Ford truck guys, 2009. Weird problem here. When all light switches are off, brake lights and the third brake light are normal. But when I turn the park or the head lights on, the third brake light turns continuously on, whether or not my foot is on the brake pedal. The rear left and right brake lights don't light up when I press the brake pedal and left and right rear park lights don't light up. In other words, the left and right rear park lights and brake lights don't work if I turn on the park or head lights. (Front park lights work fine, though.) I've replaced the rear bulbs, but no joy. I'm thinking maybe a stuck relay or something. Any tips, guys, before I dig into the fuse and relay box?
Weird brake and park light problem
- Thread starter berniedd
- Start date
- Status
Whipcitywrencher
Site Donor 2025
Does that have a in line plug in trailer wire harness? If it does I'd check that, I've had one short out on me before.
There's a bulb with both filaments shorting together. Remove bulbs one at a time until resolved. If red ones don't do it, go look at amber turns in front.
Edit, you did some of this. Also look at stuff like the turn signal and hazards switch.
Edit, you did some of this. Also look at stuff like the turn signal and hazards switch.
Originally Posted by eljefino
There's a bulb with both filaments shorting together. Remove bulbs one at a time until resolved. If red ones don't do it, go look at amber turns in front.
Edit, you did some of this. Also look at stuff like the turn signal and hazards switch.
My guess is either this or a trailer harness shorting out somewhere.
There's a bulb with both filaments shorting together. Remove bulbs one at a time until resolved. If red ones don't do it, go look at amber turns in front.
Edit, you did some of this. Also look at stuff like the turn signal and hazards switch.
My guess is either this or a trailer harness shorting out somewhere.
Ground. Your third brake lights up because there is only one ground serving all the rear bulbs.. With your park lights on, the power going through the park filiment on the rear light(s) is going back through the brake filiment in reverse into the brake circuit and then through the 3rd brake lights to ground. When you step on the brake pedal that ground path is eliminated as now there is +12V on the brake wire so you loose all your rear lights.
Ive seen issues with bad grounds causing light dimming or other issues, but in your case I suspect your ground is completely disconnected since usually the lights work somewhat with a bad ground.
I wouldnt bother with sockets or bulbs or anything. Trace out your ground wire and you'll find your problem.
Ive seen issues with bad grounds causing light dimming or other issues, but in your case I suspect your ground is completely disconnected since usually the lights work somewhat with a bad ground.
I wouldnt bother with sockets or bulbs or anything. Trace out your ground wire and you'll find your problem.
Last edited:
Colt, your diagnosis was spot on.
Ground problem, alright. First, I spliced in a wire to the black ground wire after it came out of the bulb socket and connected the other end to the chassis. No workie! What the????? Next, I connected the spliced ground wire directly to the bulb socket base, and .... everything works normal now. I look at the bulb socket and its ground wire is press fit to the socket base. Although it looks OK, apparently, there's corrosion or oxidation between the wire and the bulb socket base itself and it is now not electrically allowing current to get to ground. I jury rig it so my new ground wire wraps around the socket base.
Although it works fine now, I still don't understand why the left and right brake lights worked OK without a working ground when the light switches are off. I also don't understand why i did the fix only on the left brake light and now the right brake light is also back to normal.
Ground problem, alright. First, I spliced in a wire to the black ground wire after it came out of the bulb socket and connected the other end to the chassis. No workie! What the????? Next, I connected the spliced ground wire directly to the bulb socket base, and .... everything works normal now. I look at the bulb socket and its ground wire is press fit to the socket base. Although it looks OK, apparently, there's corrosion or oxidation between the wire and the bulb socket base itself and it is now not electrically allowing current to get to ground. I jury rig it so my new ground wire wraps around the socket base.
Although it works fine now, I still don't understand why the left and right brake lights worked OK without a working ground when the light switches are off. I also don't understand why i did the fix only on the left brake light and now the right brake light is also back to normal.
With my old Honda, the metal bulb socket can be pushed out of the outer plastic part. Then solder the ground wire to the socket cup instead of having it pressed together.
They weren't working OK. Only the small tail light filament was lighting up, which is not as bright as the brake lights should be. When the bulb is not grounded, the brake current goes through the tail light circuit and eventually finds ground through the other lights. Your unfixed right bulb is probably still dim.
They weren't working OK. Only the small tail light filament was lighting up, which is not as bright as the brake lights should be. When the bulb is not grounded, the brake current goes through the tail light circuit and eventually finds ground through the other lights. Your unfixed right bulb is probably still dim.
Last edited:
Originally Posted by mk378
Your unfixed right bulb is probably still dim.
Fortunately, the right side bulb (which I didn't touch) is just as bright as the left one. Defies my understanding...!
Your unfixed right bulb is probably still dim.
Fortunately, the right side bulb (which I didn't touch) is just as bright as the left one. Defies my understanding...!
- Status
