Thanks to everybody who offered advice on this problem. I am happy to report that the issue is now resolved and everything is functioning properly.
The root cause of the tachometer failure was that I had simply hooked the tach wire to the wrong terminal on the alternator. Initially, after the reinstall of the alt, this didn't raise a red flag because the tach had already quit on me before the alt swap. I now know that this must have been when the first alt went bad.
I got a tip on another forum about looking into the battery isolator that set me on the right path. I became convinced that the bad isolator had killed my new alternator. But after testing the isolator with my multimeter, it proved good. After further investigation I realized that the terminal on the isolator that should run to the (+) on the chassis battery was in fact run to the (-).
This could only have happened when I changed the chassis batteries about a week ago, another fact that I left out of the conversation because I didn't think it was relevant.
So, to make a long story short, I initially hooked up the tach wire wrong, then later when I hooked it up correctly, it still didn't work because the alt was not functioning properly at that time because of the battery swap.
Thanks again to all who helped me out with this matter.
BTW, on my alternator, the correct terminal was marked (R)
