Is this alternator W RPM input circuit ok and how can I improve it?
You may use an RC filter with a time constant of approx 50% of worst case max pulse width RPM period then use a CMOS Schmitt trigger inverter which has 33% hysteresis to reject noise between approx 1/3 and 2/3 Vcc after low pass filter and proper divide ratio to scale.


But for CMOS ground the unused inputs.
But you can also make a one-shot out of these gates with a diode cap resistor and 5Vreg to make the Tach voltage directly by averaging the 1-shot output in DC with a similar filter as before, except now RC will be about 20ms. depending on the desired slew rate. ...vroom...  except at idle the freq. may be too low and flutter he voltage so a CMOS transmission gate can be used as a S&amp;H to eliminate the flutter using the new clock and filtered DC Tach signal.


Thus if the maximum interval of the pulse or frequency is 10kHz or 50us "high" then use a 5us time constant = RC.  

Also, use twisted pair (UTP or STP) to reduce spark plug crosstalk or whatever is causing this.







simulate this circuit &ndash; Schematic created using CircuitLab


or use 3.3V
Don't forget different vehicles have different pulley ratios that affect Alternator/Motor RPM ratios.