Ignition scope & Cranking KV?

I'm t-shooting a single inductive coil, Hall-Effect electronic ignition Saab 900 4 cyl

During hot idle, I'm seeing what seems to be normal 6KV - 9KV firing line peaks across all cylinders.

During a Cranking KV test, however - w/fully charged batt, fuel pump disabled and WOT - I'm seeing 10KV firing line peaks across all cylinders (the eveness corresponding well to a 'regular' compression test shows all 4 cylinders between 192 PSI - 205 PSI)...

But shouldn't that figure be substantially higher, like +15KV to +20KV or so?

Thank you, Lance

Reply to
Lance Morgan
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10 KV seems OK to me why do you think it should be double that amount?
Reply to
saeengineer

Just thought that w/no fuel, and spark gap under significant pressure, would drive KV up more vs having fuel to ignite...?

I guess, on avg, I'm seeing a ~2 KV bump

Reply to
Lance Morgan

The slower the RPM the longer coil charge time. The ignition module`s maximum charge time is usually 14ms at the lowest rpm. At idle the charge could be around 13ms. A scope could tell you this. Koji

Reply to
Koji San

And this addresses his question how?

Reply to
Neil Nelson

Thanks for the reminder and the coil charge time reference, which I've not specifically measured. The 'dwell' was changing appropriately. My scope is currently in the shop getting calibrated.

This is what prompted my question - read several weeks ago - and refound

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My stock plug gap setting is only 0.028, and I've got a different ignition system than the above, but it would appear 10 KV is too low.

Another consistent pattern that I see, that I think (but am not sure) is abnormal, is, after the spark line/burn duration, the peak of the first coil oscillation amplitude markedly varies/is erratic, on both secondary and primary displays (the first subsequent coil oscillation 'valley' is stable). This, of course, pulls the spark line slope up into a positive angle, but only during the latest stage of the burn. Trying to discern if this is really a form of lean misfire.

Reply to
LanceM

Good! You could do a lot worse than Bill Fulton. Back when I worked for Snap-On, I arranged for a Councellor2 DSO for Bill to tote around the country putting on seminars. I got to sit in on a lot of seminars and give him feedback

-and- the freebie scope helped sell more scopes. 8^)

Forget the hardware differences, the dynamics are the same.

10KV could very well be too low, or it could be okay. This is where you have to get inventive, systematically working your way downstream in the system to see if in fact the spark energy is finding an alternate (easier) path to ground. IIRC, you do have an adjustable spark-gap tester which can be set to a value higher than 10KV (12-14), you can then systematically position the spark tester along the secondary in place and see if the spark makes it that far, or if it is shunting to ground. Start at the coil, then at the distributor end of the coil wire, then at the end of the sparkplug wire. If you can fire 12-14 KV at these points, you now know that the 10KV you saw during your cranking test wasn't because the spark was finding an alternate path.

Whenever you suspect a lean condition based upon what you see on the spark line, supplimenting in some fuel can confirm or deny the condition. Disconnecting vacuum from the fuel pressure regulator is one method (plug the vacuum leak), a modified propane bottle/propane torch head with a length of vacuum hose is another. If there is a MAF or VAF sensor in the way of the throttle body, connect in with a "Tee" to a convienient vacuum source and add in extra fuel while you watch the spark line. If the hook settles down, suspect a lean condition. Short burn time is usually associated with a lean mixture, so you'll want to watch the burn time also.

In almost all cases, there is a rise in the spark line voltage at the end of the burn, this is because eventually the cylinder -does- run out of fuel and the absence of HC molecules will cause an increase in voltage as the spark is sustained up to the point where the spark energy is depleated at which point, the coil oscillations begin.

Reply to
Neil Nelson

Thanks for the, once again, precise help. I also reread some of your earlier suggestions - studied them along w/some other ref material - and spent some time on the scope, using two vehicles

I did indeed get the adjustable spark tester direct from

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just yesterday (some other cool stuff there, which you've probably already seen), and now have a much better/more effective idea of how to use it. I've used propane and induced vacuum leaks to test O2 sensor rise time, test fuel injector PW response, etc: it should have occurred to me to apply a similar technique while evaluating the spark line slope (will do so when I get the scope back)

I have IDed some component failures (and other waveform abnormalities) with the scope: some components have been re-replaced and tested (particular emphasis on night misting components & ground probe). The root driveability problem still remains, the bucking, and new inner distrib cap contacts are still noticeably burning with less than an hour of run time.

I'm not able to perform a reasonable snap-throttle, as the engine bogs down

W/o the extensive list of other (normal) system/component tests, I'm beginning to think the root cause of the problem may be a leaking head gasket, more specifically possibly head or block erosion/pitting (which my model/engine Saab has somewhat of a rep for).

Altho compression is even across all cylinders, both on the scope and by gauge; and a chemical test, for exhaust gas, at the coolant expansion tank was negative, I've just very recently noticed a slight coolant level drop (which I've been closely monitoring for a couple of months now), and have caught the faintest whiff of coolant, both in the cockpit and exhaust (nothing so dramatic as white smoke, oil/coolant cross-contamination, extra clean plug or two, etc). I'll also further pursue cross-testing of the HG, head & block, as think trace amts of coolant, being sucked in at moderate-to-high vacuum, may be causing the combustion chamber misfire

Reply to
LanceM

p.s. I forgot to mention that I also have steady timing retardation, on all 4 cylinders, at low-to-moderate RPM, advancing properly at higher RPM. So maybe multiple faults at play here

Reply to
LanceM

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