Re: Adding a tach to a diesel...
I haven't tried one, so this is just the theory, which is sound:
The alternator windings put out AC, which is turned into pulsed DC by going through the diodes. It should be no problem to count the pulses and determine accurate alternator RPM. To be accurate displaying engine RPM,there would need to be a calibration built in to compensate for the speed change of engine pulley versus alternator pulley size. A tach might just be preset for an average or common pulley pair. Also, if the alternator had one or more bad diodes, the number of pulses would be off (and your battery would have trouble charging).
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.