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Question:

Help with trailer wiring (4 wire)?

I have installed a new wiring harness on my trailer, and two new LED trailer lights. The tail lights work, both turn signals work, but for some reason, I do not have a brake light on the right side. Both lights are grounded to their own post, and I do have a good ground on the coupler of the trailer. If a turn signal and brake light in a 4 wire system are using the same wire (green in this case)- how can one work, while the other is not? Any good ideas? I really appreciate the help.

Answer:

Not all cars (imports) can run on a 4 wire system. (they look like they will work when testing or locating wires, but don't when all hooked together). Adapters are at the car parts store, but not cheap.I would first try running the trailer lights with one power wire touched to the three wires, then hook up a stabber type elec tester and see if the problem might be at the car circuit. Do the car lights work right? Would get a jumper wire and jumper the trailer to the car and each light on the trailer to the frame. You may be getting a back feed ground on the circuit somewhere. All the temp grounds will tell you where the problem is.
assuming it was working ok before....... funky symptoms like that (where you can't imagine the circuit that would make 'em behave like that) almost always turn out to be a weak ground -- like the ground is ok for turn signals because only one side is on at a time, but when you demand enough current to light up both sides, it just ain't happening. you can rule out a ground issue between the vehicle and trailer by hooking a jumper cable between the frames. if that don't produce a change, then run an oversized wire from the fixture direct to the connector.
all sound good, but try checking your fuses also. some veh. have a different fuse for each side, and trailer lights may be separate also.

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