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I am about at my wits end. I have a 2016 F-350 Lariat w/the 6.7 Powerstroke. Everything has been perfect up until last year, I started getting an occasional "Trailer wiring fault" when hauling my fifth wheel (2018 Jayco Pinnacle) Now it is happening more often and when it happens I lose the trailer brakes. (almost rear ended a guy who cut in front of me then stopped short to make a left had turn) Needless to say it has make me very apprehensive and almost afraid to tow my fiver. I have inspected, cleaned, and applied fresh dielectric grease to all the connections under the truck with no improvement. I have an IPA electric brake force meter and discovered something that at least I think is unusual. When I am traveling down the road an apply the trucks service brakes the meter shows power going to the trailer brakes and depending how much force I ally to the service brakes the gain increases. As soon as I come to a complete stop the power suddenly shuts off going to the trailer brakes. If I leave the brake pedal depressed about 5 to 7 seconds later the power returns. Is this common, or do I need to look elsewhere. (I am about to sell both units and buy beer)
I have never used dielectric grease to fix a trailer fault issue. As you drive the controller is constantly sending a very small voltage signal down the brake lead and back to insure you have a trailer in tow. If that connection breaks for any number of reasons such as a failing connection at the trailer brakes you get the fault signal warning you the trailer is off line.
While it is possible the controller is failing, more often it has been my experience a connection issue. Controllers quite often get blamed for things that may not be their fault. I do not thing your finding when stopped is causing your problem.
Since the most common issue is a failing ground, if you can reproduce the problem in any reliable way, take a jumper cable and make a good connection between the fiver frame (the pin box should work) and to the truck frame and see if anything improves. It also would be a great help diagnostically to see if the problem recurs with another trailer. Intermittent problems can be maddening to locate.
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