Ignition Control Module: Description and Operation

CIS-EIII Control Units:

The CIS-EIII system uses two separate ECUs. One for fuel control and one for ignition control. The ECUs share information from various engine sensors and use this information, compared with information stored in their individual memories, to precisely control the fuel and ignition systems for lower exhaust emissions, better fuel economy and better engine performance, under all operating conditions.

IGNITION CONTROL UNIT

Ignition Maps - Knock Sensor Control:

The ignition ECU controls the ignition timing according to complex "maps" programmed into the computer memory. The ECU monitors engine speed, load, and temperature, then plots these points on a complex 3 dimensional graph (ignition map) to determine the degree of ignition advance. A knock sensor is used to detect spark knock. If a knock is detected, the ECU retards the ignition timing for that cylinder from the mapped point, in steps, until the knocking ceases, then gradually steps the timing back to its previous setting. If knocking persists when the timing has been retarded to the maximum (12 degrees), the ECU will switch to a second ignition map programmed for fuels with lower octane ratings. If the knocking still continues, a trouble code will be stored in the computer memory.

Primary Current Vs. Charge Time W/Respect To Voltage:

The ignition dwell is controlled according to a similar map. With a constant dwell angle, the charging time changes depending upon the engine speed. At high engine speeds, the charging time is significantly reduced, and consequently, spark voltage is reduced due to insufficient coil saturation. At low engine speeds coil saturation is reached well before spark occurs, resulting in wasted energy and unnecessary coil heating. Since coil saturation is directly proportional to the amount of current flowing through the primary windings, by controlling the dwell angle and voltage, charge time (length of time required to reach nominal current flow and coil saturation) can be controlled. A low voltage results in a slow charge rate, and a relatively long period of time required to reach nominal current flow. A higher voltage has a faster charge rate and correspondingly shorter time to reach nominal current. The ignition ECU monitors the engine rpm, and charging system voltage, determines the required charge time for optimum spark at that engine speed (according to the "map" programmed into the computers memory), and adjusts the dwell angle and the voltage across the ignition coil, to maintain adequate charging time for optimum sparking voltage at all engine speeds and loads.

For more information about the ECU and its functions, refer to COMPUTERIZED ENGINE CONTROLS.