Ignition System: Description and Operation

The Electronic Spark Advance provides electronically controlled spark advance and eliminates the need for mechanical and vacuum advance controls in the distributor. The Spark Control Computer uses as many as five sensors to monitor the engines conditions so that it can fire the spark plugs at the proper time.

SPARK CONTROL COMPUTER (SCC)
The Spark Control Computer is the heart of the entire system. It gives the capability of igniting the fuel mixture according to different modes of engine operation by delivering an infinite amount of variable advance curves. During cranking an electrical signal from the distributor is fed into the computer. This signal will cause the computer to fire the spark plugs with a fixed amount of advance. Once the engine starts, the timing will now be controlled by the computer based on the information received from the various sensors.
The amount of spark advance is determined by three factors, coolant temperature, engine speed, and manifold vacuum. The computer determines spark advance in the following manner:
1. Coolant temperature modifies the vacuum advance schedule. There is a different schedule for hot and for cold engines.
2. Advance from vacuum is programmed into computer. In a cold or warm engine vacuum advance depends only on amount of manifold vacuum. In a hot engine vacuum advance depends on both manifold vacuum and engine speed.
3. Advance from speed is programmed into computer and depends only engine speed.

SENSORS
The Electronic Control System incorporates the following sensors:
1. The Hall Effect Pickup assembly, located in distributor, supplies basic timing signal and engine speed to computer.
2. The coolant sensor, located on thermostat housing (front left side of intake manifold on V6-239 engine), gives computer information on engine temp.
3. A Carburetor Switch located on end of idle stop tells the computer when the engine is at an idle.
4. The Vacuum Transducer is located on the Spark Control Computer and tells the computer how much vacuum engine is producing.
5. The Oxygen Sensor (used with Feed Back Carburetor) is located in exhaust manifold and informs the computer about oxygen in exhaust so it can alter air/fuel mixture.