Have you tried bypassing the regulator? I'm assuming you've got a 10SI alternator with the built-in regulator. There's a D-shaped hole in the back of the housing near the position of the 2-wire plug on the outer perimeter. If you use an ice pick or a thin-bladed screwdriver to ground the field circuit to the case by putting it through the hole and touching the tab on the regulator, thh voltage should go up a noticeable amount. If the engine is revved up a little with the field grounded, voltage will go to 15-20 volts, so don't race the engine. Also be sure the #2 wire in the 2-wire plug has battery voltage on it at all times. The #1 wire (usually white) gets switched voltage from the ignition, or it serves as a ground for the idiot light.

I'm not surprised at the unit passing the check at the parts store- - - -I've seen them pass one with the brushes removed!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!