Trailer light and brake problems

I am having problems with my trailer lights and brakes. I am towing the trailer with a 2002 Chevy 1500, that has automatic headlights. During daylight hours, the turn and stop lights, on the trailer, work as they should, and the brake controller shows power. When the headlights are on, the controller goes dark, and the trailer lights do not work. I am sure there are no trailer brakes working either way.Have tested trailer lights and plug on truck and all seem to be ok. I would appreciate any help anyone has to offer. Thanks. Fred
 
Yes, it is wired to a plug, under the dash, to a panel on the firewall. Maybe I should of stated that I've had this truck and trailer for 3 or 4 years now, and have not had problems. Seems like someone told me years ago, not to plug the trailer plug in with the truck running or the lights on, because of damaging something??? Thanks. Fred
 
You have all the signs of failed/weak ground connection. With headlights off, the indicator on the brake controller is backfeeding through the trailer lights & the truck tail/marker lights, eventually reaching the truck batt ground post.
When you turn on the truck headlights you are feeding the trailer lights from both sides, so nothing on trailer works.
Most likely bad spots to check would be ground wire to trailer frame, ground pin in plug, & ground wire from plug to truck frame. For test purposes, try a jumper cable from clean spot on trailer frame to clean spot on truck frame, or direct to batt.Willie
 
sidebar- with automatic headlights, when the sensor on the dash detects daylight outside, it won't turn on the tail lights or trailer lights, have to turn them on manually.
WJ
 
Yep. The truck at work with the brake controller was doing this. Got blamed on the controller, dealer swapped it out with another one and sent another 7 to 6 adapter. Swapped the wires in the first adapter, ended its issues. Re-hooked the wire in the trailer cord to the block in the junction box on the trailer, and problem was solved. Someone had pulled away without unhooking the trailer plug at some point.

Had to do it again with the same trailer a couple weeks ago. Wire got pulled on again, ground wire in plug was intermittently hitting the post for the signal wires, and blowing the signal fuse in the truck.
 
Thank you fellows. I think I found my problem as being a bad connection on the ground wire on the back side of the pickup plug. Thanks again. Fred
 
Found something like that last night.

We've had a bad ground on a hopper for the last year, but it only got looked at when it was loaded- and grounding through the fifth wheel. Hooked the truck to the other trailer yesterday and was having the same issues. The lights worked perfectly on the other truck, so that pointed to something in this truck.

I determined ground wasn't getting through the cord somehow, so I tore the plug ends apart, and found the ground wire was loose in both ends of the cord.
 

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