HITACHI Electronic Ignition: Operation
With ignition on, a magnetic field is created. As reluctor's external teeth approach and then pass stator's internal teeth, the field builds and collapses. This generates an electrical signal in the pick-up coil, which in turn is conducted to ignition unit. The ignition unit turns the current flow in ignition coil primary circuit on and off as the teeth pass each other. This results in a high voltage surge in the secondary circuit, which fires spark plugs.
The ignition unit contains 5 circuits that perform the following functions. The Spark Timing Signal Monitoring Circuit monitors and amplifies signal from distributor pick-up coil. The Lock-Preventing Circuit cuts off ignition coil primary current when ignition on and engine is not running.
The Duty Control Circuit controls ratio of ignition coil primary current on-off time (equivalent to dwell angle). The Power Switching Circuit makes or breaks primary circuit current of ignition coil. The Current Limiting Circuit controls current value so excessive current will not flow through power switching circuit. All 5 circuits are contained in ignition unit. Failure of any circuit requires replacement of the entire ignition unit.