BOSCH Electronic Ignition: Operation

Inside the distributor, a trigger wheel turns with distributor shaft. The trigger wheel has one tooth or lug for each engine cylinder. As trigger wheel rotates past lugs of magnetic pick-up coil, a magnetic field is built up that continually builds and collapses. This produces a low voltage electrical signal.

This signal passes to the electronic control module, which controls dwell angle and at the same time interrupts ignition coil's primary current. This induces high secondary coil output voltage that fires spark plugs.