Auto Service Professional

OCT 2018

Magazine for the auto service professional

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Page 23 of 69

22 A S P O c t o b e r 2 018 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 affect on the KV demand. On COP-type ignition sys- tems, remember we have only one authorized air gap being the plug gap. In addition, the coils are charged with nearly double the amperage values compared to conventional systems. A good secondary ignition waveform from a GM coil near plug system can be seen in Figure 17. In another example is Figure 18, where we are looking at KV de- mand during a WOT (wide- open-throttle) cranking clear flood mode on a distributor- equipped engine, using the bar graph function. Notice the 24 to 28 KV demand. is is another way of doing a rela- tive compression test. Another example is from a 3.8L Mustang with an en- gine misfire from #5 cylin- der. Look closely at the sec- ondary ignition waveform from #5 cylinder in Figure 19. Do you see the split fir- ing line and the extremely shor t spa rk duration pe- riod? There a re on ly t wo possible causes, one being an open plug wire or, two, an open spark plug resister. e cause in this case was an open plug wire. Notice the good secondary waveform aer the plug wire was re- placed in Figure 20. e low inductive cur- rent probes used with a lab scope is a powerful diag- nostic weapon. e amper- age waveform will help us determine if the coil was properly saturated. We sim- ply clamp the current probe around the B+ feed wire to the coils. e most popular attenuation factor is for ev- Figure 13: OEM engine decal An underhood label may indicate that dual ground electrode spark plugs are to be used. Figure 12: 3.4L Toyota COP DIS ignition system (misses under load — see schematic) Some V6 Toyota engines feature a combination COP/DIS system. The system uses an external igniter to control the coils. A coil is mounted directly on top of a spark plug, with a secondary lead going to the companion cylinder. This engine had a miss under road load conditions. This is a good example of the diagnostic power of accessing the primary side of the coil.

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