Auto Service Professional

OCT 2018

Magazine for the auto service professional

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O c t o b e r 2 018 A SP 11 a lack of spark from say a bad COP coil will not cause a significant shi in long-term and short- term fuel trim values. However, a misfire caused by a lean density condition will create double digit positive fuel trim additions. In contrast, a rich condition causing a rich density misfire will create negative double digit fuel trim corrections. You will only see this if the PCM is maintaining closed loop strategies. e modern day PCMs will force the engine back into open loop if the misfire is severe and consistent enough. So if your freeze frame indicated that the engine was forced into open loop, the fuel trim values are worthless. In the old days, Sun Electric and Allen Test Products made the large analyzers so that tech- nicians could easily pinpoint these problems by reading the waveform. As you have heard before, "a picture is worth a thousand words" and this will apply to the good, the bad and the ugly wave- forms we will cover in this article. First it is important to know the critical points of a secondary ignition waveform. A sec- ondary ignition waveform is indicated in Figure 2. Pont A is the point of primary turn-on and we are starting to saturate the coil. Point B is the point of primary turn-off. So effectively we have charged the coil. At Point B as we turn off pri- mary, the magnetic field collapses and gets mutu- ally inducted into the secondary windings of the coil and the voltage potential is multiplied by 100 to give us the high voltage needed to arc across the pressurized plug gap. Notice that the scope is set at 2 KV per division on the vertical scale. Pont C is known as the firing line and measures a good 12 KV demand. Point D is known as the Figure 1: Spark testers 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 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 ¾-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.

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