[hot_sd]

I've never looked at spark plug signals this way. Have you tried a spark plug tester. I suppose that would be hard to use on the plug that is under the coil.

You cannot get meaningful results on signals that are not dc or a single sinewave (ac). Things like pulses can only be correctly interpreted on an oscilloscope. If you want to look at this kind of signal buy yourself an automotive scope. The meter will give you some form of averaged reading - either the mean or root mean square reading.