So here's what I got..I ran the truck over to adv auto they checked my alternator for me and it came up 12.15 volts..guy says yup it's bad. You need a new one. My question...do I? Is it my voltage regulator? Basically the issue is I can charge both batteries (they're new) run the truck for 2.5 days and turn it off go to start it and nada..jump a battery and it runs.. So what to do..Also 86 6.9 f250 stock everything. Lol

 

12.15 volts is not good, there is a problem. Could be regulator though, might try pulling the alternator and having it tested independently. If it still fails, junk, if it passes it's either the regulator or wiring.

 

In that case..I have an 88 bronco 5.0. Does the alternator fit on the 250? I was planning on using the bronco as a donor

 

Offhand I'd guess no, but just bring up Rockauto's or Autozone's website and look the two vehicles up and compare part numbers.

 

Sounds like an excellent oportunity for a 3g swap

 

Honestly, alternators all do the same thing. As long as your donor alternator will physically fit(and you have a pully that will work with your belt), you can make it work. Depending on what the other alternator is, you may need to find a controller for it, or come up with some new wiring for it. You can check the wiring diagram from your donor vehicle to see what you need.

That all being said, what I'd do is first check that the external regulator is in place, intact and that it's case is grounded(perhaps run a wire from the mounting screw to someplace else on the body, or battery -)
If it's still not charging(you might take it to the auto parts store if you don't have a volt meter), replacement regulators are relatively cheap(especially compared to another alternator). Your other rig /might/ actually have the same regulator, which you can try instead.

If you end up having to get a new alternator, you can look at a '1-wire' 3G swap, regular 3G, or a stock 1G(which is what you have now); anything should work. The newer 3G is 'better', but a properly working 1G works just fine.

Also, I seriously recommend getting yourself a multi-meter; troubleshooting electrical stuff without it is a real pain, and they are real cheap -- Harbor Freight has 'good enough' multimeters for under $8; Home Depot/Lowes, auto parts stores or most any place that sells wiring stuff should have them as well(usually in the $20 range)