posted 06-30-2001 01:13 PM ET (US)
Generally on older 2-stroke engines the tach derives its pulses from the same coil that provides current to the rectifier for battery charging.The pulsating current from the coil (of referred to as a STATOR coil) enables the tachomoter to indicate crankshaft speed.
The tachometer will usually have a selector switch to match the tach calibration to the number of pulses per revolution of the flywheel, which is in turn determined by the number of magnet poles contained in the flywheel.
The tachometer also generally needs battery voltage and ground applied to it, and most also have another terminal for gauge illumination.
Measuring the tachometer lead with an AC Voltmeter will show about 12 Vac. This is because the 12-volt battery clamps the voltage output from the stator coil at 12 volts. Any higher voltage and the rectifier conducts current into the battery.
Six poles is a typical number, so at a rotation speed of 1,000 RPM, the tachometer lead should be producing about 6,000 pulses per minute, or 100 pulses per second. If you happen to have a frequency counter (often built into better Digital Volt Meters) you can count the pulse output.
--jimh