Hello guys. I recently purchased a 2018 F350 SRW Powerstroke to pull my RV with. This is my first Ford. I love the truck but I have a major problem. I have trailer running lights but no turn signal or brake lights. I learned this when a nice police officer pulled me over and entered me into a "raffle." I have checked all "trailer" related fuses and they are fine. One unrelated issue is the horn doesnt work but I havent started checking it out yet. Thanks for your help.

 

I assume all the vehicle lights work, you can buy a 7 pin trailer plug tester, its got separate LEDs for each function, might be worth checking the 4 pin connector, though I imagine its the same exact wires. Does your vehicle have the factory trailer brake connector? Have you tried a different trailer by any chance?

 

I tried two trailers and yes I have tested the plug with a test light and I am getting nothing. Yes to factory brake

 

Ok thank you for confirming youve tested on more then one trailer. So running lights are the only thing that works. Brake, left and right turn, reverse, license plate, 12V battery are all inop?

 

Correct

 

I'll get back to you in a day or two with wiring diagrams and what not, work is crazy busy this week.

 

OK first up here is the horn diagrams for if you have adaptive steering or not. Remove horn relay, check for voltage at pins 1 & 3. Jump relay pins 3 & 5, see if horn blows. Theres also connector view and location for the horn itself.
Easiest test for the horn is to use the key fob panic button to see if it works, basically if the horn is inop from both the steering wheel and the key fob its likely the horn itself.

 

OK here is the trailer wiring harness diagrams. I would first go through and check every fuse. Do you have the connector in the bed or on the bumper?

 

Sir, since you bought this truck and finding this problem it was probaly doing this before. your truck has a module for body control and trailer control. it sounds like you just need to make sure trailer lights are fine and truck lights are fine then you just need to reset the body control module or the trailering module. I have a 2011 f250 pulling trailers when bulbs go out and short I fix them then I have to plug in Forscan to reset the module.