Chesapeake Black 4 Tachometer - 4000 RPM Diesel Mechanical Takeoff Var Ratio AltPerimeter-lighted Black dial with stylish Blue and White graphics, polished stainless steel bezel, contoured white pointer and domed glass lens.Electronic 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