Copyright Goodheart-Willcox Co., Inc. Chapter 10 ∣ Ignition Systems 215 Original equipment plug wires contain a built-in resistance to reduce radio interference. Most replacement wires are also resistor types. Resistor wires should always be used in late- model vehicles to prevent radio and computer system interference. Ignition Timing and Advance For correct ignition timing, each cylinder should receive a spark at the plug electrodes as the piston nears the top of its compression stroke (a few degrees before TDC). In distributor ignition systems, the distributor shaft turns at one-half crankshaft speed. The distributor shaft is often turned by one-to-one gearing with the camshaft, which is already turning at one-half the engine’s rotational speed. On some engines equipped with a timing belt, the belt drives the distributor. The distributor shaft gear is timed so that the spark is produced when the cylinder is ready to fire. The rotor will then point toward the cylinder’s cap plug terminal. A plug wire is attached to this terminal. The wires are attached to the cap starting at the number one cylinder and following the firing order in the direction of distributor shaft rotation. As the engine turns, the distributor shaft revolves. Each time the distributor shaft has turned enough to cause the rotor to point to a plug terminal, the ignition system produces a spark. This cycle is repeated over and over. The engine manufacturer specifies timing in regard to the number of degrees before top dead center (TDC) that the number one cylinder should fire. All other cylinders will fire at the same number of degrees before TDC. If the plug fires later than the specified setting, the timing is said to be retarded. If the plug fires earlier than specified, the timing is referred to as advanced. In distributorless ignition systems, the ECM controls ignition timing based on inputs from the crankshaft position sensor. Some engines also have a camshaft position sensor. The ECM determines piston position from the sensor inputs and fires the proper coils when each piston is at the top of its compression stroke. Coil wire Coil Distributor cap Distributor shaft turns rotor Rotor Carbon rod Spark plugs Spark Battery Spark plug wires Ignition switch Goodheart-Willcox Publisher Figure 10-21. A distributor cap and rotor positioned in an ignition system. As it rotates, the rotor transfers current from the center cap terminal to the outer terminals.