Why alternator not showing any voltage or charging battery?

had same problem with my 91' 'computor' the alternators are internally regulated- but to save fuel, dodge had a feature that would only turn on the regulating of the alt. when the voltage was at min. there is one little ground wire from the computer that controls this- want a much cheaper fix? take back your alt . and buy a internally regulated alt for a 'chevy' simple to wire - and this does not set off any new codes -or confuse the computer. i had the auto-parts stores scratching their heads too- brand new alt -wouldnt charge-on the truck- but kept passing on their bench test! I admitt this sounds like a weird fix- but when you price a new pcm-its the way to-go. thats " of coarse" if you have encountered the same as i did.