Model A Ignition Waveform

[Richard Lorenz]

Would the digital scope set-up display the ignition conditions while the engine was being cranked (before actual engine running)? What I have in mind is looking for what causes a weak spark that makes starting difficult. The particular problem that I have in mind is misaligned points that might roll or drag one on the other. This type of behavior would show up mostly when there is a slow separation of the points. A fast, clean break is needed in order to get the full coil performance, I believe. The above is just my opinion with what might happen.

Bad points show up while testing the spark with the distributor open and the coil wire moved close to ground. Screwdriver make-and-break gives a good spark, but point make-and-break doesn't, especially with a slow break. It would be nice if the digital scope set-up would reveal the cause of the weak spark without opening the distributor.

 

[Doug in NJ]

I'd really appreciate that! One problem with modern information is that it covers mostly modern electronic ignitions, which behave quite differently than the 'A' system.

Doug

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.

 

[Doug in NJ]

A modern digital oscilloscope or a PC-based setup ought to be good for this, since it could capture several seconds of waveform as you are cranking, and then you browse each individual firing for problems.

Doug

Would the digital scope set-up display the ignition conditions while the engine was being cranked (before actual engine running)? What I have in mind is looking for what causes a weak spark that makes starting difficult. The particular problem that I have in mind is misaligned points that might roll or drag one on the other. This type of behavior would show up mostly when there is a slow separation of the points. A fast, clean break is needed in order to get the full coil performance, I believe. The above is just my opinion with what might happen.

 

[Jim Mason]

I'd appreciate a copy. I'd post it on my site..

 

[mach0415]

A modern digital oscilloscope or a PC-based setup ought to be good for this, since it could capture several seconds of waveform as you are cranking, and then you browse each individual firing for problems.

Doug

Yes it would. Many come with a "flight recorder" option that makes a movie of your data to be played back. You would have to experiment with time and threshhold minimums and maximums to get the full picture. Here is a great example:
http://www.picoauto.com/automotivetopics/primary.html

Inductance leads to each spark plug "wire" would provide a signal on the secondary side to identify the suspect cylinder. Modern engines use a crankshaft position sensor (CKP) , which is nothing more than a hall effect sensor or "AC signal generator" for cylinder identification. Camshaft position sensors (CMP) work the same way. When a modern engine management system monitors for misfire, it looks at CKP, CMP, and RPM logic. A misfire causes an anomoly in the speed the the crankshaft within the 2 revolutions it makes in the 4 stroke process, and the processor can identify which cylinder is misfiring, based upon when the speed of the crankshaft slows in the cycle.

How would this be applied to an A? If it had electronic ignition, it would be cake. You will need to go with the amp clamp-style inductance pickup lead, I believe.

Before you go at it, it might be good to check for distributor shaft play and points gap for each lobe of the distributor cam. Coil saturation time can vary if there is an issue with these.

One other piece of food for thought... disconnect the copper leads from the spark plug, but leave a gap so the plug will still fire. Perhaps your cylinder can be identified by one not always seeing the spark in relation to the misfire...

 

[QGolden]

K Folks, seems you would like the Waveform data. I got more than a few PM`s too on it. Glad to share. Give me a couple of days to get the scannin` done. I have a pretty full schedule tomorrow. I don't know how big the file will be, so I will put it on one of my websites, and post a link. You can view it or download it or whatever.

 

[Richard Lorenz]

I looked up the PicoScope automotive digital oscilloscope (USB connection for your laptop) on the internet. It looks like the cost would be between two and three hundred dollars. I sent them a query about which model was needed for the Model A, but have not received an answer.

 

[van Dyck]

Richard Lorenz (#21) is correct about a crisp and clean point opening. On occasion (over the past 50 years having to deal with breaker point ignitions) I have encountered sloppy point arm pivots or loose pivot posts. These cause the contacts to slide sideways relative to each other and prevent a crisp, clean separation. The primary waveform would show this abnormality as "hash" marks at the "point open" portion on the right hand side of the oscilloscope screen (depending on the make and features of your scope). With a good contact point setup, correctly installed and gapped, the primary wave form will show a clean and crisp right-angle upward trace. This allows the ignition coil to fully collapse the magnetic field built up during the dwell period (points closed).