I suppose you can have TWO "good" junkyard alternators both suddenly fail though it seems kinda unlikely. For the price for a fresh rebuilt alternator ($40 or so) you might be ahead to get one instead of another junkyard unit.
However a healthy alternator will be ruined if the charge circuit (alternator BAT to ammeter to battery positive) goes open while it's charging - if even for an instant. Also if the sensing circuit (wiring to the #2 terminal) goes open while running. So I'd suggest double checking all wiring for good tight connections - including the battery cable terminals
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As for determining exactly what's failed on a dead alternator here's a couple things to check:
1 - With the engine running at 1/2 throttle bypass the regulator by grounding shorting tab inside the back of the alternator. If it now charges hard the regulator has failed.
2 - If you have a VOM or DMM set the meter on the lowest ohms range. Now disconnect all wiring from the alternator and take a resistance reading between the BAT terminal and the case. Note the reading then reverse the meter probes and measure again.
A "good" reading is infinite resistance with the probes one way and very low resistance with the probes reversed.
Infinite ohms in both directions means an open circuit in either the stator or diode bridge. Low ohms in both directions is a shorted bridge. Only fix for either is to replace with new.
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