Hello All,

I am having an issue with the lights on my trailers. I know this is an issue at the truck, because it. is happening on multiple trailers I tow (4 and 7 pin). I am getting my running lights to turn on no problem, but I cannot get brake or signal lights to work. They work on the truck, but on the trailer nothing at all.

Where would you start. to diagnose this problem? I'll take the truck into the dealership if I have to, but I am hoping this is just a quick fix.

Thank you!

 

Are you seeing any faults reported on the dash? I'm not sure if this is the fix, but I have read that the ground has to be very good for the Trailer Control Module to work. I have had a similar problem with 3 different trailers connected to my 2016 XLT. Each trailer's right directional and brake light do not work intermittently. I've had the electrical system on the truck and the one my new camper tested and they both work, but connect them and the above referenced lights sometimes work/don't work. I read recently that I should run a separate ground wire from the front of the trailer to the lights in the rear. I will try it this week.

 

Thank you for the reply. Nothing on the dash at all. I've read about guys pulling a fuse for the Auto/Start/Stop feature causing issues, but I haven't modified the truck in any way at all.

 

Have you tried testing the circuits to see if they are powered at the plug? A cheap $12 tester or a $10 Harbor Freight VOM will do.
As Bronze 150 pointed out check your grounds. I believe there is one ground point on the chassis under the bed on the passenger side that has caused problems because it's more exposed.
Also check the harness plug under the bumper by the receiver. Unplug it, clean with electrical cleaner then give a nice coating of dielectric grease to protect it. This also is exposed to elements

 

First, I assume the trailer lights work on other trucks?

With most trailer lighting issues, I'm suspicious of the grounds first and foremost; if you have an accessible ground point on the trailer, plug in the harness (but don't put on the ball) and check to see if you have continuity between the trailer chassis ground and truck chassis ground. If not, start by resolving that issue.

Second, are the 7-pin connectors actually wired to separate brake and turn circuits, or are they 7-pin connectors on 4-pin wiring? In my experience, the latter can be an issue; I had an open sled trailer (no brakes, combo brake/turn lamp on each side, plus running lamps) that had a seven-wire plug on it. It worked fine on my XTerra (with its not-so-smart trailer lighting system), but on my F-150, it wouldn't work (although it would throw an error on the dash screen) because the brake circuit was open. Removing the seven-pin connector in favor of a four-pin solved that issue (although that was a bit disappointing, because the seven-pin connectors seem to seat more reliably).

 

The truck provides the same ground for brake lights as it does for clearance and running. If you lost the ground from the truck, you'd have no lights on the trailer. Check the plug for any voltage on the left/right wires. If it's there but low, clean the connectors under the bumper then re-check. If it's not there, check the output at the source.

 

It should be standard on all trailers ,green right, yellow left ,brown running white grd ,if you need blue for trailer brakes or rev.lights.

 

Do you have a key fob? Make sure when you testing your lights that the key fob is in the truck and all the doors are shut.

 

This may sound stupid but has the Ford dealer "activated" your tow package.

I couldn't believe they needed to do it to my truck.

Figured you buy a brand new F-150 with a Max Tow package it would be ready to tow a trailer right off the showroom floor.

Nope, they needed to activate something stupid for it to talk to the tow harness or some ridiculous crap.

Same thing, I only had running lights. No brake lights, turn signals or signal to the trailer brakes. Tried two different trailers same thing. My Nissan everything on the trailers worked just fine.

It's the truck.

 

Have a look under the dash above the brake pedal, there is a white plastic box with 3 or 4 plugs going in to it. Check and see if the smallest (or any) plugs are undone, what you described is exactly what happens when someone unplugs the smallest plug to get rid of the auto start/stop feature and then has some lights not working on the trailer.