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  #1   IP: 137.103.82.194
Old 08-05-2017, 02:34 PM
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A4 ignition trace

This is what my A4 points ignition looks like on my scope.
The second photo with the sweep turned to a real slow setting is showing the "second pulse" after the main event. I think this is the points closing again,but I am not sure.
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  #2   IP: 174.192.3.94
Old 08-05-2017, 03:08 PM
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What is the time scale for each division of the top trace?
Same, but for the bottom trace?
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  #3   IP: 137.103.82.194
Old 08-06-2017, 07:21 AM
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I can't recall the settings on those photos, but here is one at 2ms/division.
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  #4   IP: 174.192.3.94
Old 08-06-2017, 08:09 AM
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Thanks. A few more questions:
1. What was the scope's vertical scale (volts/div)?
2. What scope probe did you use (1:1, 10:1, 100:1, etc.)? Or if you used a voltage divider, what ratio?
3. Where did you measure? Across the points? One side of points to ground?
4. What was the engine RPM?
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Old 08-06-2017, 08:30 AM
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I don't recall the RPM or voltage setting. I used a 10:1 probe and there was no physical connection. I just used a test lead with one end clipped to the scope probe and the other end wrapped and clipped to the outside of plug wire #1. This is showing what you get with inductive/RF coupling. I also did the coil wire and saw - obviously - 4 times as many sparks and the traces looked about the same.
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Old 08-06-2017, 08:39 AM
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  #6   IP: 174.192.3.94
Old 08-06-2017, 02:26 PM
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Joe:
1. Thank you for your initiative and the photos. Using a scope lead attached to a spark plug wire, or the coil high tension lead, is an neat old trick. Because the plug wire and the test lead/scope probe are so short, and because self-inductance of straight, small gauge wire is so small, the coupling from plug wire to test lead is likely to be capacitive. Both the test lead and the plug wire are conductors, and they are separated by dielectric (their insulation), making a simple capacitor. See,

https://en.m.wikipedia.org/wiki/Capacitive_coupling

2. I think your photo 3 was done using the coil HT lead, not #1.

3. If this trace were of #1 plug wire, then:

Using the 2 msec time divisions in photo 3, the time between firings is about 13 msec (.013 sec).

At .013 sec between firings, there would be 1 / .013 = 77 firings/sec.
77 firings/sec = 77 x 60 = 4615 firings/min.

Since the distributor turns at half the crank speed, this would mean the engine would be running at 4615 x 2 = 9230 RPM.

4. If photo 3 was instead taken on the coil lead, then it would be showing two of the four cylinders firing, and the engine RPM would then be 9230 / 4 = 2307 RPM.

5. The waveform for each firing in photo 3 lasts about 2 msec (or so). In photo 1 the waveform lasts about 4 divisions. This would mean that the time scale in photo 1 is about 4 div for 2 msec, or 500 μsec/div (.5 msec/div). Then the period of the oscillation, from zero crossing to zero crossing, is about 1 division, or .5 msec. That means the frequency of oscillation is 1 / .5 msec = 2kHz.
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  #7   IP: 24.152.132.65
Old 08-06-2017, 06:26 PM
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Interesting pictures and chatter about O-scopes and RPM calculations but I'm hoping this leads to practical information about our A-4 ignition systems. Out of blind curiosity it would also be interesting to compare an EI trace to that of points ignition, perhaps performance of different coils too.
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  #8   IP: 137.200.32.6
Old 08-07-2017, 08:49 AM
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This started out as just for fun, the scope was down there to check the inverter output. I'll see what else I can come up with. I suspect a bad condenser would change the appearance of the trace a lot. Note the spark is not a DC event, as tac mentioned a post or two up, it appears to be a pulse of about 2 KHz high voltage RF. The first highest pulse is likely what ionized the path from electrode to ground and reduces the resistance for the next few pulses.

Found this: https://www.princeton.edu/ssp/tiger_..._waveforms.pdf
This is really cool and I was right - the pulse after the main spark is the coil closing. I may have to try the setup they show.

Last edited by joe_db; 08-07-2017 at 08:58 AM.
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Old 08-08-2017, 01:52 PM
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If you do any more scope work, could I suggest:
1. Keep a record of the X (time) sweep setting, engine RPM, Y volts/div and trigger mode and level.
2. At the same RPM, do the voltage across the points. Check to see what the maximum voltage input the scope is rated for. Depending on how good the ignition condenser/capacitor is, you might see spikes as high as a couple hundred volts.
3. If you have a dual trace scope, do the points on one channel and either a plug lead, or the coil/tower lead, on the other. Trigger on the rising edge of the points voltage.
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