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).
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