My 90 f350 is having tailight issues. Electrical is s not my forte so im just wondering what exactly is causing my issues and how to address them and just to further my understanding of how it works.. anyways.. i keep blowing tail light fuzes but only for my brake lights and hazards. I put the power probe on it and i dont really understand how to make out the readings i got because i dont know what wires are what. The issue im having with my brake lights now is that my passenger side works good but my drivers doesnt work at all. The blinker on the drivers side doesnt work either but i think thats because its connected to the brake light but im not 100% sure. I have pictures of all the stuff from the powerprobe. I just had it hooked to the battery the whole time. I hooked it up to the green wire with the red stripe and turned the blinker on and the power probe starting beeping in sinc with the blinker but the light doesnt blink. I can jump power to the green wire with it from the battery and the light starts to blink. I can jump power to the black wire and make the light blink and if i jump power to brown it makes all my running lights come on. So i am assuming brown is for my running lights and green withbthe red stripe is everything brake light related including blinkers? But if so why does the power probe solid beep on the green wire without the ground on the little probe itself hooked up without the brake pedal being pressed? This whole thing is flabbergasting me lol

 

green is power to brake and blinker.
brown is power to running light.
black is ground.
if putting power to the black wire makes the lights work, you have a bad ground wire and are back feeding the light through the other circuit.
apply a ground to the black wire and all lights should work.

 

Be aware that all of the Bullnose-OBS era taillight sockets I've seen are covered in congealed dielectric grease. Make sure you use a soft bristle brush and electrical cleaner/Simple Green to clean all the sockets before you start chasing electrical gremlins.

 

i didnt see any electric grease in the socket when i had it apart. The guy that had the truck before me was active on these forms.

 

so after I fix the ground is it supposed to still turn on back feeding power?

 

no, if you back feed power into the ground it will blow the fuse or burn the wires or blinker switch.

 

so because i can backfeed it to make it turn on without blowing fuzes that makes a bad ground? And if so why do my running lights still work if its a bad ground?

 

So i re did the ground and when i turn my left signal on to turn it makes my volt gauge on my dash bounce from 0 to normal every time the signal is supposed to come on. It also dims all my lights whenever its supposed to blink.