87 f150 alt. overheating

I've got a 1987 F-150 with 82,000 miles. The alternator is overheating, like it's working to hard. I've got a brand new one from NAPA on there and it was doing this with the old one too. I've checked the wiring and all looks good. I've even tried a different battery. Nothing seems to help. It only takes about 30 seconds and it's to hot to even touch. I took it to O'reillys and it checked good. I'm not sure where to start next.

You probably have a short in the system somewhere, check the amps stamped on the alternator and then put a clamp on amp gauge around the big charging wire (usually labeled B+) coming from the alternator and see what the load is.
Start pulling fuses until you see a dramatic drop in the amperage that should tell you what circuit is creating the huge load.

The new alternator should of came with a new plug in,Ford has problems with them burning up.Carquest's alternators come with a new plug in on their new alternators and the warranty is void without replacing that plug in.

I don't have one of those testers, but what I did was take one fuse out at a time to see if one would stop the big spark that occurs when you touch the main charging wire from the alt. to where it goes on the selonoid. The weird thing is , it will only do it when it is running. It's fine with the key on or off as long as you don't start it. I have ruled out the starter and the selonoid. Would the coil or the capacitor on the side of the coil have anything to do with it? Thanks again. And I did put on a new connector on the alt.