I have a 2018 F150 without an integrated brake controller. When I connect my camper (20' single axel), all the trailer lights work just fine. The strange part is that if the headlights are on, I get a "trailer disconnected' warning every time I apply the brakes and sometimes I will also get a trailer wiring / bulb burned out warning when I use the turn signals. I have searched quite a bit but haven't found any discussions or solutions about this. I have no idea where to start with troubleshooting.

 

I would start by looking at the various trailer harness plugs under the rear of the truck. Unplug, look, maybe spray with electrical cleaner.

 

Has it rained a lot recently or did you go through a car wash? Mine did that and it turns out a bunch of water got into the 7-pin connector on the truck. I blasted it and the trailer plug with a water displacing spray and it’s worked fine ever since.

 

I replaced the trailer 7 pin connector and cable and there is no change. One thing I didn't mention is that I replaced the trailer taillight bulbs with LED bulbs. I didn't think it was related because there is no issue when the headlights are off. I suppose I will need to install resistors in the taillight housing to fix this?
I don't get it. This is a 2018 Ford, not a 2008. There should be a setting or even a jumper to switch the detection mode between incandescent and LED bulbs.

 

FWIW, I had a similar issue on a sled trailer and it turned out to be a lack of grounding wire in the harness on the trailer (the trailer was effectively grounding through the hitch, which worked on my older rig but irritated the F-150).

If I understand correctly, the message is most likely to occur when you use the brakes and the truck doesn't detect a load on the trailer brake circuit. If your camper has electric brakes, and those are working, I wouldn't think switching the bulbs would make a difference; however, in your shoes, I'd give swapping back to incandescent bulbs a shot to see what happens.

 

I found a couple of standard bulbs and swapped them out. The result is that the disconnect warning does not occur. Basically, the trailer detection circuit is poorly designed.
Now I need to figure out how/where to install parallel resistors. I see lots of things to consider. First, the taillight housing is all plastic so, if I put the resistors in there, I have to make sure they don't get too hot and melt anything. Next, do I need a resistor for the running lights and the brake lights? For the incandescent bulbs, the running light draws 400mA and the brake light draws 2A. For the LEDs, it's 50mA and 250mA. I'm pretty sure I don't need 2A to trigger the detection circuit so I will start by trying 12 Ohm resistors on the brake lights which will give 1.2A total.

 

I tow a travel trailer and I have found I need to clean the truck 7 pin connector and the trailer end really well every year to remove any corrosion. I also use some dialectic grease on the connectors too. These trucks sense really small differences in resistance monitoring bulbs etc, so poor connections only add problems. Since I started doing this every spring I have eliminated all my former issues.

 

I am 100% convinced that the issue is in the trailer detection circuit and not poor contact in the 7 pin connector. I just replaced the connector/cable assembly on the trailer and the pins on the truck connector are clean.

I'm still trying to make sense of this puzzle though. One more thing that I wasn't thinking about is that the running lights are all over the trailer so replacing the tail light bulb with LED has minimal affect on the current draw of that circuit. The brake/turn-signal bulb on the other hand only goes to the taillight. It looks like the truck wants to see the same , if not more, current in the brake lights as the running lights? Maybe that's why I only get the disconnect message when the lights are on?

 

Do you get a trailer disconnected message with LED "bulbs" in place and the turn signals activated, sans brakes?Just wondering since the turn signals are the same circuit as the stop lights.

 

I get a different message. I forget the exact wording but basically it says to check for burned out trailer bulbs.