How to Check Your Tachometer

I was doubting my tach and was wondering how to test it to see if it is actually accurate. On the alt.autos.rod-n-custom newsgroup from 2001 (used www.dejanews.com to search), I found a discussion that talked about using a 12 volt battery charger to check the tach. So what the heck, I tried it.

The theory goes that a 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

  • 1800 rpm for an 8 cylinder,
  • 2400 for a 6 cylinder
  • 3600 for a 4 cylinder

Well it works! When I hooked it up to my tach's input (+ve to tach +ve and -ve to gnd), it showed exactly 1800 rpm. The technical theory goes like this. The battery charger doesn't put out a DC signal, it is a 120 Hz full rectified waveform. When a battery charger is connected to a battery, the battery acts like a huge capacitor and smoothes out the waveform (simple explanation). I used a frequency counter to verify that the battery charger was putting out 120 Hz.

Here's the math:

120 Hz = 120 cycles/sec

120 cycles/sec * 60 = 7200 cycles/min

There are 8 cylinders, so 7200 / 8 = 900 cycles/min

but it is a 4 stroke engine which only fires on 1 firing stroke (combustion)

It fires once every 2 cycles

So 900 cycles/min *( 4 strokes/1 firing stroke) * (1 firing stroke/2 cycles) = 1800 cycles/min or 1800 rpm

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Copyright June 2011 Eugene Blanchard