I have a 2017 SD that has the factory tow package with 5th wheel connector on the LH side of the bed. Every time I tow I keep getting intermittent "Trailer Disconnected" Warnings on the dash. I have replaced the trailer plug although there didnt seem to be anything wrong with it. Shaking the plug and wiring while trailer is hooked up does not produce the warning. You would think the problem is on the trailer side of things but I just cant find any problem there and Im wondering if it could be on the truck side. I guess one question I have is What pin on the truck side of the connector needs a ground or to complete a circuit to tell the truck that a trailer is connected? I havent pulled the plug out of the bed yet to check the wiring on it, not sure if you can see anything anyway.

 

check the backside connectors on the truck. Both up in the bed, and down on the bumper. There have been reports of them being not fully latched.

 

Since my truck detects when it is connected to a boat trailer with surge brakes, I would say the lighting circuit is what is used for trailer detection. Don't know that for a fact, though.
Take a look at the pins inside the socket. I was surprised how quickly they showed corrosion. A spray with contact cleaner and a coating of dielectric grease fixed my intermittent "trailer disconnected" issue.

 

I have had this happen with other vehicles. Spray the connector and the truck plug with some dielectric grease to see if you've got a slightly flaky connection.

 

I have to agree...must be the lighting circuit. My truck recognizes my small, single axle/no brakes enclosed dirtbike trailer.

 

The lights on my trailer are not flickering and if the Trailer Connected circuit went through there you would think the lights would flicker?? IDK, lol , and I dont see any corrosion on the connector pins. I guess I will pull the connector out of the bed side and inspect the connections on back of plug. If I find the problem I will take Pics and post the findings. Thanks

 

I had this same problem towing an old car trailer. I have to assume the car trailer had a bad ground. Trailers are famous for that. I also had no light flickering.

 

Any chance you have a bed mat? My bed mat, while at speed, would flip up, occasionally, with enough force to not only slap the connector to give me he intermittent disconnect warning, but would also knock it out completely every now and then.

 

Like wrvond mentioned, I will add some dielectric grease to the contact points. That cured my 2016 F350. Also check the tabs on the trailer plug end, sometimes they need pinched back together.

 

I have read on here that some trailers the have LED lights may have to have a resistor installed, the LED's may not give enough resistance. I have not had that problem and mine are all LED's.

 

I converted all my trailer lights (interior and exterior) to LED and haven't had any issues. All my connectors under the truck got the dielectric grease treatment a few months ago. I didn't have any issues, just preventive.

Check for a cut in the trailer cable.

 

OK I got a chance to pull the in bed trailer connector apart. I see no corrosion anywhere, I checked the back of the connector for loose wiring like someone suggested. All looks good to me. My 44' 5th wheel has all LED lights but it hasnt been a problem for the last 11 months. Trailer ground is good. IDK? Just drives you crazy with the warning going off all the time. Wish I could fix it or find a way to deactivate the warning because the trailer lights work fine.

 

What about the terminals on the trailer plug? How wide are the terminals spread? Do you have dielectric grease on connections?

 

Do you have a metal wiring box attached to your king pin? I have a bunch of ground wires bundled (white) together. I had an intermittent issue and found a loose wire in the bundle.
I removed some insulation on the one wire and bundled it all with a wire nut and tapped it real tight. Now my truck immediately recognizes the 5th wheel. I have all led lights btw.

hope that helps. Check other grounding points also...

 

I installed LED tail lights in my trailer and get "trailer disconnected" every time i touch the brakes, and 'trailer connected' every time I let off them - only if the headlights are ON. As other's suggested, you have some resistance or ground issue w/ trailer lights/wiring.