OK, so I have a 91 F350 7.3 4speed, out of the blue two days ago all my dash and exterior running lights stopped working and the RPM gauge to. I unplugged two relays on driver side and plugged back up, jiggled them and they all worked, then I got home, when I shut the lights off, they refused to shut off until I shut off the truck. Next, they quit again after retrying, messed w/the relays again.. Nothing, but jiggled the switch and after several times they blinked. I replaced the relays but still.. Nothing.. So now I go out the next day, tried to crank it, turned over LIL slow, but never caused a problem before,, now, the alternator is new but don't start charging until bout ten MIN. or more, then charges at 13.78v, so I know the batteries can't charge driving short distance round the yard unless u hold the throttle half way for several MIN. , both batteries are good, new GP's.. Now with my batteries almost dead now, I put a 1700amp jump start box (snap-on) on and sprayed ether in the duct,, turned over faster but not hitting a lick. I'm lost, bear in mind, I'm a certified auto mechanic w/over 15yrs in the game, and worked on Terex's,Euclid's, and road graters with pure basics, no computer crap and harnesses but I the only differ in this truck is it's a diesel, not gas burner so however most everything is identical except for maybe some extra wiring, extra relays, etc... is there anything I may be over looking or someone with the same experience??? Keep in mind it went from auto to 4 speed granny but no issues from that I'm sure, it's been perfect for years after that. Plz help if u can as I'm not mechanically inclined on diesels and possible relays for differ things! I would greatly appreciate it.😤😤

 

I could be wrong, but it does sound like there are some bad ground connections from the engine and negative battery terminal to the sheet metal of the vehicle. Consider checking (loosen then retighten) all the ground wires you can find/see to the sheet metal and see what you get.