Does anyone here happen to know what kind of a signal our tachometers use? Is it just a positive 12 volt square wave that has it's pulsewidth modulated based on engine rpm and how fast the ignition coil is discharging?

 

The tach is connected to the coil primary, so it's FM not PWM.

 

Alright, thats even better It is a 12 volt square wave correct?

 

12V give or take, but I wouldn't count on it necessarily being square.

 

Yup, the tachometer wire (usually white) is connected to the coil's primary side, the groudning wire, not the pink power wire.

Hope this helps.
Lou

 

Awesome! Just got my handy dandy 25 dollar Princess Auto tach working on the bench here.

Now to start generating the crazy AC sawtooth waveform my diesel's engine speed sensor puts out, and condition it to output the same 12 volt square signal I've built here with the 555 timer then calibrate it in such a way that I have a reliable tachometer setup on my diesel without having to worry about tapping into my alternator, or gluing stuff to various rotating components on my truck...

Thanks for your help guys!