Electronic Control Unit (Ecu)

Ignition firing signals from ECU are transmitted through terminal "B" of connector No. 2 on ICM. The ignition signal from ECU is received by ICM in form of a 5-volt square wave. See Figure. As leading edge of wave contacts ignition circuitry, ICM charges coil primary windings.

When coil saturation occurs, ICM circuitary opens primary windings, which collapses magnetic field. The high voltage in coil secondary windings is then transmitted through coil wire, distributor cap and rotor to the correct spark plug.