Trailer lights backfeeding to truck

[wedek]

While towing my enclosed trailer one night coming home from the dragstrip my trailer brakes pulsated on and off with the turn signal on. Craziest thing having the trailer brakes locking up and releasing in snyc with the blinker. On the way home trying to figure this out in my head I blamed it on the trailer at first but then I hooked the truck up to my dump trailer and it did the same thing. I even noticed that both the truck L&R brake lights blink with the turn signal on and trailer connected...even both of the dash indicators blink as if the hazards are on. With the trailer(s) disconnected everything is fine and working normal. So two different trailers exhibiting the same "backfeeding" problem.

I replaced the 7 way receptacle on the truck side with a used factory Dodge 7 way connector/receptacle thinking it may be shorting internally. Did not help the problem. I disconnected the trailer brake controller which is wired to the factory plug under the dash and still did it. I checked the ground wire at the 7 way plug and its getting a ground with a test light. I cleaned the battery terminals thinking the battery ground could be it...nope! So I am almost at the point of cutting the ground wire that feeds the 7way connector and making a dedicated ground to the truck frame. Before I go that far has anyone had this issue and or have any advice to give. Oh, I did try a new turn signal switch and it still did it.

Truck is a 1999 CTD ive owned since new and I am the only one driving it and working on it so I know it wasn't something caused by someone else.

 

[Wiredawg]

To me it sounds like your turn signal circuit is crossed with your trailer brake circuit. Get your self a multimeter and check your trailer conductors for a short. you can also visually inspect the trailer wiring for smashed or damaged wiring.

Hope this helps. Let us know what you find.

Ron

 

[GAmes]

My '97 has two wire connectors behind the driver's side tire. One of mine wasn't waterproof. The water that seeped in created a path for several of the wires inside to receive power at the same time. Don't know if a '99 has them but is fairly easy to check.

 

[DavidC]

My '97 has two wire connectors behind the driver's side tire. One of mine wasn't waterproof. The water that seeped in created a path for several of the wires inside to receive power at the same time. Don't know if a '99 has them but is fairly easy to check.

Yes 99 has 2 or 3 there

 

[Wiredawg]

And my 02 had moisture problems too... had to replace 7 pin connector hardwired.

Cheers, Ron

 

[PBJ2500CTD]

It sounds like an adequate ground return is not being made between the truck and trailer connector. If the potential is sent to the turn signal lights and there is no ground back to the battery from the trailer, the potential will be present at the high side of the trailer turn signal bulb and follow the path of least resistance to ground. This would flow through the turn signal element and out the low (ground) side through the trailer's tail light and into the low (ground) of the opposite side turn/brake elements and out of the tail and turn signal into the truck's tail light and turn signal elements then to the truck's ground.
A few questions:
Does the issue occur with both left and right turn signal?
Can you see any dim illumination of the tail light elements (truck and trailer) when the turn signal is activated?
Do the front turn signals/parking lights appear to work correctly?
How do the turn signals react if the lights are on?
What do the trailer brakes do with the 4 way flashers energized?
Check your ground connection between the trailer and truck with a multimeter. Ideally, it should read less than 1 ohm. With the truck and trailer hitched and electric cable hooked up try clipping a ground jumper wire from the trailer's frame to the truck's frame. If the problem goes away look at the ground connection between the truck and trailer.
I believe you said that you swapped out the multi-function switch.

On EDIT: I re-read your issue and agree with Ron. Sounds like your wiring may be crossed/shorted since it was still messed up (brake pulsing) after you removed the brake controller. If that was the case I would think a fuse or two would let go from the brake current.

 

[PBJ2500CTD]

Also, check the filaments in in the bulbs. I have seen a filament break on one end then fall and arc to the second filament causing similar issues.

 

[Missouri Mule]

Try pulling the back off the trailer lights plug in. In the past members have corrosion inside the box.