Helped my friend try to fix one of the brake lights on his snowmobile trailer. Right side works fine, has both running lights and brake/turn signal. Left side, only running lights work. Thinking it might be the bulb, we swapped fixtures. Same result, no brake on left side, only running lights. Remove the light fixture and measure the voltage levels at the connector. The one plug for the running lights had proper voltage when those lights are turned on. The other plug for stop has a constant voltage when brake pedal applied, and a on/off voltage when turn signals on.

We did some resistance testing too and found the ground on the left side wasn't good. There but a higher resistance, so we spliced in a new line and grounded it better. That only gives us a better resistance reading.

Anyone have any ideas? Not like it's rocket science, just DC wiring.
[Reply]

Six pin, seven pin, four pin?
[Reply]

Sorry, four pin.
[Reply]

Looking at diagrams on the web, we're setup right. Yellow/brown on left side, brown/green on right. We ran a lot of new wire, replacing the front half of the wiring. I wish he'd have rewired the whole thing, but he had already spiced in the left side by the time I got back from a run home.
[Reply]

Well, I think I found the answer. Probably the plug on the harness that foes into the back of the light. Something with the ground inside it. My friend is going to see if the trailer store has a replacement and give it a whirl.
[Reply]

Me and my BIL had a similar issue a few years ago with a boat trailer. We spent the majority of an afternoon about to pull our hair out. As your buddy is doing, we took back both lights and the wiring harness (4-pin). New ones went in within 10 minutes with no problem. We never found out what the real issue was. It will drive you crazy!
[Reply]

Thanks Jamie. It definitely is frustrating. Especially when it shouldn't be that hard.
[Reply]

Check your signal at the outlet, brother. The harness may be goofed on the vehicle.
Once I've done that, I just push my probe through the wire covering and test everything on the back side of the trailer plug.
It could very well be a bad contact in the plug. They get pretty gross. Clean them up with some sandpaper and a little file, then spooge everything up with electric grease.

Once you know you've got the right signals on the back side of your plug, you can see if it's getting to the wires behind the light.
If you do, then the fixture ground is bad. Pull it off and clean it up.
If the ground is carried on a wire, and from what you said, I don't think it is, check your continuity between the frame and the fixture. (I think you already did that).

It's all super easy if you go in order. It's the guess and chase game that'll drive you mental.
[Reply]

Thanks Scott, we've done all of this. Plug on back of vehicle has power on the one line in question. We used his volt meter and traced it all the way back, doing like you said, pushing the probe thru the wire. We even have power at the plug at the back, where it connects to the light. The only thing we can think of at this point is the ground in the plug for the brake wire is bad. Instead of going out that way, it's going back thru the filament for the running light, and out it's ground.
[Reply]

Also, make sure that the trailer is hitched to the vehicle and that there is clean metal to metal contact at the hitch. I used to do this at U-Haul when I first got out of the Navy. We sometimes used to have to run a ground wire from the vehicle to the trailer.
[Reply]

I re-read your initial post, Bob.
You should only have two wires at the left side. One is the tail/marker and one is the flashed lead for the turn signal.
So the brake/running light (tail/marker) goes back on one lead. Should be brown, IIRC.
The flasher should be yellow on one side and green on the other, I think.
The ground should be white.

What you describe acts like a grounding problem. The low amp lights will work, but when the max amps are applied the ground is insufficient to get the job done.
If one side is good, I'd think it's the ground at the fixture.

In my experience the best place to lose a ground, aside from at a grounding point, is where guys splice wires with a crimp connector. You can pull on it and it seems connected, but the corroded wires inside aren't grounded well enough.
It'll make a guy lose his mind cause everything looks good.
I've been bit enough times that I take time to solder wires and heat shrink everything nowadays. I even bought a heat gun. :-)

Good luck, brother.
Let us know what it was, will ya?
[Reply]

Yep, you got it right. The yellow wire on the left is the one giving us problems. I'll let ya know what it is/was if we find the culprit.
[Reply]