This message is also displayed during manual
activation without a trailer connected.
This is what he said he did....

 

so i monkeyed around with this issue for a bit doing a little troubleshooting last night. hooked the wiring harness up and this time the "parking aid" said <on>, so the reverse sensing system beeped while truck was in reverse. all of the trailer lights worked just fine, so i didn't think there was a bad connection at the 7 pin plug in, but i decided to clean the female plug on the trailer anyways. i used a small strip of sandpaper to shine up all the metal contacts. a few had a bit of corrosion but nothing i felt was of any consequence. i noticed there was a little curly cue of black rubber clung onto the center pin, which must have been a remnant of the original molding process, so i picked it out with an x-acto knife. i put a touch of terminal grease on the connector and plugged it back in, and lo and behold the "no trailer" message went away and everything seems to be working just fine now. so even tho all the lights worked fine during this issue, the plug in connection must have still been the problem. so i'm happy now!

 

on another note, i checked the fuse panel under the hood and it was missing the fuse and relay for trailer battery charger. it just so happened that there was a small plastic bag with the proper amp fuse and a reley in the passenger door compartment when i first got the truck. i always wondered what they were for. so i put those in last night to. i had assumed my truck would always be charging the trailer battery, but i'm sure that wasn't the case not having the fuse and relay in place. why wouldn't these items have been installed either at the factory or at the dealer?

 

That is bang on. The Ford integrated brake controller will not work with hydraulic brakes and you will get the disco message.

 

I checked the fuse panel under the hood and it was missing the fuse and relay for trailer battery charger. I found the small plastic bag with the proper amp fuse and a reley in the glove box. I have two questions.
1) Is there anything special that I need to do for the truck to charge the trailer battery?
2) Why wouldn't these items have been installed either at the factory or at the dealer?

Mustang1964s

 

Now put some dielectric grease in there so you don't have anymore corrosion issues. I had this same problem and it was a couple of dirty pins at fault.

 

1.) No, as long as the fuse and relay mentioned above are installed where they are supposed to be AND the trailer and its battery are wired properly.

2.) Good question. Some dealers do this I'm told but I haven't seen this be true in my past two new purchased Ford trucks from two different dealers.