RE: Calibrating Tachs
Diesel electronic tachs when set up correctly and matched to *the alternator, work well. My OEM*VDOs matched with OEM Prestolite alternators have done fine over the entire RPM range. As David, FF's 3rd line*and JayN suggest and state, something isn't right in your setup.
Lots of alternators, chargers, tachs and electronics get fried by experimenting. Be careful of internet advice too* - now may be the time for a accurate schematic to aide in troubleshooting. With dozens of*similar engines, alternators, chargers*and tachs*on this site, I'm sure posting a schematic would get you lots of feedback on not only tachs, but other issues too.