Trailer brakes not working, truck says no trailer connected?

[B & J]

So just started towing with a 2019 F350, every other trailer that I connect to the truck shows trailer connected and everything is working. When I connect our "new to us" 2022 Montana Fifth Wheel trailer the truck does not show that the trailer is there. All the lights work but no breaks. Do I need to look at the truck for a problem or to the trailer for a problem? What tells the truck that the trailer is there? The brake system or the lights or?

 

[buck119fa]

I had this issue with my 22 F350 and the 5ht wheel. After a lot of trouble shooting, taking it to the dealer for inspections and trial and error I found it was the wiring on the trailer. I could hook up to other trailers, campers and a different 5th wheel and everything was fine. l dug into the brake wiring and found shorts on the trailer, bad connections at the wire from the main harness connecting to the brakes at the drums, bad wire inside the axle from side to side.
I removed all the connectors and soldered the wires and used heat shrink over it for all the connections.
The big issue was the wires that run through the axels from the left side to the right, the wiring had rubbed through inside the axle tube causing the short. They run that wire through there with a lot of excess and it just bounces around in there rubbing the coating off. There's others that have posted the same issues, it's fairly common.
For my repair I ran new wire up and through the underbelly and dropped it down to the axle, soldered the connections up, heat shrink, added some wire loom for protection where it comes out of the underbelly to the axle. I left slack on that for the axle movement so id won't pull apart. Issues solved. Haven't had an issue since, we've put a lot of mile on it.

 

[scottandsusan]

Buck119fa,
Our brakes will occasionally not work. I assumed my controller on the my older truck was going bad. Thanks for the heads up on checking the trailer wiring.

 

[ChuckS]

Quickest way to check the brake wiring and magnets is to use an ohm meter set to X1 scale.. On the RV trailer 7 way plug measure the resistance of the brake magnets..

Note - tandem axle all four brake magnets are wired in parallel...

Your resistance reading should read around 1.0 ohms or less of resistance when measured at pins 1 and 2 on 7 way plug of RV..

Each magnet measured separately would measure around 3.2 ohms of. resistance if you remove the two wires on the backside of the brake backing plate and measure across the two brake magnet wires coming out of the backing plate.

** If the initial Ohm measurement with all four brake magnets connected did note read 1 ohm or less I would start checking each magnet at the back side of the backing plate to find the one thats an issue..

** If all four magnets are within 3.2 ohms then you know your issue is with the brake wire harness from the 7 way plug to the pin box junction box where the 7 way cable connects to and then along the main wire feeding each brake magnet..

Video I put together that might be useful

** If you have a clamp on DC amp meter you can quickly measure the amperage draw of al four brake magnets as well as each one separately...

 

[jsb5717]

On ours it was the wire that is inside the axle that connects the brakes side to side. I pulled it out and found the shielding was worn, cause intermittent shorts, giving me the same “trailer disconnected” message. It was easy to run new wire and strap it to the axle. Problem solved

 

[Daryles]

Excellent write up and video Chuck. I'm saving that for future reference.

 

[ChuckS]

Just wondered if the OP has had any time to further diagnose the issue with the "trailer disconnect"?

 

[Daryles]

One time we had intermittent "trailer disconnected".
Very frustrating.
I took some 200g sand paper and cleaned the contacts in the 7 way connector.
Been great ever since.

 

[twindman]

I think I used a can of electrical cleaner(?) and spray the connection points. I haven't done anything much lately, as I now insert and remove the plug 2 or 3 times when hooking up. I think it cleans the connections.

 

[laverdur]

DeOxIt is an easy way to remove oxidation and corrosion from both sides of a 7-pin connector. I'm going to go out on a limb and guess that your 2019 F350 has the same thing as my 2013 in that the truck sends a pulse to the trailer brakes every few seconds. If a short circuit or open circuit is present, the truck will report that the trailer is disconnected. My truck/trailer gave me fits when the brake wires that run through the axles were shorting to ground intermittently due to chaffed insulation. To verify that the truck was OK I bought a Tekonsha Trailer Emulator. It showed me that everything on the truck side, including the pulse to the brakes, was good. Walmart sells them for $47.06. Google "Tekonsha 7-way Blade Tester and Trailer Emulator"

 

[rickybobby]

I used to have that problem with my chevy 3500. Sometimes the inside of the plug gets squished you need a small blade to open back up. Haven't had issue since. Be conscious that you pull out straight, not to side