So how's the darned thing work?  Understanding electronic speedometers. 

We've already talked about mechanical eddy-current speedometers, which seem quite simple and easy to service especially if you know what you're doing.  But what about electric or electronic speedometers in the newer Mustangs?  How do they work?

Electronic speedometers work on a principle similar to a tachometer.  They work on a electrical magnetic pulse principle much like the tachometer does.  Like a mechanical speedometer, an electronic speedometer gets its input from transmission tail shaft or drive shaft speed.  Instead of a mechanical cable from transmission to speedometer, the electronic speedometer gets its "signal" from a magnetic pick-up at the output shaft/drive shaft.  This is known as a "Hall effect" system.  The faster your Mustang rolls, the more pulses arrive from the pick-up per minute.  A tachometer works the same way off the ignition coil as the primary circuit collapses and regroups between spark plug firings.

A Hall effect module works like a magneto in your lawn mower's ignition system.  As magnets on the rotating shaft pass the pick-up module connected to your speedometer, electricity is generated in pulses as magnetic fields happen again and again via rotation.  The faster they happen, the higher your speedometer will read.

What makes an electronic speedometer (digital or analog) more complex is integrated circuits and microprocessors that convert that Hall effect electrical pulse to rotary motion in a speedometer needle, which is attached to a small electric motor that moves it about the face.  A motor also runs the odometer.