Secondary Ignition Trace: Measurement Artifact? Or Evidence?

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Secondary Ignition Trace: Measurement Artifact? Or Evidence?

Postby 928NE » Wed Jan 22, 2014 1:54 am

Hi Folks, first post. Looking for a little advice. Go easy on me please :)

Pertinent details: twin-distributor V8, old-school: two coils, one feeding each distributor. Chasing a misfire. The only evidence I've been able to find is displayed in the traces below. (These are all from one of the coil (King) wires.)

We consistently but randomly see this characteristic trace ...

2014-01-18-RS-Coil-trace.JPG


... with a far too-high firing line followed by this 'decaying dwell' voltage.

Here's Primary vs Secondary. You can see that the primary is fine, but the secondary is still 'weird.'

pvss2.JPG


Most of the time we see the pattern on both coil (King) wires every once in a while. Sometimes it happens repeatedly like in this one:

trace2.JPG


I've googled high and low and haven't seen this. Is this a measurement artifact? Or is this indicative of a real problem?

Thanks in Advance
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby Robski » Wed Jan 22, 2014 9:45 am

So why don't you sync a cylinder & home in, then use an adjustable spark tester to take you further ?

What condition were these taken ?
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby Autonerdz » Wed Jan 22, 2014 4:50 pm

That's an unfortunate effect of the secondary leads and will occur on a vehicle with no problems..
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby Robski » Wed Jan 22, 2014 5:17 pm

Autonerdz wrote:That's an unfortunate effect of the secondary leads and will occur on a vehicle with no problems..

hmmm, every days a school day Tom :|
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby 928NE » Thu Jan 23, 2014 2:24 am

Autonerdz wrote:That's an unfortunate effect of the secondary leads and will occur on a vehicle with no problems..


Ah. Thank you Sir. I'm not surprised. A bit disappointed. But, not surprised.
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby Robski » Thu Jan 23, 2014 8:36 am

928NE wrote:
Autonerdz wrote:That's an unfortunate effect of the secondary leads and will occur on a vehicle with no problems..


Ah. Thank you Sir. I'm not surprised. A bit disappointed. But, not surprised.

?

do you have a misfire or not ?
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby 928NE » Sat Jan 25, 2014 1:45 am

Sorry for being tardy replying. I didn't realize that e-mail notifications are off by default.
Robski wrote:do you have a misfire or not ?


Yes.

The pico seems to be telling me that the misfire is a 'system' issue and not isolated to a single cylinder. We see too-high firing lines randomly across all cylinders. The misfire is evident by ear and by eye. The evidence of misfire is not as evident during warm-up. Thus, a rich mixture somewhat abates the symptoms. But, even a super-rich mixture does not cure the primary issue.

Diagnosing a misfire is a second-order diagnostic. We are actually attempting to diagnose a harsh harmonic vibration that occurs at 1250 RPM increments with severity increasing with load. So, we are looking for mechanical issues as well.

I could write several pages on the steps that have already been taken with respect to both mechanical and engine-operation diagnosis. Acquisition and use of the Pico system is the first step in looking for the more-bizarre failure modes. More-correctly, the Pico system is an upgrade from an old, poorly-operating 'scope that wasn't up to the task.

The car in question is - as might be evident from my user name - a Porsche 928. This 928 - an '84S (ROW not US/NA-spec) - is old but has been extremely well-cared for. It has rarely seen rain and has only 43k-miles on the odometer. Virtually every component attached to - or part of - the engine that might be subject to degradation due to time or mileage is new or less than three years old.

We'll be attacking the problem again tomorrow looking for evidence that might point in the right direction.
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby 928NE » Sat Jan 25, 2014 1:57 am

Autonerdz wrote:That's an unfortunate effect of the secondary leads and will occur on a vehicle with no problems..


When I replied the other day I was too tired to do so cogently.

Is it purely an observation of a benign electrical effect? Or is it the result of the higher-than-normal firing lines - or in other words - would we expect this 'post-firing too-high voltage' to follow misfires of certain types?

Is it uncharacterized noise that requires no explanation? Or measurement of a phenomenon that might lead to diagnosis?

New plugs. New plug wires. New distributor caps. New rotors. New coil wires. Coils have been swapped with known-good coils. Amplifier stages also. Grounds are clean. (The new components were installed a year before the vibration symptom began. Not in an attempt to 'throw parts at the problem.' Component swaps were part of diagnostics.)
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Re: Secondary Ignition Trace: Measurement Artifact? Or Evide

Postby Autonerdz » Sun Jan 26, 2014 4:39 pm

928NE,

I am not sure exactly what causes the waveform jump up, as you illustrated in your first image, but it is not something you would want to consider as part of a diagnostic process because it is related to the equipment and not the vehicle. This issue with the secondary leads has been around for over a decade. It's an effect we have observed in other capacitive secondary test leads as well. It is aggravated by the 10,000:1 attenuation and is less evident in other leads with a 1000:1 attenuation but still present.

My advice would be to ignore the effect and move on.
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