My 75 that my sons driving wouldnt crank today when he left school....he ran a hot wire to the coil and crossed the solenoid and cranked it....would only crank if the key was off or in acc. position. When I got off tonight I went and got it and had to do the same to get it to the shop. I figured out that, first off the fusible link to the alt. was blown, I cut it out and put a fuse inline and it worked to crank 10 or 12 times then it blew the fuse, I can replace the fuse and itll do fine for a few times then it starts overcharging as in 17 volts. I replaced the voltage reg. and then the alt and itll do fine till you ground the regulator then it starts overcharging again....checked all the wires to the alt and reg. and they all look good (unwrapped them and made sure none were shorted together).....guys really need some help here.....I'm stumped, checked everything I know to check and can't find a thing and I've dealt with some wiring doozes before but never ran into this one. Later Whit

 

I would check the ground cable to the engine block connection. I would then look around either at the battery ground, or somewhere on the engine block for a ground strap that goes to the sheetmetal and make sure it's good or not missing. I would also take the large connection on the starter relay, that goes to the battery, alt, and wiring harness, apart and make sure it's clean. Make sure you put a decent size fuse in the alt output line. A large amount of current can flow through there depending on the size of the alt.

 

A new regulator usually has a note that says to be sure the regulator is grounded or damage to it might happen..Monitor the field term of the reg or the alt. You will see close to battery voltage if it is calling for max output.

What size fuse are you using? It will have to handle the full output of the alternator. And yes, check the ground wires. Let us know what you find.

 

Cleaned the Bat. connection at the sol. and all the grounds, also where the braided ground goes to the body. Got agrevated after about 5 hours last night and 4 this morn. and was gonna just pull the 81 Bronco in and fix what was wrong with it and drive it today, hooked everything together and bolted the reg. back up, when I cranked it up to pull out of the shop everything worked fine.......started it up and shut it down a couple of dozen times went and drove it around for about an hour and stopped and started it another dozen times. Went ahead and drove it to work today and everything seems fine (might add that I took most of my wiring tools along for the ride....you know, just in case). I guess this is gonna be one of those gremlins that cause so much heartache. I'd rather it just quit or something.....always makes it easier to find the problem that way. I reckon I'll just drive it a few more days and then let my son at it again.....at least he knows how to hotwire it to get it home, LOL.
By the way I wired in a 30 amp fuse where the fusible link was.
Thanks for all the advice and I'll holler if I finally find whatever it was. I figure I'm gonna end up finding a bad wire somewhere. Later Whit

 

I would go with a bigger fuse. If the battery gets low, you will pull more then 30 amps when it starts charging the battery. You probably have a 60 amp alternator.

 

I really appreciate all the advice, it's still doing ok right now. Mike W. I understand that it will put out more than 30 amps but whats worrying me is when it jumps up to 17 volts....if I don't unhook the field wire off the alt. pretty quickly it stays at 17 and then blows the fuse. Any more ideas would be welcome....I'm still worried that it'll happen while my son's driving it and leave him stranded. I've put a supply of fuses in the truck for him but you know a dad, it still worries me. Later Whit