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International Automotive Technicians Network
Posted to Technical Tips Forum on 10/7/2009 13 Replies

When DIS ignition was first introduced, the hangover from distributor ignition still existed with the believe that: "you-got-a-have" a paraded pattern and scope analysis is useless without it. At great expense, equipment designers scrambled to come up with a variety of adapters to satisfy the cry of the scope users. It takes a lot of ingenuity and talented engineering to unscramble: positive & negative -- Waste & Compression -- different time bases -- inverting signals & high voltage radiation in close proximity with each other, all in one box. Some adapters were fairly successful in doing so, others missed the mark completely. Even those somewhat decent outputs worked great on a good running vehicle, but where all over the map on the problem car. So the question is: "Why do we need parade?"

The Ironic part is that the need for KV readings is obsolete, and DIS ignition offers a very unique alternative. 1. If you are looking for a bad plug-wire, that same plug-wire. serves the waste as well as the compression stroke and you can see a High KV demand a lot easier and sooner on the waste stroke when it is not under compression.

What about wide plug gap? Worn plugs are detrimental to any further combustion analysis and may lead to inaccurate diagnosis. So it is important to know if it is excessive. The effect of a worn plug under compression may be 10X times more than waste which is the same plug not under compression.(File 45237) What is acceptable? Compression KV should not be more than 5X waste, or waste should be about 20% to 25% of compression KV at idle. For diagnostic purposes it is not even required to have a KV scale. Every Lab-Scope has a Voltage scale. That solves also another problem. Calibration of the so called Secondary clip is not critical. If the secondary clip is normally 10 Pico Farad, this may rise on a humid day to 15 pF. However, the ratio still remains the same. If you don't belief this, spray some water on the plug wire and convince yourself.

Once we know that the ionization KV is within reason and not effecting firing time, we can qualify the coil output as OK as long as the spark duration is over 1 Ms. Now we can concentrate on evaluating the combustion behavior inside the combustion chamber at idle -- 2000 RPM -- and Load test. To qualify each cylinder good or fail under all driving conditions should take less than 3 minutes. To interpret a failure may take another minute or another step.


Mac from Michigan

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