with a bolt.  (By the way, it took me a while to figure out I even needed the ground wire, which I originally thought was redundant.) The angle grinder did a great job of removing the surface rust and ensuring a solid electrical connection.

I hooked up the trailer to the truck, turned on the turn signal, ran back to the trailer, and expected to see the light flashing.  And like the Millennium Falcon’s hyperdrive in The Empire Strikes Back, again the darned light remained off.  I was now at hour 7 in this project, where I probably could have purchased a new trailer if I was doing billable software development work at my day job.

Back to the voltmeter.  I verified I had power at the truck’s plug when the trailer was disconnected.  But then with the trailer plugged in, I wasn’t getting voltage at the light assemblies.  And I had brand new wire.  Weird.  I then disconnected the light assemblies from the trailer wiring and checked voltage at the end of the trailer wires.  Voltage!  Huh?  Looks like the wiring is fine.

Could the light assemblies be somehow shorting out the circuit?  That was the only explanation, since without them attached, the voltage was fine on the wiring.  I spent the next half hour carefully going every nook and cranny of those light assemblies, looking for anything that could be a short.  Nothing.  I even wondered if the bulbs could somehow be shorting out the circuit.  Perhaps I had bad bulbs?

I pulled the bulb out and checked the voltage inside the light assemblies.  Now it’s fine.  So somehow the bulbs themselves were shorting out the circuit.  Just about when I was going to buy new bulbs, a little voice in my head reminded me of many hours troubleshooting electric horse fences.  A typical problem with electric fences is grounding.  This just smelled like a grounding problem.  And it made total sense that the voltmeter would detect voltage, but there wasn’t a strong enough connection to ground to run the current needed by the bulb.

I took a pair of jumper cables and connected the ground from the trailer coupler back to the light assembly, as shown below.


Running ground to the lights

Running ground to the lights

The lights worked!  OK, I had a grounding issue.  But what was the issue?  I then noticed that the utility trailer is a tilt trailer and is made of two separate pieces.  Hence, the frame is not a single, solid piece.  It became obvious that the electrical connection between the two pieces must have deteriorated due to rust and wasn’t good enough to power the lights.

I ran the ground wire from the truck plug all the way back to the second half of the trailer frame.  Voila!  The lights worked, and I learned quite a bit about trailer wiring.



  1. Wow…you nailed it correctly…I was having the same exact issues with my tilt bed trailer 4×8!!! Now it works fine with the better ground!


  2. well, you pretty much summarized every thing i went through with my trailer wiring debacle….and what was the problem? you guessed it – poor ground. i also have a tilt trailer and think i was loosing ground at the pivot point.
    i ran ground wires to each light and my problems are no more : )

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