There are wires carrying the tachometer sign - one is the grey wire you see, and the opposite is the dc floor buss. The tachometer measures the ability distinction among the gray cord and dc ground. It truly "counts" pulses on the gray cord, and, for an anlog tach, converts the heart beat rate to a voltage or contemporary to power the tach meter. In the picture, there is a single gray wire coming off of the unrectified aspect of the rectifier. That gray cord runs into the wiring harness, up to the dash of the captains chair, and right into a 12v tachometer. I have been told that it's far measuring ac cycles with a view to extrapolate engine pace but i am uncertain of the technique through that you use to degree cycles with only one cord?.