Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 214 The resistor shortens both stages, lengthening elec- trode life and suppressing electromagnetic interference. Figure 10-19 illustrates the difference between the spark produced by a nonresistor plug and a spark produced by a resistor plug. No additional voltage is required to fire the resistor plug. Directional Spark Plugs Some of the spark plugs used in certain distributor- less ignition systems are fired from the side (ground) electrode to the center electrode. These plugs, known as directional spark plugs, are equipped with special electrodes to prevent misfires. On many engines with waste spark systems, half the plugs are conventional and half the plugs are directional. When servicing the plugs on these engines, a technician must be sure to install all spark plugs in their original locations. Replacement spark plugs for these engines will usually be the same for all cylinders. Distributor Cap and Rotor A distributor cap is used in ignition systems equipped with distributors to route secondary voltage to the spark plugs. It contains one center terminal and a series of outer terminals arranged in a circular pattern around the center terminal. High voltage from the coil is carried by an insulated wire to the center terminal of the cap. On some vehicles, the coil is installed in the distributor cap. Heavily insulated wires are used to connect each outer terminal with its corresponding spark plug. The distributor cap is made of a plastic material, sometimes mica-filled to reduce flashover tendency. The materials used must provide excellent insulation. On the inside of the cap, the plug terminals have aluminum or brass lugs that extend down past the cap material, Figure 10-20. A distributor rotor carries and routes the second- ary voltage from the distributor cap’s center terminal to an outer terminal. The rotor body is made of an insulat- ing material, usually plastic. It is attached to the top of the distributor shaft. Its outer edge has a brass terminal that passes, as it turns, very close to the distributor cap’s side terminals. This terminal is connected to a spring that rubs against the cap’s center terminal (carbon rod). When current from the coil arrives at the center terminal of the distributor cap, it travels down the carbon rod, through the spring, and out to the rotor’s outer edge terminal. From this point, it jumps the small gap between the rotor and side terminal and goes on to the plug. See Figure 10-21. Jumping the gap between the rotor and distributor cap termi- nal requires about 3000 extra volts, well within the coil’s capacity. Spark Plug Wires Spark plug wires, also called secondary wires, are used on some engines to carry the high- voltage current from the coils or distributor cap to the spark plugs. Spark plug wires are cov- ered with a thick layer of insulation to protect the wire and to reduce the possibility of arcing. To plug From coil Snap fitting cast into hold wire Carbon rod Outer terminal Anti-flashover grooves Nonconductive material Goodheart-Willcox Publisher Figure 10-20. Cross-sectional view of a distributor cap. Note the fixed carbon rod that contacts the center of the rotor. Ignition point Time Inductive Capacitive Capacitive Inductive Voltage Nonresistor plug Resistor plug Goodheart-Willcox Publisher Figure 10-19. Spark plug discharge graph. Note how the resistor plug reduces the undesirable inductive portion of spark.