The purpose of the ignition control module is to distribute the signals from the electronic control module to operate the coils. The computer gets various signals necessary for proper engine management and makes many decisions every second to keep the mixture within parameters regardless of load.
The sensors responsible for the signals are the crankshaft sensor, which supplies the computer with top dead center on the number 1 cylinder. It is also used for rpm. The coolant temperature sensor signals the computer as to the temperature of the engine so the mixture can be adjusted. The mass air flow sensor uses a hotwire in the intake path that the computer supplies power to. The object is to maintain the same temperature of the wire. The more air that passes the hotwire (cooling it), the more power necessary to maintain the temperature. The knock sensor sends a signal if the timing is too advanced or any pre-ignition or detonation occurs. If this signal is seen, the computer will automatically retard the spark to prevent detonation.
The manifold air pressure (MAP) or barometric (BARO) sensor (depending on the year, make and model of the vehicle) determines and signals the altitude above sea level relative to location or temperature and humidity. The oxygen sensors sense the amount of oxygen in the exhaust, which relates to the relative mixture. The optimum mixture is 14.5 to 1. The computer receives the signals from the sensors, then calculates a plan of action based on these results.
The ignition control module supplies the power to the coils. The computer sends a signal to the ignition control module to ground the coils, allowing them to fire. In many vehicles, there are multiple coils. This is called a distributorless ignition. When this system was first introduced, there was one coil to operate two cylinders. This was called the wasted spark system. One spark plug would fire on a combustion stroke, and the other would fire on an exhaust stroke. This just kept the number of coils down. Many vehicles still use this system, but many have one coil per cylinder situated on top of the spark plug.
Another widely used arrangement of the coils is called the coil pack. Two to four coils are placed side by side directly on top of the ignition control module. They use conventional spark plug wires to convey the spark.
