Ignition Control Module: Description and Operation

MPI Electronic Control Unit (Early Production w/2 Connectors):

MPI Electronic Control Unit (Late Production w/4 Connectors):

The MPI engine management system is new for the 1990 model year and, according to information at the time of this release, is available only on the 20 valve 2.3 liter DOHC engine (also new for 1990 in the U.S.A.). The system is produced in two basic versions during its introductory year. In each version, a single ECU performs both fuel management and ignition control. The early and late versions can be identified by the connectors on the ECU. The early version (produced before March 1990) has two ECU harness connectors. The late version (produced after March 1990) has four ECU harness connectors. Aside from physical differences in the wiring harness and some software changes, the sensor inputs and control output functions of the ECU are the same for both versions. The ECU monitors various engine sensors and controls the air/fuel ratio, ignition timing, and ignition dwell, through output devices and signals.

Ignition Maps And Knock Sensor Control:

IGNITION CONTROL
The ECU controls the ignition timing and dwell, 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, 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 the maximum number of steps, 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 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.