BOSCH Electronic Ignition System: Operation

Inside the distributor, a trigger wheel turns with the distributor shaft. The trigger wheel has one tooth or lug for each engine cylinder. As the trigger wheel rotates past the lugs of the 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 control module, which controls the dwell angle and at the same time interrupts the ignition coil's primary current. This induces the high secondary coil output voltage that fires the spark plugs.