Ignition Control Module (ICM)
An electronic component that switches the ignition coil on and off to generate high voltage at the spark plugs.
The Ignition Control Module (ICM) is an electronic switching device that controls the timing and delivery of the spark in the engine's ignition system. It receives signals from the crankshaft position sensor or distributor and uses this data to switch the ground circuit of the ignition coil on and off. Turning the coil's primary circuit on and off collapses its magnetic field, inducing the high-voltage spark required to ignite the air-fuel mixture in the cylinders.
Because the ICM handles high electrical currents, it generates a significant amount of heat. It is typically mounted on a heat sink, either inside the distributor, on the firewall, or directly on the engine block. Heat is the primary enemy of the ICM; over time, thermal stress can cause internal electronic components to fail.
A failing ICM can cause the vehicle to stall once it warms up, fail to start entirely, or cause random misfires. Diagnosing a bad module involves checking for spark and verifying control signals to the ignition coil.