Dress White 4″ Tachometer w/Systemcheck Indicator – 7,000 RPM (Gas – Johnson / Evinrude Outboard) Tachometers work by counting pulses generated by the ignition 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 position you let the tachometer know how many pulses are sent per each engine revolution. These tachometers have a light system built into the dial that show off