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Why alternator not showing any voltage or charging battery?

(1990 Dodge W150)
in Harrisburg, PA on April 25, 2010
1987 Dodge ram 150 or 1500 pick up truck. I've put 2 new alternators in & neither one is showing voltage or what ever & isn't charging battery & my mechanics & I can't figure out why. please help?
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on January 23, 2011
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.
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on February 19, 2011
i also have a 91 dodge ram and am having the same issues. what size alt should i get gm product wise with 318motor?
on June 23, 2010
i have a simalar problem except mine shows minus 12 volts no matter what i change it has not helped
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