Doug,
There are three main differences between a normal electronics
oscilloscope and an engine scope.
The oscilloscope has a fixed time base (horizontal scan) while the
engine scope scan is based on the speed of the engine. When you speed up
an engine you see the pulses get closer together, then the scan widens
out to where the pulses were before.
The vertical scan of an oscilloscope is linear, while the engine scope
is something like a log scale so you can see the small voltage changes
near zero and the top of the peaks at several KV at the same time.
The oscilloscope normally has a fixed brightness to the trace (some
intensify the faster beam movement) while the engine scope will always
intensify the higher speed trace movement so yo can see everything clearly.
Having said that, I have used an oscilloscope for ignition work for
years. You just can't do everything as easily as you can with a
dedicated engine scope.
The ringing you see starts right after the points open and the plug
fires. This ringing is caused by the collapse of the magnetic field in
the coil, coupled with the condenser across the points, which are now
open. This is normal and will be on all ignition systems with points,
and probably early (non-CDI) ignition systems. The current flow through
the spark will also affect the waveform. Try pulling a plug wire and
both ground it and leave it open and see the difference in waveform.
Experimentation is the best method of learning! You should be able to
wrap a wire around the plug wire you are messing with and connect it to
the sync input of the scope. Go to external sync and then the trace will
start with that particular plug so you can see the results easier.
I'm not sure what you mean by the ringing switching between two levels,
but suggest that you verify that your points are in good shape, gapped
correctly and are properly grounded. Most distributors have a ground
wire from the contact plate to the distributor housing that can break
from the continual rotating of the contact plate due to vacuum advance.
Likewise there is a wire from the points to the coil connection on the
outside of the distributor that can also break. Finally, make sure the
condenser on the distributor is grounded solidly and connected to the
points wire.
If you have other questions, please feel free to contact me.
Peace,
Pat
Thusly spake Doug Braun
> Hello,
>
> I had borrowed an oscilloscope from a friend, so it occurred to me to hook
> it up to the primary ignition circuit of my '31 Ford. I never realized that
> so much information could be gotten from the waveform!
>
> Here is a video of what I saw: http://youtu.be/Tj4ixoUG87Y
>
> Has anybody else ever scoped an antique car ignition? The waveform doesn't
> look like the sample waveforms I have found on the web. The main difference
> is the fast ringing during the burn time. The intensity of the ringing
> randomly switches between two levels, as the engine misfires. Has anybody
> ever seen that?
>
> I thought that maybe that ringing comes from the car's non-resistor plugs.
> But after this I hooked it up to my '72 Spitfire, which of course has
> resistor plugs and wires.
> That high-frequency ringing during the burn time was present on that car,
> too. Any idea where it comes from?
>
> Thanks,
> Doug
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--
Pat Horne, Owner, Horne Systems
(512) 797-7501 Voice & Text 5026 FM 2001
Pat@HorneSystemsTx.com Lockhart, TX 78644-4443
www.hornesystemstx.com
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