How Does an Ignition Control Module Work?

Ignition control modules were introduced along with fuel injection. Electronic fuel injection requires a computer coupled with a host of sensors for situational input from the engine to operational strategies compiled within the computer. These strategies are formulated to optimize the engine's performance under all density altitude conditions as to the mass of the air and for real-time power demand. Its strategies also include the proper operation to minimize harmful emissions into the atmosphere. It does this by not only maintaining a 14.5-to-1 optimal mixture ratio for power, but by monitoring the amount of unburned fuel and adjusting the on time of the injectors and the spark advance.

The oxygen sensors are the primary sensors indicating the emission level through the amount of free oxygen in the exhaust system. When they encounter free oxygen in the exhaust system, they give off varying electrical signals in the millivolt range. These electrical signals are called counts. The computer monitors these counts and adjusts the on time of the injectors accordingly.

The ignition control module controls the ignition coil or coil's firing time, whichever the case may be. Some of the first ignition control modules in the mid-1980s controlled only one coil. With the introduction of the wasted spark system, the control module controls a series of coils--one coil for two cylinders. In the case of a six-cylinder engine, for example, there are three coils. The coils are usually mounted directly on the top of the ignition control module. Regardless of the location of the ignition control module, the operation is the same. The ignition control module receives power from the ignition switch on one terminal and has a ground terminal for ECM to ICM control of the ground signal. There are ground wires to all coils so they can be fired individually. On this type of system, each coil fires two spark plugs simultaneously. One spark is useful, and the other is not. The wasted spark usually fires on the exhaust stroke.

Ignition control modules have been known to go bad on all makes and models of vehicles. When this occurs, there is no spark or a very erratic spark. It is easy to diagnose if this should happen. Use a voltmeter to check for power at the control module. The ignition should be on while checking for power. If there is power, check for a pulsating ground at the module while cranking the engine. This signal is from the computer. If there is a signal, the module is bad. If there is no signal, the computer has an issue with a sensor or itself.