Chapter 8 Ignition System Fundamentals 153 Copyright by Goodheart-Willcox Co., Inc. reach TDC and start the downward stroke before the air-fuel mixture is ignited. To properly ignite the air-fuel charge, the ignition timing advance system adjusts the spark earlier (advances) or later (retards) in relation to engine speed and load to prevent spark knock. To start the engine, the spark must be near top dead center. After the engine starts, the spark must be advanced before top dead center for best performance, fuel economy, and driveability. On older vehicles, vacuum and centrifu- gal advance mechanisms were used. Now, the ECM adjusts the timing. The ECM can change timing much faster than older systems. It also compensates for changes in engine operation or outside factors, such as air temperature. Computer control systems are discussed in Chapter 7. The ECM bases the timing setting on information provided by the ignition triggering device and other various engine and vehicle sensors: • Engine temperature. • Engine and vehicle speed. • Manifold vacuum. • Throttle position. • Intake airflow. • Intake air temperature. • Knock sensor. The ECM processes these inputs and controls the ignition control module to vary the timing as required. By varying the timing of the triggering signal, the ECM advances or retards coil firing and therefore spark timing. Ignition timing changes are precisely matched to engine speed, load, and temperature. Under normal oper- ating conditions, the ECM advances the ignition timing as much as possible for maximum fuel economy without increasing emissions or causing detonation (pinging). Timing is constantly advanced or retarded as sensor inputs indicate changes in engine operating conditions. During wide-open throttle operation, the ECM may retard timing to a preset amount to reduce the chances of detonation and engine damage. In some cases, the ECM disregards input from the knock sensor during wide-open throttle operation for maximum power output. Unusual conditions, such as engine overheating or the failure of a computer system component, may cause the ECM to go into open loop and reduce timing to a preset amount. The advance may be preset or only move a small amount when the engine is cold and the ECM is in open loop. Many ECMs also control the amount of time that the primary current flows in the coil(s). This is done to reduce coil heating by producing only enough current flow to fire the spark plugs. When a hotter spark plug is needed, the computer increases primary current flow time. The time that current flows is called dwell. The term “dwell” originated from old contact point ignition systems and was adjustable on these systems. Dwell is no longer adjustable, but can be measured as part of ignition system diagnosis. Secondary Ignition Circuit Unlike the primary ignition circuit, which creates and times the high-voltage current, the secondary ignition circuit distributes the current from the coil to the spark plugs and creates the spark. The primary function of the secondary circuit is to ensure that the electricity reaches the right plug to create a spark at the right time without shorting or arcing to ground. Defects in the secondary circuit usually cause engine missing, backfiring, or other symptoms that are similar in both older and newer engine designs. Many parts of the secondary ignition circuit contain small resistors or resistance-increasing material. Increasing resistance reduces the size of the magnetic field around the secondary ignition components. This, in turn, reduces electromagnetic interference or static in sound systems, on-board computers, and cellular telephones. Specific resistance-reducing devices are discussed in the next sec- tions under each secondary system component. Secondary Wires Secondary wires, commonly called spark plug wires, are used to conduct high-voltage electricity from the coil to the spark plugs, as shown in Figure 8-21. The conductor is made of fiberglass string impregnated with graphite or car- bon. The graphite-impregnated string has enough electrical resistance to reduce the magnetic field around the wires. Copper and aluminum conductors have not been installed Figure 8-21. Secondary, or spark plug, wires carry the high sec- ondary voltage from the coil to the spark plug. They come in sev- eral sizes and are made out of carbon or silicon impregnated fiberglass string with a thick covering of insulation. (Ford) Insulator Insulators To coil Secondary wires