If its a factory 2 field alt with the electronic regulator the reg has to be grounded for it to charge at all since the reg grounds the field to make it charge. Many times on that system it is the blue wire at the reg either lost volts or is not making contact on the reg and it does not know what battery volts is so it keeps on charging. I have seen alot of them break the wire on the inside by the reg connector. The wire itself is still hooked up but the wire strands break inside the insulation and the reg does not see battery volts. Thats the blue wire at the reg as it should be 12 volts with the key on and when running it should run around 13 to 14.5 volts the same as its charging. You said it stops charging with the reg unplugged and you replaced the reg so I would check that blue volt sense wire real close on the reg. Ron