Troubleshooting Running Lights Not Working with 7-Way on Trailer that Only has 4 Wires

Question:

I have a 6x12 flat deck trailer. Brake lights work, signal lights work, running lights don’t work. When I use other trailers, all their lights work, so problem isn’t the truck It is the trailer . Trailer has a seven way plug, but only 4 wires .

asked by: Scott

Expert Reply:

Since we know that the problem is isolated to the running light circuit on your trailer then that is what you need to inspect. When wired correctly, the running light circuit on your trailer should be connected to the pin in the 2 o'clock position on your trailer side 7-Way (see attached picture).

Start off by making sure that pin is clean and free of corrosion. Then look along that wire and check for any places where the wire may be damaged, exposed, or bent and replace as necessary. Checking your grounds for your running lights is another great idea as that is typically an issue. They should be securely connected to a clean, bare-metal surface. It wouldn't be a bad idea to go ahead and check the main ground for your tow vehicle as well since the other trailers you hooked up may have had less lights and required less ground.

If everything looks good, then you can try testing different sections of your wiring with a circuit tester like part # PTW2993 as well. I have attached a helpful article that you can check out. It references 4-Way and 5-Way setups, but it will still apply to your situation.

expert reply by: Jon G