Chesapeake Black SS 4 Tachometer 4,000 RPM Diesel Mechanical Takeoff Var Ratio Alt . 4 Hole size 3.375 85mm . Electronic tachometers work by counting pulses generated by the ingition system, alternator, tach signal generator or magnetic pickup sender. The tach is hooked up to 12VDC, Ground and one of the signal sources listed above. By selecting the right tach and setting the switch on the back to the correct postion you let the tachometer know how many pulses are sent per each engine revolution. Benefirts Wide range of scales. Multiple signal sources. Rugged, Reliable construction 4 Hole size 3.375 85mm . Electronic tachometers work by counting pulses generated by the ingit By selecting the right tach and setting the switch on the back to the correct postion you let the ta Wide range of scales. Multiple signal sources Rugged, Reliable construction