Auto Service Professional

OCT 2018

Magazine for the auto service professional

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10 A S P O c t o b e r 2 018 I In the normal approach to a misfire code or misfire symptom, most techs initially sus- pect the ignition system is at fault primarily from a bad coil on a COP type ignition sys- tem. 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 fore- most the modern COP, HEI and DIS ignition sys- tems have the ability to crank out 25 KV. Notice the three separate spark testers in Figure 1. e unit on the le is known as a ST125 which re- quires a true 25 KV. e unit on the right is known as a ST115 which requires 15 KV. e 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 abil- ity to deliver 25 KV. e objective from this article is to explain the diagnostic value of a secondary ignition wave- form or a primary ignition waveform to pinpoint lean density misfires, open circuits in secondary, internal coil carbon tracking problems, rich den- sity 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 Ignition waveform diagnostics Ignition waveform diagnostics By Bill Fulton I g n i t i o n W a v e f o r m D i a g n o s t i c s Scoping out secondary ignition to detect causes of engine misfires

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