Understanding the scope wave forms

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  • David Green
    Senior Member
    • Oct 2009
    • 382

    Understanding the scope wave forms

    I recently had a Vauxhall Astra none runner, I don't profess to be an expert understanding the scope patterns. I carried out a starter cranking test after having completed a code read producing the codes P0351, P0352, P0353 and P0354, all relating to the ignition primary coil circuits.

    Battery voltage was available at the coils positive terminal and a good ground connection was also available. The voltage at the coils positive and negative connections showed about 10.8V during engine cranking.

    The primary ignition trace shown did not seem to be seen when scoping it using the Vantage Pro, later due to my lack of experience using the scope I learned that the scope did record the trace, which now appears to have been a glitch on screen and then gone, but after loading to shop stream and using the movie viewer I found the trace.

    If the ECM switching circuit was working correctly I would have thought that the ignition primary wave form should have been observed on the scope at all times the engine was cranking?

    The ignition primary trace shown has what I refer to as ash at the part of the trace, which if that was the secondary trace I would of been asking what is happening when the spark plug is arcing during the two mill seconds burn time?

    Is it the scope settings that cause this ash effect on the trace or is there another valid reason for it?

    Thanks

    David
    Last edited by David Green; 10-30-2016, 01:06 PM.
  • antthecat
    Junior Member
    • Apr 2014
    • 29

    #2
    David, I haven't looked at the waveform yet but (I'm guessing) your sweep was too short, try two channels, one on the crank sensor (have the screen sweep set so three wider gaps can be seen on the screen, then you know 720 degrees crank rotation has took place) and backprobe any of the four coil primary switched earths.

    If you are in any doubt may I suggest a bit of scannerdanner on YouTube, what he don't know about the snappy scope and scanner (and American cars) could be written on a postage stamp.

    I'll have a gander at your waveform shortly and see what I think but I'm not a fan of analysing secondary faults from a primary waveform as its a mirror image.

    Be a bit careful, some primary switched earths can have a back emf of up to 400v, a good scope killer.

    Hope this helps

    Anthony Pickering AMIMI

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