Originally Posted by Mark Oomkes

First off, I have 2 Tow Commands, both on '05's and love them. One came with the truck, the other was added because my sales rep was (he still is, just not my rep anymore) an idiot. They are great, no more herky, jerky at lights with a heavily loaded trailer and high brake setting, no more locking the brakes at low speeds. Ford did this one right. However:
Warning

I am having issues with both of these trucks with certain trailers. I own a landscape management, so I have numerous trucks and trailers. I can hook up a trailer to an older truck with a aftermarket brake controller and have no problems, brakes work as advertised. I hook it to one of my 2 with TC and I will get intermittent "Trailer Disconnected" or "Check Trailer" along with the accompanying beep almost constantly. I took the plug apart on one of the trailers, looked the wiring over, and everything is as should be, at least according to the diagram on the truck side plug. When I rewire a truck side plug--after it becomes corroded--I always run a ground wire to the frame at the back of the truck so not only is the trailer grounded through the hitch (which doesn't always work) but also has a wire through the plug from the trailer frame to the truck frame. I just had one of these trucks into the dealer for a FICM and had them check the TC while it was there, they cleared all codes and checked it on their simulator and said it was working fine.

So can someone tell me what causes the truck to 'think' the trailer is disconnected?