Strange truck/trailer electrical problem.
 

Earlier this year I replaced my old ramp truck with a F350 dually and 24 foot Wells Cargo trailer. On Saturday morning I was all loaded up for a race, but when I was ready to leave home, as soon as I turned on my trucks headlights, the red light on the brake controller came on and the trailer brakes were engaged. Turned the lights off, and the truck would move, turn the lights on, and the trailer brakes locked up. When I unplugged the trailer harness from the truck, the controller light went out, and the brakes were released. I have used the truck and trailer before with no issues, so it can`t be anything wired up wrong, but I`m guessing power is being fed back thru the electrical system. Anybody experianced this before?

Re: Strange truck/trailer electrical problem.
 

More than likely your trailer has been grounding thru the hitch ball. Trailer sat awhile without being hooked up and some corrosion formed in the coupler and on the ball. Most strange electrical problems when you first hook up can be traced to bad grounds between the truck and trailer.

Re: Strange truck/trailer electrical problem.
 

Another area to lock is the 7-way plug. Look for corrosion tracing between the terminals.

Re: Strange truck/trailer electrical problem.
 

CHECK the wiring to make sure you have the wires in the correct position in the trailer plug in relation to the truck receptacle--- look inside the cover flap on the truck and it will give you the position number and the wire color for each position make sure it matches the trailer pattern--- some use the RV pattern and others may have used the commercial truck wiring pattern--- also if your truck is over about 10 yrs old the factory wiring receptable is probably corroded inside & you can get a replacement unit on ebay for less than $10--- my experience on this!!!!---FED 387

Re: Strange truck/trailer electrical problem.
 

Most all "strange" electrical problems come from bad grounding. I always recommend running a separate ground wire for the trailer wiring and not rely solely on the hitch to make the ground.
Dan

Re: Strange truck/trailer electrical problem.
 

My son just had a similar issue with his rig...only when he turned his ignition on the trailer brakes would apply (turning the ignition on makes the Aux. battery terminal hot in the 7 pin plug, at least on the fords) He unhooked the plug a few times and it stopped but I'm certain there's corrosion in either the vehicle plug or the trailer plug or both. The only other option is one of the 2 is miswired. If the truck has the factory 7 pin plug then that would be wired correctly, at that point I'd look closely at the trailer wiring. Good luck Joe BTW any modern enclosed trailer has a dedicated ground wire in it's plug...so grounding thru the ball is a thing of the past.

Re: Strange truck/trailer electrical problem.
 

Poor ground.

Re: Strange truck/trailer electrical problem.
 

You might also check the trailer wiring where exposed for rodential munching.