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An ignition control module is a component that can regulate spark generation in many internal combustion engines. The main purpose of the ignition module is typically to open or close a ground circuit to the primary winding inside the ignition coil. When this occurs, the coil can generate sufficient voltage to fire a spark plug. In order to ground the coil at the correct time, the ignition control module typically receives an input from a sensor inside the distributor. Ignition modules are often located inside or near the distributor, and are often coated in insulating compounds to protect their internal electronic components.
In mechanically timed engines, the ignition coil's primary winding is grounded through contact points inside the distributor. As the distributor shaft rotates, the points open at predictable intervals and break the circuit. This causes the magnetic field of the primary to collapse, which allows the secondary wiring to generate a high voltage. Electronic ignitions replace the functionality of the points with an ignition control module and an optical or magnetic sensor inside the distributor.
When the distributor shaft in an engine with electronic ignition rotates, the internal sensor sends an input to the ignition control module. The module can then break the ground circuit to the ignition coil primary, after which the system functions in much the same way as mechanically timed ignitions. Electronic ignitions may also allow the timing to be adjusted by the on board computer in order to achieve better fuel efficiency or fewer tailpipe emissions.
Ignition control modules typically consist of one or more transistors or other electronic components that can be heat sensitive. Some modules are located inside or near the distributor, where they are often subjected to high temperatures. Many control module designs include some type of insulating material to protect against heat damage, though failures are relatively common. When an ignition control module fails, the engine typically will not start, as the coil primary ground circuit will not be interrupted correctly.
Though malfunctioning ignition control modules typically cease operating altogether, resulting in the engine dying and not restarting, failures are often heat related. A common failure pattern for an ignition control module is for the engine to die once it gets hot, only to start and run just fine after it has cooled down. These types of malfunctions are often difficult to diagnose, as the module may test fine when it is cold.
You nailed it -- a malfunctioning ignition control module can be hard to diagnose because they can test perfectly fine when cold. I was having engine trouble a few years ago and it took the mechanic some time to figure out what was going wrong. I thought the car must have hated me because it worked fine for him but would fail on me.
It turns out the ignition control module had gone bad. It wasn't too expensive to get it fixed, but getting to the point where the mechanic figured out the thing was broken was aggravating.