676 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Th e spark line shows the voltage used to maintain the arc or spark across the spark plug electrodes. Once the spark is started, less voltage is needed to maintain the arc. Th e spark line should be almost straight, clean, and about one- fourth as high as the fi ring line. Intermediate Section Th e intermediate section, or coil oscillations section, shows voltage fl uctuations aft er the spark plug stops fi ring. Typically, the voltage should swing up and down several times. Th is section of the pattern will indicate problems with the ignition coil or coil pack. See Figure 34-33. Th e voltage oscillations will disappear at the end of the intermediate section as the ignition amplifi er or PCM begins to conduct. Dwell Section Th e dwell section starts when the ignition module conducts primary current through the ignition coil. In an older contact point system, it is the time when the points are closed. Th e ignition coil is building up a magnetic fi eld during the dwell section. The dwell section will indicate problems such as a faulty ignition module, burned contact points, or a leak- ing condenser. An electronic ignition can have diff erent dwell peri- ods from cylinder to cylinder. However, if the dwell varies in a contact-point type ignition, it indicates distributor wear or damage. To introduce the basic sections of an ignition scope pattern, the primary and secondary patterns for one cylin- der will be explained. More complex patterns for specialized tests will be covered later. Primary Ignition Scope Pattern Th e primary ignition scope pattern shows the low- voltage, or primary-voltage, changes in an ignition system. A primary ignition scope pattern is shown in Figure 34-32. Th e primary ignition pattern has three sections: fi ring, intermediate, and dwell. Note how the voltages change in each section of the pattern. The ignition secondary circuit cannot work properly unless the primary circuit is in good condition. A prob- lem in the primary circuit will usually affect the second- ary circuit. Secondary Ignition Scope Pattern Th e secondary ignition scope pattern shows the high voltages needed to fi re the spark plugs. Figure 34-33 illus- trates the secondary pattern for one cylinder. Firing Section The secondary pattern starts on the left with the firing section. The firing section will pinpoint problems with the spark plugs, ignition coil(s), PCM, the plug wires, and distributor. Th e fi ring line is the tall spike or line representing the amount of voltage needed to cause the electric arc to jump across the spark plug gap. It is normally the peak voltage in the ignition system. Figure 34-32. Typical primary waveform for an ignition system. Study the various sections of the trace. Firing Section Intermediate Section Dwell Section Primary Oscillations Secondary Oscillations Ignition Control Module Allows Primary Current Flow Ignition Control Module Stops Primary Current Flow Figure 34-33. A secondary waveform for one cylinder. The fir- ing line is voltage needed to fire the spark plug. The spark line is voltage needed to maintain the spark across the plug gap. Intermediate oscillations show the coil and condenser action. Dwell is the amount of time primary current flows through the ignition coil. Intermediate Section Dwell Section Intermediate Oscillations Ignition Control Module Stops Current Flow Ignition Control Module Allows Current Flow Firing Line Firing Section Spark Line