I have a 2012 F150 Max tow. Recently I have had an issue with the truck display saying the trailer is disconnected. At that point the trailer brakes do not operate, however the trailer lights still operate. I have checked the electrical connector and it is plugged in, hence the trailers lights working. I have looked at the fuses and they seem fine. not sure what the truck is looking at to determine connection.

 

I've run into this problem recently. I think what's happening is that the plug is a little bit loose in the socket, so it loses connection momentarily. I used a small screwdriver to bend the connectors in my plug over a little, and it seemed to help. I think I might try some dielectric grease the next time I use the trailer, as well.

Anyhow, to answer your original question, I think it looks for the trailer brake circuit.

 

That is what I sort of thought must be happening. I am going to replace the trailer plug and if it happens some more work on the truck side connection. Thanks!

 

Does it happen when you turn? You might be getting a little bit of tension on the plug and losing connection.

 

No it is there even from the start and then can pop on and off as we go down the road. the electrical bundle is fairly long so it is not getting pulled at all.

 

Had the same issue, receptacle replacement took care of it.

 

I ran into this on my brothers boat trailer. That trailer has surge brakes, so it's not drawing power for the brakes, and the truck cannot see it. The lights operate just fine, and the surge brakes work as they are supposed to.

That was my first time using surge brakes, and it really stumped me for a while. I guess they make sense on a boat trailer, but I much prefer the brake system on my TT.

 

I thought it reads the ground (-) through the break positive (+) wire. so if one of the to are not there it will not read and send mag to display of trailer disconnected.