Trailer light issue

I have a wiring/lighting issue with my trailer. 7 pin plug. It has a breakaway trailer break. So the run down. I used a circuit tester on the plug of my vehicle. All pins light up properly with the 12V needed. The tail lights are signal, break, and running lights. Then I have three running lights on top back end of my trailer. And two running lights on each side. Plus a light for the license plate that connects to the left tail light. My left tail light was only partly functioning and the circuit tester said only 4V going there. So I know I have to replace some wires on the left. However, this is the part that I am not able to work out. This is true on both sides of the trailer, but I did most of my testing on the right since that tail light working just fine. My right tail light was getting 10V on the running light wire. The running light, blinker, and brake light was working. However, the three top running lights connect on the right side are not working, nor are the two right side running lights. I tested at each light. I have 9-11V at each light. So there is power. The lights are riveted on. To test my ground at each light I touched the live wire and the rivet using my circuit tester. All good. Then to check the running light fixture, I touched the ground to the strip inside that the rivets go through to connect to the trailer for the ground and touched the live wire. Still 9-11V depending on the light. Then I tested the running light fixture by touching the grounding inside and the metal connector for the light where the live wire connects. Nothing. I put on new connectors. Retested. Same exact results. I had extra running light fixtures from a previous trailer I wired. They also gave me the same results. I can take pictures and send them if necessary. No matter what I replace, the running lights arent working, but each unit has ground and power.

It’s hard to tell from here, but what you’re describing sounds like the infamous “bad ground” problem. All of the running lights on a trailer are connected together; it’s all the same circuit. With a voltmeter you should see 12 volts all over the place back there. Since you’re seeing as low as 4 volts, that means you have a bad voltage drop somewhere. Wires either conduct or they don’t so I doubt it’s in the wiring. The “ground” is the trailer itself, lots of corroded mechanical connections, rivets, and other potential problems.

Your circuit tester draws a tiny amount of current compared to the lights. It will tell you if you have voltage back there, but not power. Power takes some amperage and if there are corroded connections you’ll lose power right there.

The solution here might be as simple as taking apart anything you can on the ground side and cleaning it up with a wire brush. The ultimate solution is to run a dedicated ground wire from the connector at the tongue all the way back to the lights. Once I did that, I’ve seen a solid 12 volts, bright lights, and no problems for many years now.

SR

Many of the riveted lights ground through the rivet connection to the trailer. Over time, oxidation builds up t the penetration or between the rivet head. If the light actually has a ground wire to the trailer, remove the connection, sand the surfaces and apply no-ox.

2x on the dedicated ground, fixed my problem when I did it a few years back.

Steve Root wrote:

The ultimate solution is to run a dedicated ground wire from the connector at the tongue all the way back to the lights

This!