Ignition Waveform Diagnostics
Ignition waveform diagnostics
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Scoping out secondary ignition to detect causes of engine misfires In the normal approach to a misfire code or misfire symptom, most techs initially suspect the ignition system is at fault primarily from a bad coil on a COP type ignition system. While this approach is OK, we must consider that when using a spark tester and removing the coil we have actually disturbed the circuit. In cases where the insulator boot is failing and allowing spark to arc to the plug well, we may never pinpoint that problem with the coil out of the plug well. On the subject of spark testers, keep in mind that they are not created equally. First and foremost the modern COP, HEI and DIS ignition systems have the ability to crank out 25 KV. Notice the three separate spark testers in Figure 1. The unit on the left is known as a ST125 which requires a true 25 KV. The unit on the right is known as a ST115
10
ASP
October 2018
By Bill Fulton
which requires 15 KV. The adjustable unit in the rear can be adjusted to create a larger or smaller gap to vary the KV demand. A 3/4-inch gap on this unit creates a KV demand very close to 25 KV. All COP type ignition systems have the ability to deliver 25 KV. The objective from this article is to explain the diagnostic value of a secondary ignition waveform or a primary ignition waveform to pinpoint lean density misfires, open circuits in secondary, internal coil carbon tracking problems, rich density misfires and cylinders with low compression problems. In an earlier article we explained that when a single misfire has occurred and set a DTC we should always investigate the freeze frame or fail record information from the scan tool. We went on to state that a single cylinder misfire caused by
