Tachometers all look pretty much the same, but there is some variation between the methods that tachometers get their signal information. The function of a tachometer is to display the RPM (revolutions per minute) of the engine. Typical RPMs will be in the 10s of hundreds, so the method for counting the revolutions must be quick and accurate.
By far the most common method employed in providing a tachometer signal is utilizing the alternator’s revolutions to generate a discernible indicator. Alternators generate power fundamentally as AC (alternating current) that is then rectified to DC (direct current) that can then be stored in the batteries. Most alternators have an AC signal tap, pre-rectified alternator output; that AC generation will be a pulse signal (1/3 wave form), and if the ratio between the crankshaft pulley and the alternator sheave is the standard 3:1, then the pulse signal will provide an accurate measurement of the engine`s RPM.
On some engines, especially in commercial applications, the tachometer is mechanical. There will be a gear drive on the engine that will spin a cable, and transfer the rotation to the tachometer gauge, so that it can be read. A slight addition to the mechanical approach is to use the mechanical rotation of the gear drive to generate a pulse signal, similar in nature to the alternator pulse signal, which can then be converted via an electrical gauge to provide RPM information.
The least common, and most commercial, is engines that utilize a magnetic inductive pickup, usually on the flywheel, to generate a pulse. A magnet is installed in/on the flywheel, and an inductive proximity sensor is installed into the flywheel housing. Each time the magnet rotates past the inductive sensor a pulse is generated that is then converted to electrical signal to be displayed on the tachometer.
– Lyle, PYS Marine Electrical Technician