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How the Ignition System Works

Point-Type Ignition System

An automotive ignition system is divided into two electrical circuits (The primary and secondary circuits). The primary circuit carries low voltage. This circuit operates only on battery current and is controlled by the breaker points and the ignition switch. The secondary circuit consists of the secondary windings in the coil, the high tension lead between the distributor and the coil (commonly called the coil wire) on external coil distributors, the distributor cap, the distributor rotor, the spark plug leads, and the spark plugs.

The distributor is the controlling element of the system. It switches the primary current on and off and distributes the current to the proper spark plug each time a spark is needed. The distributor is a stationary housing surrounding a rotating shaft. The shaft is driven at one-half engine speed by the engine's camshaft through the distributor drive gears. A cam near the top of the distributor shaft has one lobe for each cylinder of the engine. The cam operates the contact points, which are mounted on a plate within the distributor housing.

A rotor is attached to the top of the distributor shaft. When the distributor cap is in place, a spring-loaded piece of metal in the center of the cap makes contact with a metal strip on top of the rotor. The outer end of the rotor passes very close to the contacts connected to the spark plug leads around the outside of the distributor cap.

The coil is the heart of the ignition system. Essentially, it is nothing more than a transformer which takes the relatively low voltage (12 volts) available from the battery and increases it to a point where it will fire the spark plug as much as 40,000 volts. The term "coil" is perhaps a misnomer since there are actually two coils of wire wound about an iron core. These coils are insulated from each other and the whole assembly is enclosed in an oil-filled case. The primary coil, which consists of relatively few turns of heavy wire, is connected to the two primary terminals located on top of the coil. The secondary coil consists of many turns of fine wire. It is connected to the high-tension connection on top of the coil (the tower into which the coil wire from the distributor is plugged).

Under normal operating conditions, power from the battery is fed through a resistor or resistance wire to the primary circuit of the coil and is then grounded through the ignition points in the distributor (the points are closed). Energizing the coil primary circuit with battery voltage produces current flow through the primary windings, which induces a very large, intense magnetic field. This magnetic field remains as long as current flows and the points remain closed.

As the distributor cam rotates, the points are pushed apart, breaking the primary circuit and stopping the flow of current. Interrupting the flow of primary current causes the magnetic field to collapse. Just as current flowing through a wire produces a magnetic field, moving a magnetic field across a wire will produce a current. As the magnetic field collapses, its lines of force cross the secondary windings, inducing a current in them. Since there are many more turns of wire in the secondary windings, the voltage from the primary windings is magnified considerably up to 40,000 volts.

The voltage from the coil secondary windings flows through the coil high-tension lead to the center of the distributor cap, where it is distributed by the rotor to one of the outer terminals in the cap. From there, it flows through the spark plug lead to the spark plug. This process occurs in a split second and is repeated every time the points open and close, which is up to 1500 times a minute in a 4-cylinder engine at idle.

To prevent the high voltage from burning the points, a condenser is installed in the circuit. It absorbs some of the force of the surge of electrical current that occurs during the collapse of the magnetic field. The condenser consists of several layers of aluminum foil separated by insulation. These layers of foil are capable of storing electricity, making the condenser an electrical surge tank.

Voltages just after the points open may reach 250 volts because of the amount of energy stored in the primary windings and the subsequent magnetic field. A condenser which is defective or improperly grounded will not absorb the shock from the fast-moving stream of electricity when the points open and the current can force its way across the point gap, causing pitting and burning.

Fig. 1: A schematic of a typical conventional breaker-point ignition system.

Fig. 2: A conventional breaker-point distributor

Fig. 3: Cutaway view of a conventional coil. The primary windings connect to the small terminals on the top of the coil, while the secondary winding connects to the central tower

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