Tralier Lights Stop Working When Headlights Are On

[SteveCA]

Here is a weird one. Trailer lights work fine so long as the headlights/parking lamps are off. Brake lights, turn signals, flashers and backup lights work perfect, until I turn on the parking lights/headlights. Ass soon as I turn on parking/headlights, none of the trailer lights work. As soon as the parking lights are turned off, everything works again. the parking lights do not work on the trailer end. All lights work fine on the truck.

 

I have checked the trailer light fuses under the hood. seems something is shutting off the trailer circuit when the parking lights are turned on.

 

2004 2500HD 6.6 diesel LB7

[mark1223]

Does the ball look rusty? We had had this problem before with our trailers. If the ball looks rusty, take a wire wheel on a grinder or drill and clean it up. Then drive it around a little bit. If it looks clean, try to just drive it around. It sounds like it may be a grounding problem. This works better if you have bumpy roads.

[gmtech4]

the ground is not thru the ball,sry mark,there is a separete ground thru the connector,this guy has a problem that makes no sense,check your grds steve

[SteveCA]

the ground is not thru the ball,sry mark,there is a separete ground thru the connector,this guy has a problem that makes no sense,check your grds steve

 

I checked the ground wire, even moved it around to ensure decent contact. I've diagnosed ground problems before and I know they lead to some very whacky stuff. This one is weird though as it always works so long as the headlights are off. seems like the trucks system is shutting down the trailer connections whenever the headlights are on.

 

I will check the truck 7 way plugs with a voltmeter to see if it happens when the trailer is not attached to isolate the problem.

 

Steve

[mark1223]

I never said it grounds thru the ball. I was just trying to help him. We had this problem and that is how we fixed it. This probably has nothing to do with it, but is the trailer 4 or 7 pin ?

[gmtech4]

my mistake Mark