Posted to Technical Tips Forum on 10/7/2009
MISCONCEPTIONS of SCOPING IGNITION
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The Ironic part is that the need for KV readings is
obsolete, and DIS ignition offers a very unique alternative.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.
[Comparing WASTE KV Vs. COMPRESSION STROKE]
Mac from Michigan
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