driver's side trailer taillight

[pestrem]

My driver's side boat trailer taillight is partially working. The turn signal and brake light work; the taillight when lights are on doesn't. Have an LED set on the trailer. Ohms check shows continuity all the way back to the wire coming out of the trailer at the fixture, so it seems the harness is good. Right side taillight works in all modes: taillight, blinker, and brake light.
Thought it might be the light assembly itself. Changed it out and am getting the same problem. Have read elsewhere that the ground might be bad on to the fixture (old and new), but if that were the case, why would the brake and blinker work? Also, paid special attention to connecting the ground wire on the fixture to make sure it was secure. Any ideas?

[Donald Garner]

I've had that same situation occur a couple times on my Stratos Trailer.  I wound up replacing the light fixture a couple times which was frustrating.  My issue I found was a bare wire inside of the trailer frame and a loose ground connection.  The bare wire was caused by rubbing etc. it was a PIA chasing it down and getting it fixed.

Good luck with getting it figured out.

[FD]

Ohms will not tell you the whole story.  Check the voltage coming out of the harness when you have the running lights on.

It's likely a partially broken wire.  Since running lights are not left/right specific you could jumper over drom your right side wire and if it works, then you know for sure you have a bad wire going to the left harness.

[TechAdmin]

What FD said above. And if you're ever in doubt about a ground you can just grab any cable and squeeze it underneath any bolt to the frame to test, given the whole frame is permanently negatively charged.
(I'm no fisherman but I do love and deal with electricity :D )

[topdsm0138]

As our trailers are galvanized or "hot dipped", here is some reading material on using galvanized metals for grounding.

https://southatlanticllc.com/blog/galvanized-grounding-myths/#:~:text=This%20misconception%20likely%20stems%20from,an%20effective%20choice%20for%20grounding.

Sent from undisclosed location. -Tiller Mafia

[J.W.]

If you haven't figured it out yet, try a multimeter or test light on the vehicle's wiring harness, like FD said. If you're not getting 12 v on the running lights pin, you might have a separate towing lights fuse on the vehicle that has blown. I know Toyota does this now, and I think a lot of other makes do, too. Confused me the first time it happened, but I kind of like it. If you pop a fuse because of water intrusion or a dirty pigtail or something, your vehicle's lights still work.

Edit: Nevermind, I just realized it was only one side that isn't working. Obviously not a fuse. Sorry. Sounds like a grounding issue. Do you have a plastic license plate holder that the ground wire ring is up against? I've seen several trailer mfgs doing this, and mine is that way. If so, put a washer between the ring and the license plate holder so that the ring is sandwiched by the metal on both sides, and not touching plastic. I had to do this with mine because the ring wasn't making good enough contact to ground.

[Princeton_Man]

Have you hooked to any other trailer to see if the lights work properly?

I had the very same problem back around 2014 or 15. All the tests said everything should work, but the left tail light on the trailer didn't. My tow rig at the time was a 2006 GMC Sierra 2500HD which I bought used. BigWrench went over it with me and discovered a colossal jury rig that had been done by the original owner of the truck. My GMC had several fuse panels/boxes. Two under the hood and one on the side of the dash between the instrument cluster and and the door. There were fuses for every light on the truck and for the trailer. The previous owner had blown the left trailer tail light, could find the fuse, and tapped the trucks left tail light with a jumper wire run down to the trailer wiring socket.

That must have worked until the internal backside of the trailer socket became filled with all the crap from road spray and salt. He removed the jumper wire, disassembled and cleaned out the socket, and replaced the left tail light fuse. Problem solved.

[pestrem]

Based on comments received, I've done some more troubleshooting. 
-Fuse in the truck is okay. 
-Volts coming out of the truck connector good - to include the adapter I use
-Had put on a new front end of the harness about a year ago. Peeled back the protection, cut the splice to the left side light, and am getting 12+ volts from the wire there.
-Reconnected that and cut the wire to the plug going into the tail light connector. Get 12+ volts when the signal is on but 0 volts when testing the tail light wire at the back of the trailer (this is a Stratos trailer btw).

What did I learn? To save future troubleshooting till after the weather cools down here in the panhandle of Florida. Was 111 head index when I wrapped things up this morning about 10:30. I'm working under a carport in the shade but it's still HOT!  I've put the wiring back together and still have the original problem. Gonna run it like that till late fall then put a new harness in. Based on what I see, there's a break somewhere in the channel of the taillight wire. (By the way, all the side running lights come on when I turn the lights on).

Thanks for the suggestions. 

[pestrem]

Update on the taillight not working.  As mysteriously as it quit working, it started working again.  Hooked up my trailer the night before a tournament a couple of weeks ago, and when I turned the lights on to check the ones that worked, all lights worked.  So, I'm not going to try to 'fix' anything anymore till, and if, it becomes a problem again. Go figure!

[Marky1]

Does your trailer have surge brakes (disc/hydraulic) ? Are the brake lines steel inside a metal tube the that is attached to trailer ? Brake lines were rotted,I mean ROTTED. If so ,the problem could be a bad connection to the harness. That is how I found my problem. harness was old ,cracked. Brake & directionals still worked  at the time even if the fuse was blown.