Just fixed one with a similar problem. After 10 hours of diagnostic work!
We installed two NAPA reman alternators and were getting the same results as you. We installed a Carquest reman alternator and it was fixed. Apparently NAPA's rebuilder doesn't realize that the Isuzu's ECU needs a reference signal from the Alternators internal regulator or it will ground the circuit and prevent the alternator from charging. The NAPA alts were not piutting out the voltage at this wire, the CQ Alt. Was.
P.S.-Took 10 hours to figure this out, good luck.
Wednesday, January 18th, 2006 AT 7:32 PM