Guys, I'm stumped by a lighting problem with my brother's 2012 F250 pickup and its flatbed gooseneck trailer.

He just replaced the round-pin plug on the trailer so it would match the 7-flat-pin factory trailer lighting socket on the truck bumper, without using an adapter.

We had issues with the trailer lights until we figured out that he maybe used the wrong pin in the trailer plug for the ground, and then he had the left and right turn signal reversed. And then had to deal with a bad ground connection on one of the LED taillights, as they are mounted onto these flip-up ramps with rusty hinges.

After he put in jumpers in the taillights to insure solid grounds, by-passing the hinges, all the lights on the trailer worked great!
Running lights, brake lights, turn signals; all good.

Until we tried the lights again later..... Nothing would work then.
We tested the trailer lighting with a 12v battery and jumpers; the lights all worked.

I checked the output from the pickup socket with a meter; the outputs all looked good and correct, according to the diagram on the cap of the socket.

I even disconnected the battery one time for a minute to try to reset the truck's ECM or CPU, in case it somehow got corrupted data in the processor.

Still no lights, save for the turn signals, alittle weakly.....but not if the running light switch is turned on (still no running lights).

Any clues for me? Thank you much in advance.
Troy

If I were getting any power through even though as you say weakly, I would go back to looking at a grounding issue. It could be from the truck plug since you have been messing with it.

Make sure the wiring on the inside of the truck plug is still good and not corroded.

Have you checked fuses? It will may have different fusing for the tail/stop and turn circuits. Don't think it's that but I would verify.

Sounds like a ground issue. On the very back of the truck receptical, Normally the ground is the white wire. Check from that wire to actual ground on the frame of the truck. normally the White wire is the largest in the seven pin configuration whether it is a 7 flat blade or a 7 round pin setup.Also recheck all the fuses in the fuse box with a test light or a DVOM.

Conventional layout for reference .

This message has been edited. Last edited by: TD3550,

“Still no lights, save for the turn signals, alittle weakly.....but not if the running light switch is turned on (still no running lights).”

That tells me it’s a ground issue probably at the truck. Locate all the grounds truck-trailer and clean them. The ball ground is not enough and erratic. It can be tedious and a pain in the azz!

Have you got another truck you could plug in or another trailer that truck does have separate fuses for the trailer but my money is on a ground problem take your jumper cables and clamp from trailer to truck bet they work

most led lights need a dedicated ground(at least some do)tie all the grounds together to the plug and on to truck

Thank you much for all the great responses, guys. It seems that an intermittent ground is the common denominator.

I had to drive that rig a short distance today, and the dash display kept displaying the message "Trailer not connected" and "Faulty trailer wiring", off and on.

The idea of testing by jumping between the truck and the trailer chassis with a booster cable is brilliant, and I feel dumb lol for it not occurring to me.

The problem may be a loose connection in the trailer plug, though my brother assured me he had those connections tight (ahem).

Thanks too for the great wiring diagram that I can probably download.

Again, thanks for all the tips; I will follow up on all of them
Cheers!
T.

Even with the ground wire in the connector, most lights on a trailer ground through the frame and also use the truck to trailer connection at the ball. Make sure that it's not all rusted up. My son's flatbed's lights won't work unless there is a good connection there....

Correct to above statement got so all my customers landscape trailers I started running ground wires from light assembly right to trailer plug because they would rust up so quick

ford likes to use a ground wire on the side of the frame rails near the trailer hitch I bet the issue is rt there

Thanks for the extra tips, guys.