Join Date: Mar 2002
Location: Austin, TX, USA
You'd need one of two things. A) an accurate signal generator of some sort or B) a frequency counter. The signal generator could be used to generate a known frequency signal for the tach which could be verified on the display. The frequency counter could be used to measure the frequency of a running engine and translate the result into a tach signal. Although, you'd have to protect the frequency counter from ignition spikes. For a V8 motor, RPM = Freq(Hz) * 60 / (Ncyl / 2). An AC power line could be used as a signal generator if you know how to do so safely. 60Hz AC would generate a 900RPM signal. You would need a resistor network which would limit the voltage to the tach to a half-wave-rectified voltage of less than 30 volts peak or so.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI