Trailer Stop/Turn Lights INOP

Purchased a new (to me) boat and trailer which needed the right tail light assembly to be replaced. It has a 5 pin flat connector which I connect to my 2019 F350 via a 7 pin -> 5 pin connector.

I replaced it with a new LED assembly. Testing the installation I get tail lights but no stop or turn signal lights.

Suspecting bad wiring, I disconnected the new light (at the 6" pigtail) and checked for voltage.

The brown lead (tail/running light) gives me 12V+

The green lead (stop/turn signal) gives me a steady 12V+ with the brake pedal depressed and a flashing 12V+ in sync with the turn signal.

I reconnect and no stop light, no turn signal.

I checked the ground lead and observe 0.1 ohms resistance between the ground lead and a point on the trailer frame.

I connect the truck to my horse trailer and all lights work fine.

Any ideas?

SeaRayPilot said:

Purchased a new (to me) boat and trailer which needed the right tail light assembly to be replaced. It has a 5 pin flat connector which I connect to my 2019 F350 via a 7 pin -> 5 pin connector.

I replaced it with a new LED assembly. Testing the installation I get tail lights but no stop or turn signal lights.

Suspecting bad wiring, I disconnected the new light (at the 6" pigtail) and checked for voltage.

The brown lead (tail/running light) gives me 12V+

The green lead (stop/turn signal) gives me a steady 12V+ with the brake pedal depressed and a flashing 12V+ in sync with the turn signal.

I reconnect and no stop light, no turn signal.

I checked the ground lead and observe 0.1 ohms resistance between the ground lead and a point on the trailer frame.

I connect the truck to my horse trailer and all lights work fine.

Any ideas?

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Was having issues with my trailer as well. I cleaned all the light connections and used dialectic grease when reassembling. All lights started working except for 2 running lights.

Thank you Ryan for the suggestion. I cleaned all of the connectors prior to reassembly. I have new connectors from the trailer wiring to the new light pigtail. I have voltage at the trailer wiring where the pigtail connects, just no lights.

The port side works normally? Is the port side a regular bulb type light? It's possible your truck doesn't like the difference.

I had the same issues. So I ran a ground wire from the plug to the frame. Then to each ground point for each and every light. This put ground to every fixture and the frame. I found that just cleaning the frame ground at the fixtures worked on a few. It was a PITA but when the lights stop working on the boat trailers, it's always ground related.

Not sure of the OP location but we have the salt air, salt water, rain water and spray, and calcium chloride or what ever they can get for deicer. That deicer will corrode coper wires faster than salt water will. And Super Lube from Harbor Freight is your friend. Gray tube, cheap and will stay put. Good for high temps as well as cold. I found it for lubing my high power rocket motors (think shuttle SRB's on a smaller scale) to seal the o rings and make clean up easier.

Lazy Daze said:

The port side works normally? Is the port side a regular bulb type light? It's possible your truck doesn't like the difference.

Click to expand...

Great thought, so yes installed a matching LED on the port side. That works great.

SeaRayPilot said:

Great thought, so yes installed a matching LED on the port side. That works great.

Click to expand...

Good to hear - double check that your running lights on the trailer/fenders still work properly, as well.

New information, the problem has been isolated to the truck. Its not in the trailer. (I don't think)

1. My gooseneck horse trailer 7 way in the bed works fine. All functions normal on horse trailer. Since I don't use the bumper socket for the gooseneck I assumed when the horse trailer worked, it all worked.

2. So this afternoon, I plugged the horse trailer into the bumper socket, no right turn signal, no stop lights, fault message on the truck display.

3. So once again, multimeter/test light time. You guessed it. I get voltage from the right signal blade to the ground on the bumper socket. It looks normal until it is attached to a trailer.

4. This is what originally confused me as I started tracing power at the trailer and had voltage on the signal line at the trailer light unit as well. So power is flowing through the bumper socket, through the trailer wiring and to the lights but no turn signal on the boat trailer or horse trailer.

I hate to throw parts at a problem but I'm seriously considering a new socket with new connection to the harness. What stops me is, if the bumper socket or harness connection was bad, why would I be getting power (12V) on the multimeter at the trailer light unit?

Before I went with the full ground I would get power to the fixture but not the light. I was getting power because I was putting my light and meter to a good ground. But the fixture still didn't have a good ground. I beat my head on that trailer for days trying to figure it out. It wasn't until I finally said, enough is enough and replaced all the wires. And if there was corrosion and the wires couldn't be soldered, then it was replaced. No butt connectors and everything covered with heavy marine sealed shrink tubing. Salt and deicer will travel the wires bad.

EDIT: On every vehicle that had the in bed socket, I had a sealed box under the bed on the frame. Wires would run into the box and I had terminals for each wire. The terminals were riveted to the box so not to bounce around. That gave a place for all wires coming in to be separated and clean. No having to pull a bunch of sticky tape off of wire bundles and hours tracing each wire. bundle in from the cab and wires out to the tail end of the vehicle then wires on the other side for the trailer sockets.

That and if I get stopped with a light out, I pay three times the fine. And they can pull my over sized permit for the boat and/or put points on my CDL A. So I prefer not to give the man a reason.

Check the fuses on the truck. I had a similar problem on my f-350 and found there were 2 fuses labeled as trailer stop /turn and one was blown.