Strange truck/trailer electrical problem.

[drgrcr1156]

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

[FED 387]

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

[voltdr]

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

[buzzinhalfdozen]

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.

[Wayne W]

Poor ground.

[Run to Rund]

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