Alternator charging Question

I have a nice spare dual field alt kicking around that has some gremlins. I had a 72 swinger that had a perfectly good working alt but thought I would swap it out for the nice new looking one. When I did it was over charging, into the 18volt range. I took it in and it tested fine {twice}, tried it in the car again and still 18volts. So I replaced it with the old one. Shoot ahead a year and now I have a 73 Cuda with a ugly old alt "that works fine"...yup put the nice shiney one in and still the same problem...18volts. The guy who rebuilds them could not tell me the problem...I am sure it is not the cars wiring since it has been in two cars and still over charges with the wires hooked up the same way.
What it the alts problem???? would be nice to fix it.
Thanks
Al