Trailer Brake Lights Fail when Headlights on
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Joined: May 2003
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Trailer Brake Lights Fail when Headlights on
I have an issue I have been stuggling with for a while....
I have a 2001 F-250 v-10. I tow a gooseneck trailer with this truck. I have used this combo for 5 years with no issues until the past year...
Every light, brake, turn signal, etc... functions perfectly on the truck and the trailer until you turn the the truck headlights. When you turn on the headlights all the lights operate as normal but when you hit the truck brake pedal the trailer brake lights go out...they work fine with the headlights off...even the rear running lights work but the brake lights on the trailer only go out with brakes applied when the headlights are on.
The truck brake lights work fine with or without the headlights on. I have tested every fuse in the truck.All are good. Is there a relay or something I need to check? I have traced the problem to the factory wiring harness and have the same results. I am all but positive that it is not a faulty wiring issue.
Thanks for the help...
I have a 2001 F-250 v-10. I tow a gooseneck trailer with this truck. I have used this combo for 5 years with no issues until the past year...
Every light, brake, turn signal, etc... functions perfectly on the truck and the trailer until you turn the the truck headlights. When you turn on the headlights all the lights operate as normal but when you hit the truck brake pedal the trailer brake lights go out...they work fine with the headlights off...even the rear running lights work but the brake lights on the trailer only go out with brakes applied when the headlights are on.
The truck brake lights work fine with or without the headlights on. I have tested every fuse in the truck.All are good. Is there a relay or something I need to check? I have traced the problem to the factory wiring harness and have the same results. I am all but positive that it is not a faulty wiring issue.
Thanks for the help...
Elder User
Joined: Apr 2004
Posts: 797
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From: Wickenburg, AZ
The trailer will do turn signals with the headlights on just fine but it won't for brake lights? That's odd.
I would start by looking for a short across the tail/running light circuit and each rear brake/turn signal circuit. You should never have a completed circuit there, but if you do maybe current is being backfed from the brake lights to the running light circuit. That would explain why they go out there in the back, because with both circuits hot you'd have current on both sides of the bulb, therefore no flow.
Did you check the quality of the ground on those back turn/brake lamps for the trailer, by the way?
I would start by looking for a short across the tail/running light circuit and each rear brake/turn signal circuit. You should never have a completed circuit there, but if you do maybe current is being backfed from the brake lights to the running light circuit. That would explain why they go out there in the back, because with both circuits hot you'd have current on both sides of the bulb, therefore no flow.
Did you check the quality of the ground on those back turn/brake lamps for the trailer, by the way?
Senior User
Joined: May 2006
Posts: 181
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If I had to guess, I would check for the grounding problem first and the first place I would look is the main ground to the truck harness, then work back to the brake lights. That's a quick guess but I've had some real "issues" over time with grounds.
Then go with the short possiblity as suggested.
Good Luck
Then go with the short possiblity as suggested.
Good Luck
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