The tach is driven by keyed AC (pulses) from the charge coils on the stator. Depending on the configuration of the bobbins and the magnets you get anywhere from 4 to 20 pulses per revolution.
While there is specs out there for the ignition side of things, I have never seen any for the charge side (at least coming directly from the stator charge/lighting coils), before conversion to DC.
The charge bobbins produce AC that is rectified to DC either with a rudimentary rectifier (which outputs between 13.5-16.0 volts DC on average depending on rpms - never dissected one but suspect it combines the peak AC both positive/negative after filtering with high/low pass circuitry into a single positive keyed DC stream ) or is rectified/regulated to 13.5 volts by a voltage regulator.
The "signal" for the tach is parsed off before the voltage is rectified, so can only assume that it's AC pulses with a peak power somewhere between 13.5 and 16'ish volts.
The 175 Merc produces 12 pulses per rotation, and again, never stuck it into a scope so can't say if would look like a pulsed stream, or due to distortion, a keyed square wave.
(now making me think here - dangerous
)
It should be simple to produce a digital tach. Would only need a "counter" that divided the received pulses by 12/per minute..
