No alternator seems to work!

[Gibson]

Ok this problem has stumped all the car guys in my neighborhood. The fundamental issue is: no alternator (tried three) will pump out more than 12.5 volts. When not hooked to the battery main, the alternator will put out 19v-45v depending on engine RPM. We've checked a laundry list of things which I'll describe below. (sorry so long)

Its a '78 c10 with a 350 and a for a week or two it was squealing really bad and when it did, the voltage would drop down to 8 volts. So I ordered a new alternator. Before the new alternator had arrived, it made a bad squeal, then threw the belt into the fan clutch. I went and got a new alt from the local parts store and a new belt, but on testing the voltage after it was hooked up I noticed that the voltage basically didn't change when I started the truck. So I took the alt and battery to the parts store - both tested good. I checked the voltage on the leads to the alt and the primary and the post 2 (bigger red wire) have full voltage and post 1 (smaller white wire) has a few volts less which I understand is what it is suppose to have.

We got a different alternator and it was the same deal. We tried running new charge/ primary leads and and double checked the ground. I had done some painting in the engine compartment so I double checked all the grounds and cleaned them up. I pulled all the fuses too. No significant change.

I am absolutely stumped. Any help is greatly appreciated!
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