weird trailer issue when activating brake controller
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4th Gear
Joined: Jan 2026
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weird trailer issue when activating brake controller
I've just made the switch to a 2020 F250 for towing purposes.
I was testing out the trailer (just a typical 6x12 trailer with elec brakes).
Plug the trailer in, test indicators, brake lights by pushing the brake pedal, emergency lights, everything works, trailer is 'connected'
I squeeze the brake controller (the manual thingy) and the trailer IMMEDIATELY 'disconnects'.
What gives?
Trailer worked fine on previous truck (F150) - though that truck did not have a brake controller.
Visually inspected the truck plug on the F250, no damage to wires, inside of the plug is extremely clean, zero corrosion.
Anyone else had this?
I was testing out the trailer (just a typical 6x12 trailer with elec brakes).
Plug the trailer in, test indicators, brake lights by pushing the brake pedal, emergency lights, everything works, trailer is 'connected'
I squeeze the brake controller (the manual thingy) and the trailer IMMEDIATELY 'disconnects'.
What gives?
Trailer worked fine on previous truck (F150) - though that truck did not have a brake controller.
Visually inspected the truck plug on the F250, no damage to wires, inside of the plug is extremely clean, zero corrosion.
Anyone else had this?
Lead Driver
Joined: Jul 2003
Posts: 9,355
Likes: 1,722
From: AKRON ohio
I can't explain it as I don't fully understand but the system on the trailer monitor/controller is very sensitive.
my example:
my 2005 f150 everything worked fine, hook up to my then new 2022 f250 and the same EXACT thing occurred.
long story short, I had shorted wires. The wires that on my trailer were routed thru the axle tube for the other side brakes had rubbed thru inside the axle tube.
these new trucks somehow sense when there is a problem
I didn't believe the dealer because the trailer still worked on my f150
i pulled the break away cable out of the switch and about 10 minutes later I had a melt down in the break away switch.(I had wired a battery direct hot and ground)
I then started re-wiring and that's when I found the bad wires in the axle tubes.
all good now.
I can just about guarantee it's a trailer problem
my example:
my 2005 f150 everything worked fine, hook up to my then new 2022 f250 and the same EXACT thing occurred.
long story short, I had shorted wires. The wires that on my trailer were routed thru the axle tube for the other side brakes had rubbed thru inside the axle tube.
these new trucks somehow sense when there is a problem
I didn't believe the dealer because the trailer still worked on my f150
i pulled the break away cable out of the switch and about 10 minutes later I had a melt down in the break away switch.(I had wired a battery direct hot and ground)
I then started re-wiring and that's when I found the bad wires in the axle tubes.
all good now.
I can just about guarantee it's a trailer problem
Last edited by bigwin56f100; Jan 3, 2026 at 09:01 PM.
Thread Starter
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4th Gear
Joined: Jan 2026
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Thanks for the reply brother!
I crawled under the trailer, and yep, sure enough, a couple of the wires coming out of the trailer hubs are broken.
Ill test by hooking up to a friends trailer in the next few days to verify there is no truck side issue.
I crawled under the trailer, and yep, sure enough, a couple of the wires coming out of the trailer hubs are broken.
Ill test by hooking up to a friends trailer in the next few days to verify there is no truck side issue.
Lead Driver
Joined: Jul 2003
Posts: 9,355
Likes: 1,722
From: AKRON ohio
so long as the trailer # 2 doesn't have any issues. I fought this for a whole summer because the 2005 f150 worked fine and the superduty didn't. I just couldn't believe it was a trailer problem but it was.
Logistics Pro
Joined: Jan 2010
Posts: 4,085
Likes: 1,623
Many older trailers are showing issues with newer trucks, just like this. It's barely a short, but enough for the sensitive modules in newer trucks to pick them up and kick out errors like this. It's an overly-sensitive system in my opinion, but there isn't much that can be done to change it.
More Turbo
Joined: Sep 2009
Posts: 537
Likes: 198
I have this problem, only with my boat trailer that has EOH brakes. No problem with the utility trailer, the RV, or the car trailer I borrowed. The brakes still seem to work fine, but occasionally get the "Disconnected" message as I come to a stop, and if I manually apply them.
4th Gear
Joined: Jan 2022
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I'm seeing the same issue on my f450 with my 5th wheel. Trailer brakes are working normally. But this occurs when activating the brake manually...trying to figure it out.
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More Turbo
Joined: Sep 2009
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Does your 5’er have basic electric or electric/hydraulic brakes?
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More Turbo
Joined: Sep 2009
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Same… interestingly I don’t have the problem if I run the brakes under the “electric” setting, but then the EOH pump runs constant even when truck wheels aren’t rolling. (Traffic light, etc) The EOH setting drops the brake voltage to zero when the truck isn’t rolling, so the pump doesn’t run.
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