Computer Controlled Electronic Ignition System: Operation

As the engine turns the distributor shaft, the shutter blades of the rotor/armature assembly pass through the Hall Effect pick-up coil. A signal is then sent to the electronic control unit. The electronic control unit receives engine speed information from distributor and engine load information from intake manifold.

Speed and vacuum signals are processed in the electronic control unit to control spark advance and dwell. A throttle position microswitch (located on the airflow meter) provides throttle information. Under normal conditions, the throttle position signal is sent to the electronic control unit. The manifold vacuum signal is also processed by the control unit to control ignition timing. When the throttle is closed, (during deceleration and idle) the circuit is closed and no signal is received at the control unit.

The computer analyzes all information and fires spark plugs at the proper time by interrupting signal to ignition coil primary circuit. This causes a high voltage surge in the secondary circuit, firing the spark plug.

The electronic control unit can operate without the micro-processor in the "LIMP HOME" or "CRANK" mode. When the engine is first started, it runs in the "CRANK" mode until engine speed reaches 1500 RPM. At this point the micro-processor takes over, controlling all engine speeds down to 250 RPM. The vehicle operates at a fixed ignition timing in "LIMP HOME" mode to allow vehicle to be driven to a service facility.