Model A Ignition Waveform

Doug in NJ

Doug in NJ

Well-known member
Hello,

I had borrowed an oscilloscope from a friend, so it occurred to me to hook it up to my car's ignition. 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 'A' ignition? The waveform doesn't look like the sample waveforms I have found. 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?

Thanks,
Doug
 
Just curious... Where did you take the signal from? Your notes say the vertical axis is 100 volts per division, and I see about -2 divisions and +3 divisions of signal. That's -200v to +300v range, right?

I would have thought you would see 6v on the primary and 20,000v on the secondary.
 
I have an ignition scope that was used in my families garage in the 60's into the 70's. I have used it to analyze many a problem in an engine. The scope comes with Books to analyze the wave forms. One can detect everything from burnt valves to bad coils, dwell time issues, burnt, sticking points etc. Knowing where you got the signal from might help to understand the wave that you are seeing, although jumping at the head end of the wave is usually an issue with the coil. Although I am working from memory which ain't so good. I should go into the shop and crack a book first. Of course my Ignition Scope is set to do engines only and not really the same as an electronic oscilloscope.
 
Doug's Scope: Horizontal: 0.5 mS/div. Vertical: 100 Volts/div
DougsAIgnScope_zps24e1475a.jpg
 
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The signal is from the primary (i.e. the red wire to the coil). If the engine RPM were higher, you could see the transition from 6 to 0 volts as the points closed. Can you see that the right side of the trace is a tiny bit above the line on the display? That line is exactly zero volts. At the moment the points open, there is a big spike of a couple of hundred volts, and then that decaying ringing. The burn period ends after about 2 divisions, or about 1.1 milliseconds. That is apparently a typical burn time value. During the burn, the average coil voltage is about 50 volts. Once the burning is done, there is nothing left to conduct the spark, and the remaining electrical energy oscillates between the coil and the condenser, causing the second, slower ringing. That part of the waveform is textbook, indicating that the coil and condenser are a harmonious couple.

As you can see in those stills, about every fourth ignition there is a misfire, the ringing is greatly reduced, and the burn time voltage is reduced a bit.

BTW, I tried the same setup on my '72 Spitfire, with resistor plugs, and it had the same sort of ringing during the burn time. Its burn time was longer, about 2 mS.

This test told me what I already knew: the car misfires at idle. I have no idea why, except that the scope seems to say that the electrical system is OK, and the problem is not confined to one cylinder. I have checked all the usual culprits, such as air leaks. A swapping carbs did not help. Off idle, the car runs well, and accelerates and goes up hills just fine.

Doug
 
Dunno, could be just one cyl....could be distrib cap shorting to strap to plug #3....I had to lengthen that strap to stop the short no matter what cap I used...NOS, repop, whatever
Look at it in the dark.....
 
Doug,
How can you tell that the misfire occurs at very 4th ignition? I ask because I do not know how that scope is set up. My Ignition scope screen would split that wavecom into 4 displays, one for each cylinder. Without that benefit I am not sure I could interpret your results. Wish we were closer together I would gladly give you the lend of my scope and time.
 
I have installed electronic ingition on my a, I have a scope that came out of a 70's garage put was told it would not work with electronic ingition, the manual I have with the scope does not address electronic ingition, question is can it be used and will it harm the scope if hooked up to it?
 
QGolden said:
Doug,
How can you tell that the misfire occurs at very 4th ignition? I ask because I do not know how that scope is set up. My Ignition scope screen would split that wavecom into 4 displays, one for each cylinder. Without that benefit I am not sure I could interpret your results. Wish we were closer together I would gladly give you the lend of my scope and time.
Click to expand...

It's not exactly every 4th- its random-looking. If you freeze the video, about one time in four you see the reduced ringing. If one cylinder were consistently misfiring, the pattern would look more rhythmic.

The scope is a generic laboratory model, not a specialized automotive model, and it is simpy viewing the primary wave form, so there is nothing to tell me which cylinder is which. If I had a secondary pickup, I could trigger the scope from the cylinder 1 spark and get a more standard-looking parade display. But the at idle speed, it would be sort of hard to view, since the scope's tube does not have much persistence.

I am thinking of buying myself a good modern digital lab scope. These can store and replay megabytes of data, so I could record maybe a minute of trace data and browse through it.

Doug

Since
 
Doug, consider one of these. A digital scope that plugs into the USB of your laptop. They range from $70 and up if you want quad inputs and a lot of peripheral attenuated and inductive pickups. Your computer can store and replay plenty. I have a cheap one and my two HP scopes now gather dust.
Look at this LINK from just one of several manufacturers.

72-10175.jpg
 
Bigguy,
I will check my scope manual today, it mentions Electronic Ignitions. I do not recall any warning. I have seen such a waveform. I will get back to you.
 
MikeK said:
Doug, consider one of these. A digital scope that plugs into the USB
72-10175.jpg
Click to expand...


Wow, Gotta' love Technology! I remember my Dad "arguing" with my Grandfather back in the 60's because he wanted to spend "a few hundred" on that Ignition Scope. In those days they were getting about 7 dollars an hour for labor and that money came dear. But they made a lot of money from its use being the only Garage in a small town with one.
 
Does the digital scope attachment for a laptop (MikeK's picture above) actually show the high voltage ignition wave? Is there something similar that does show the high voltage wave? Thanks.
 
Yes, you would hook it up to a computer, install the software, and it would display the waveforms on the screen. There are small ones made for Auto use. If you watch eBay you often see older scopes like mine for less than 100, including freight. Anyone in my area who would like to make use of a scope, is welcome to come and use mine.
 
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Is there a vendor in this country for the digital scope that MikeK showed above (Hantek 1008B, made in China, I believe)? Also, is there a source of information about what one can learn about the Model A ignition system from using an oscilloscope. If it could be useful, I would like to get a digital setup put together to use at our Club's Spring Tune-up.
 
Do you know anybody in the automotive repair business?

At the high school,where I teach automotive students, I have a Snap-On Modis. The Modis is one of many digital scan tools that also has an oscilloscope. This particular one is a 4 channel, but it also has an inductance probe set just for ignition systems. Perhaps if you have access to somebody, they would be able to help.

FWIW computer-based oscilloscopes are coming back as a means to solve simple and complex drivability issues. It used to be considered antiquated and archaic. We teach it to higher level automotive electrical class students, because the industry demands this knowledge again.
 
Iff'n anyone has any interest, I could, if given a little time, scan and create a a PDF of the instructions, wave forms, and diagnostics book that came with mine. It is probably 45~50 years old so I don't think I would be infringing on anyone's copyright. It's not very big, 15 pages or so.

The book correlates the various waveforms with situations in the ignition system. Vacuum, Valve performance, mechanical issues in the distributor,regulator voltage too high. Swapped power leads to coil, burnt or burning points etc.

Mark can probably elaborate more.
 
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