Chapter 34 Advanced Diagnostics 675 Copyright by Goodheart-Willcox Co., Inc. low resistance and would produce a lower-than-normal volt- age trace. Figure 34-31 gives several faulty ignition system scope patterns. An oscilloscope’s controls allow it to display either the primary (low-voltage) pattern or the secondary (high- voltage) pattern of the ignition system. Some scanner scopes will only show primary coil voltage and current. However, since the secondary high voltage follows the same pattern as the low voltage primary, you can analyze ignition coil output and spark plug operation studying the primary waveform. Th e scope patterns are similar, but important diff erences should be understood. Figure 34-31. Examples of bad scope patterns. Study the shape of the trace and problems for each. All firing lines fairly even but too high. Look for problems common to all cylinders such as: worn spark plug electrodes, excessive rotor gap, coil high-tension wire broken or not seated fully, late timing, excessively lean air-fuel mixture, or air leaks in intake manifold. Maximum available voltage during coil test should be within the manufacturer’s specifications. Disconnect plug wire to check maximum coil output. No spark line. Caused by complete open in cable or connector. Long spark line. Caused by a shorted spark plug or partially grounded plug wire. Sloped spark line, usually with hash. Caused by fouled spark plug. With plug wire removed for coil output test, a short intermittent, or missing lower spike indicates faulty insulation. This is usually caused by a defective spark plug wire, distributor cap, rotor, coil wire, or coil tower. Consistently low firing line in one or more cylinders. Caused by fouled plug, shorted wire, low compression (valve not closing), or rich mixture. Uneven firing lines. Can be caused by worn electrodes, a cocked or worn distributor cap, fuel mixture variations, vacuum leaks, or uneven compression. Consistently high firing line in one or more cylinders. Caused by a broken spark plug wire, a wide spark plug gap, or a vacuum leak.