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Oscilloscope as tach
- Thread starter tom86951
- Start date
mrjaw14
Well-known member
That's slick! I know the larger spikes are spark plugs firing, I wonder what the smaller spikes are?
engineer2
Well-known member
Yes, that's known way to measure RPM.
Bear Automotive used that to read engine RPM off the battery with their 42-210 ARBST battery tester and got a patent on it around 1988. You just had to enter the number of cylinders and the ignition system type.
The other spikes you see could be the primary side of the ignition, coil oscillations, the alternator field switching on and off, injectors firing, or other electrical systems. The continuous string of little humps are voltage readings from the alternator diodes.
Bear Automotive used that to read engine RPM off the battery with their 42-210 ARBST battery tester and got a patent on it around 1988. You just had to enter the number of cylinders and the ignition system type.
The other spikes you see could be the primary side of the ignition, coil oscillations, the alternator field switching on and off, injectors firing, or other electrical systems. The continuous string of little humps are voltage readings from the alternator diodes.
Last edited:
2ndGearRubber
Well-known member
Assuming that each tall spike is the same cylinder firing, the shorter at approx. 1/3 height of the main firing event spikes would be noise from other cylinders kV events being picked up by the scope.That's slick! I know the larger spikes are spark plugs firing, I wonder what the smaller spikes are?
That's how I do RPM on old stuff too, except using a secondary lead. Lots of noise in that capture, hard to even see the burn time. Although you're not really looking at that I suppose. Pico software is a little different. Get enough time on the screen to see 720* of crankshaft rotation, set two degree rulers between the kV events, and it will auto calculate RPM. Very nice.