Copyright Goodheart-Willcox Co., Inc. 217 Summary • The maximum voltage produced by a vehicle’s electrical system is multiplied thousands of times by components of the ignition system in order to jump the gap of a spark plug’s electrodes and fire the spark plug. • The primary circuit of an ignition system consists of the battery, ignition switch, resistor (where used), ignition control module, and the primary windings of the ignition coils. • The battery is the source of electrical energy needed to operate the ignition system. It stores and produces electricity through chemical action. • The ignition switch controls the flow of electricity through the primary circuit. • The ignition control module contains electronic components that control current flow through the primary windings of the ignition coils based on inputs from other ignition components. • Ignition coils use primary windings and secondary windings wrapped around an iron core and housed in an insulated case to increase battery voltage up to 100,000 volts. • Current flowing through the primary windings of an ignition coil produces a magnetic field. When the magnetic field collapses through the primary windings due to a triggering device interruption and passes through the secondary windings, the voltage in the secondary windings is multiplied to the high-voltage current necessary to fire the spark plugs. • Magnetic pickup sensors, Hall-effect switches, and optical sensors are the three devices used to interrupt current in the primary ignition circuit. • A distributor ignition system uses an engine-driven distributor to send high-voltage current from the secondary windings of the ignition coil to fire the spark plugs via the rotor, cap, and spark plug wires. • Inside a distributor cap, a center terminal and a series of outer terminals arranged in a circle around it route secondary voltage to the spark plugs. A distributor rotor carries the secondary voltage from the center terminal and transfers it to the outer terminals. • A distributorless ignition system eliminates the distributor by using a crankshaft and/ or camshaft position sensor to control engine timing through the ignition control module. • Many late-model engines are equipped with coil-on-plug (COP) ignition systems, which is a type of distributorless ignition system. These systems, which are also called direct ignition systems, eliminate spark plug wires by installing an individual ignition coil assembly directly over each spark plug. • The secondary circuit of an ignition system consists of the secondary windings of the ignition coils, the distributor and spark plug wires (distributor-style ignition systems), and the spark plugs. • A spark plug is constructed with three major parts: the electrodes, insulator, and shell. • Electrons flowing from the ignition coil surge across the gap between a spark plug’s center and ground electrodes to create a spark. • The insulator on a spark plug keeps the high-voltage current arriving from the ignition coil from shorting to ground before creating a spark. • A spark plug shell is a steel covering at the bottom of the plug that provides a holding base for the internal parts of the spark plug, as well as threads to screw and seal it into the engine cylinder head.