NIPPONDENSO Electronic Ignition System: Operation
As the timing rotor turns with the distributor shaft, its teeth (one for each engine cylinder) pass the pick-up coil assembly. See Fig 1. As the air gap changes with the passing of the teeth, the magnetic field varies creating a signal in the pick-up coil assembly.
The ignitor senses this signal and turns the ignition coil primary circuit on and off. This causes voltage to build and collapse, resulting in a voltage surge in the ignition coil secondary circuit that fires the spark plugs.