Tail light troubles

I have a 75 Ford F150. I have a problem with my passenger brake light. If I dont have the lights on the brake lights work fine, but if I have the lights on and hit the brakes the whole passenger brake light goes out. I had a lot of problems with the trailor wiring the previous owner installed, I rewired all of it at the connector at the bumper, and that fixed it for a while but it started acting up again. I havent had any problems blowing fuses. I have heard that there is a sensor under the brake pedal that could be bad, but wouldnt that cause to have no brake lights at all? My friend thinks it is a ground problem. I am scared to drive it at night because I might get pulled over.

If you could help me it would be greatly appreciated!

Start by swapping the left and right bulbs and testing the system. There’s a small chance the problem is with the bulb itself, notably the base contacts. Send us the results. Then we can advise you on how to deal with a ground problem, which is more likely.

I tend to agree with your friend about the ground being the problem. You can see if that is true by checking the voltage between the brake light and a true ground point, while the pedal is pressed and the lights are. If you see 12 volts on both sides of the brake light leads then the ground for the brake light is bad. This may be due to a problem with the trailer wiring but I can’t say for sure. If the light just ties to the rear fender area then check to see if the fender is bonded to ground well. You can make a ground jumper lead and tie one end to a good ground and then touch it to suspected bad ground points to see if that changes things.

I think you’ve got a ground problem. They can cause a lot of weird problems. Try adding another ground wire to the socket to see if the problem goes away.

I’m assuming brake light and park light are 2 filaments in one bulb. If not I’m all wet. If so, though, they share a common ground, so if brake light has bad ground so would park light, but apparently park lights are OK. If brake light filament is broken on its ground side and the end of the broken half that has the filament is touching the positive side of the park light filament, then as long as park

light circuit’s not energized, brake light current will go through brake filament, then park filament, then to ground, so brake light will appear to work OK. Turn on park lights and you’ll have 12V going to one side of brake filament and less voltage, but some, going to the other side, effectively dimming brake filament. I think it’s the bulb.

390, you said on an earlier post that you were 15, and you just said that you were worried about gettting pulled over by the cops. Don’t get me wrong, I’m not trying to be a cop, but if you don’t have a driver’s license and you get stopped it could be years before you get license. This happened to me when I was a little older than you and the violations “snowballed” and I lost my license for about 6

years (nightmare). One more violation and I probably would have got jail time and we’re not even talking DUI here. A word to the wise, 390.