A tach-dwell meter is a combination electronic device that measures engine rpm as a tachometer and ignition point dwell angle. The tachometer function is self-explanatory; it measures engine speed in revolutions per minute. The dwell function measures the degrees of distributor rotation that ignition points are closed and is directly related to ignition point gap. In fact, the ignition point gap can be set by adjusting the dwell angle.
Attach the red lead on your meter to the negative “-” terminal on the ignition coil.