Working on a friends 2009 F150 Base model. He just pulls a boat trailer with it. He worked on it for a while and asked if I would look at it. I said sure. My bad. lol

So here is the deal. We know the trailer is wired right. It works fine on my 2000 F250 and it works fine on his 2018 Chebby and a 1989 F150. Just not on this 2009.

All the lights work fine on the 2009 till you plug a trailer into it. Then it gets screwy. Sometimes it works as it should when you first plug it in. Then switch blinker sides and the truck turn signal light on the truck will go out (dash flashes fast). Go back to the other side turn signal and it works fine. Go back to the bad side and it is still out. Turn key off and recycle and the one that was not working works first try. Then switch over to the other turn signal and the truck turn goes out and the dash indicator is flashing fast. There is no rhyme or reason to it.

Someone had been there before for some reason. They have ran new left and right turn signal wires to the trailer plug directly from the truck tail lights. They spliced into both turn wires and ran it back to where the trailer light wires go into the harness.

I had a bad ground on the trailer harness to the frame of the truck which I repaired but did not fix the problem. I unplugged both connectors o the rear harness (saw nothing wrong) and cleaned the contacts. Nothing changed.

I have swapped out both of the trailer tow relays for both the left and right turn signals. Nothing changed. The boat trailer has LEDs so I connected a portable light kit that I clamp to a trailer that has no lights and it is incandescent bulbs and the truck acts the same way with that setup.

Any ideas on what else to look at or try? Thanks

 

yikes, this could get ugly. This would be alot easier in person but we're on opposite ends of the country! I know the lights on anything recent is very fussy about seeing more or less load than is normal, and I'm not sure if it is looking at amp load or it seems it might be resistance in circuit or both. That's probably why it's acting the way it does with their rewiring at the tail lights, even the LED additional load/resistance is freaking out some module on that circuit .I think if I was doing it I would look and see if the wires are still there to put it back original and then trouble shoot that. There's some reason they did all that to begin with and you may have to find out why to ever get it right again. This won't be fun but I'll try to help. Jim

 

Remove the jumpers someone tied in. That whole truck is canbus, you can't tie into the taillights or the BCM will shut it down.

 

Good ideas. I did notice that the left turn did not work when I began but found the wire they spliced into the left tail light was broken. It went to the trailer plug. I spliced it back in and at least the trailer left turn would work sometimes then. lol thanks much I think I will yank those wires and see if I can find the original problem. I will keep you posted.

 

Ok, it works now. Removed the extra wires to no avail. Went back to the plug where the trailer connector breaks out and nothing but run lights came out of it. I remembered checking the "Trailer tow stop turn relay feed" fuse yesterday and there wasn't one there so I assumed (wrong) that it did not have the two package. After find out nothing was coming out of the harness going to the trailer plug other than tail lights (on a different fuse), I put and fuse in the slot where there was no fuse and BINGO. All works now.

One thing I did notice though. When I turn on either blinker the opposite side has a faint blink to it also. I won't worry about that for now. He is happy to having working lights.

Thanks so much for the input guys.

 

outstanding, well done!

 

The faint blink is a bad ground more than likely, it appears more on LED's and it could actually be in the trailer since most don't have a good ground into the plug and depend on the hitch.

 

Thank you sir. Yeah the truck does it by itself with no trailer connected. I assumed a ground and found the ground going to the trailer connector harness and cleaned it up. It now has zero ohms between chassis and that ground on the trailer plug. I suspect there is another ground somewhere for the actual truck rear lighting but I didn't see it.