Copyright Goodheart-Willcox Co., Inc. Chapter 10 ∣ Ignition Systems 211 Primary Circuit Complete You have now traced the flow of current through the primary system. After going through the ignition control module, current returns to the battery through the metal parts of the vehicle to which it is grounded. Be sure you understand how each primary ignition system component works and its relationship to the other parts. Secondary Circuit You have seen how the ignition coils produce high-voltage current. The job of the secondary circuit is to send this current to the proper spark plug at the correct time. The current jumps between the center electrode of the spark plug and the side, or ground, electrode. When it jumps between the center and side electrodes, the current produces a hot spark that ignites the air-fuel mixture. This section explains how this is accomplished. Some distributorless systems have no sec- ondary system apart from the spark plug. Spark Plug Construction All internal combustion gasoline engines use spark plugs. The spark plug is constructed with three major parts: the electrodes, insulator, and shell. See Figure 10-14. Spark Plug Electrodes Decades ago, a spark plug was expected to last from 15,000–30,000 miles (24,000–48,000 km) or more. Spark plugs on modern engines can last as long as 100,000 miles (160,000 km). To accomplish this, the spark plug electrodes must be constructed of material that is resis- tant to heat, oxidization, and burning. Typical materials used to make spark plug electrodes include copper and nickel alloy. Platinum and iridium, although expensive, are sometimes used. In a modern spark plug, there are two electrodes: the center electrode and the side electrode. The center elec- trode is insulated from the rest of the plug by a ceramic insulator. The top of the center electrode terminates in a push-on terminal to which the plug wire may be attached, Figure 10-14. The ground electrode is attached to the steel outer shell, which is threaded into the engine. The space between the center and side electrode is called the plug gap. On older vehicles, the plug gap may be as small as 0.025″ (0.63 mm). The spark plugs in late- model vehicles may require gaps of as large as 0.095″ (2.41 mm). Most plug gaps are between 0.045–0.065″ (1.1– 1.65 mm). Always check the vehicle’s service information for the exact plug gap. The ignition spark occurs in the plug gap between the center and side electrodes of a spark plug. The elec- trons flowing from the ignition coil and across this gap to create the spark will flow more easily from a hot sur- face to a cooler one. As the center electrode of a spark plug is always hotter than the side electrode, the plug’s center electrode has negative polarity. This ensures a Terminal connection Ribs prevent flashover Terminal stud Conductive seal Swaged and heat shrunk fitting Gasket Center electrode Side or ground electrode Insulator Insulator tip Thread reach Resistor assembly Shell Bosch Figure 10-14. Cutaway of a spark plug. All spark plugs have the same internal parts. However, high-quality plugs are manufactured with distinct designs and expensive metals that increase performance and service life.