Computer Controlled Ignition
As the engine turns the distributor shaft, the shutter blades of the rotor/armature assembly pass through the Hall Effect sending unit's pick-up coil. A signal is then sent to the electronic control unit.
In addition to receiving engine speed information from the distributor, the electronic control unit receives engine load information from the 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 sent to the control unit.
The electronic control unit analyzes all information, and interrupts the signal to the ignition coil primary circuit. This causes a high voltage surge in the secondary circuit, firing the spark plugs at the proper time.
When the engine is first started, the electronic control unit operates 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 electronic control unit can operate without the micro-processor in the "LIMP HOME" or "CRANK" mode. If a problem in the system occurs, the electronic control unit will enter the "LIMP HOME" mode. In the "LIMP HOME" mode, the engine will operate at a fixed ignition timing to allow the vehicle to be driven to a service facility.