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Re: [Shop-talk] Oscilloscope on my car's ignition

Subject: Re: [Shop-talk] Oscilloscope on my car's ignition
From: Pat Horne <>
Date: Thu, 20 Dec 2012 15:43:42 -0600
References: <>
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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.


  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:
> 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  Lockhart, TX 78644-4443
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