Troubleshoot trailer taillight wiring

WesternRoadtripper1
Sat, 30 Apr 2022 06:51:19 +0000
Maintenance/Repairs

When I noticed that directional signals and brake lights were not working on BOTH of my small utility trailers, I realized that it could be bad wiring on both of the old trailers, or something in the car. (or both).

Trailer lights DID work at one time on both trailers, but of course, both could easily have developed issues since then, I’ve been bad about checking them regularly. I only noticed the problem because I’ve recently removed the trailer lights on the smaller trailer for sandblasting and painting, and they didn’t work when I re-installed them.

That could easily be bad ground on the trailer, given new paint, though I did clean a surface where the previous wiring grounded at each tail light.

To discriminate between a problem in the trailers and problem in the car, I bought one of those small 4-flat testers at a local store. The tester indication matched what the trailers were doing: taillights work but no brake or directionals. So the problem appears to be in the car.

That’s a problem. I use the smaller of the trailers frequently. I had to look deeper.

The car lights are fine, everything works normally.

When I got this car a few years ago, it had no trailer hitch and no sign that there ever had been one. But inside the compartment for the jack was a wire harness with a square shaped connector.

At the time, I assumed that this wiring was factory installed, but looking at it now, I see that there’s electrical tape wrapped around it, casting some doubt on whether it was factory installed.

To get the car set up for towing my trailer, I bought a used trailer hitch and bolted that to the car, then bought a manufactured 4 wire harness extension to plug into the “built-in” harness. It did mate with the existing connector, suggesting that the wire harness in the jack compartment might be factory installed equipment. The reason for the black tape remains a mystery. Again, the trailer lights did work at one time.

Continuity test on the manufactured 4-flat extension shows all wires are good. So that suggests a problem in the car system. There might also be problems in the trailers, but the tester says there’s something wrong in the car wiring.

Can anyone suggest where to start in looking for a problem here? Am I right in assuming that I should have 12 volts in three of the four wires in the square connector on the “factory installed” harness when properly activated, and one functioning as ground? (I presume white is ground, is that correct?).

Again: all lights work normally on the car. The 4-flat tester shows good taillights, no brake lights or directionals, at the end of the wiring harness extension.

Thanks for any advice.

22 posts - 7 participants