Hey everyone,

2003 F250xl SD, Regular Cab, RWD, V8-5.7

I tried searching for this but I think this might be a first for anyone. I'll do my best to be concise.

I tried installing two rear lights on my truck on the same circuit as the back up alarm. So, as I throw it in reverse, lights mounted under the bumper come on, sounds like a super easy job right? Well, here's what I did..

I mounted the two lights under each side of the bumper, and used the mounting bolt to ground it out. I spliced the two hot wires together and drew my power from the blue wire on my 7pin connector (fog/reverse lamp). I also drew a line from this splice into my back up alarm and then grounded that out to the frame with a 3/8 tech screw.

I did this outside in 3 degrees Celsius in the rain, also.. I ended up blowing my 20amp fuse for the trailer battery charger terminal (center pin). I've changed that out and have since disconnected the power to the lights AND back up alarm. I also disconnected the ground wires and pulled the fuse on the lead wire.

What's happening is, my parking brake alarm and light are coming on at idle, but turning OFF when I turn on the headlights. When I put it in reverse, the warning light comes on. When I am driving, it stays on, when I press the brake pedal, it goes out.

I've disconnected my trailer brake control box AND the 5 and 7 pin connectors, I've also disconnected my switch at the parking brake pedal arm. I have plenty of brake fluid, and the sensor is tip top. Definitely a wiring issue... Also replaced a blown 15amp fuse for the ignition switch "run" circuit that blew. What I think I did, is somehow grounded out the whole trailer system, back-feeding power into the connectors under the dash and that by doing this I have somehow shorted something out between my brake pedal switch, headlight selector switch and the warning light system.... But I don't know what to do about it. My idea is to simply just remove the buzzer/chime and the brake warning light bulb. Everything still works as it should (headlights, brakes, etc) but I am afraid if I ignore the root of the problem and hook all the auxiliary lights back up, that eventually this crossed wire problem could affect something else in the system and really screw things up for me later on.

If you have an idea, I need to get this solved ASAP. She's my working girl and it's almost Monday....

 

When you have crazy stuff going on like that, it's usually a grounding problem. When you have power going to something, and it does not have a good ground, it finds a ground path somewhere else, usually a circuit that shares a ground with it. The current will actually go down the circuit like it's supposed to, go through the bulb, and try to go to ground, but when that's no good it will go up another circuit, go though that bulb backwards, and go all the way back up front till it finds a ground path. When you power up this other circuit, then the voltage coming down that circuit butts heads with the one trying to find a ground, and the problem circuit will cut off since you stopped it's backwards ground path.

The usual place this happens in the rear are the tail/brake lights. Those dual filament bulbs(one is the marker light, one is the brake/turn light) share the same common ground, they only have 3 wiring terminals on them. So the brake circuit can run backwards up the taillight circuit, and the taillight circuit can run backwards up the brakelight circuit. When you step on the brake or turn on the park lights, there is no telling what's going to happen. See if you can find a ground problem back there.