Electronic Ignition Waveform -- abnormal?

[FastWoman]

Hi all,

My second project this weekend was that I FINALLY got around to scoping my ignition. My engine still has a slightly rough idle (although at times it will smooth out beautifully), the exhaust puffs randomly (random misfires), I sometimes get a light backfire, and my tach needle gets these spasms -- twitching to higher values. At idle, the twitches are about 100 RPM in magnitude (with no change in engine speed), and on acceleration, the needle sometimes bounces wildly over a 500 to 1000 RPM range. Interestingly, the needle doesn't necessarily bounce wildly at steadily higher engine speeds. It's mostly when engine speeds are rapidly changing.

So here's a trace of the (-) terminal of the ignition coil with respect to ground -- very greatly attenuated:

I sort of expected to find random extra pulses from false triggers, not from slop in my distributor (which is new/rebuilt), but from a possibly faulty electronic ignition module. I didn't find that. The pulses ticked along perfectly with the engine. However, I did see some lower voltage hash between the pulses -- PERHAPS of a high enough voltage to false trigger my tach and PERHAPS jump a low voltage spark to cause pre-ignition.

I also note, more significantly, that the waveform doesn't resemble the normal waveform drawn in the FSM, where there is a brief voltage plateau for each pulse, preceeded and followed by ringing. Neither does it resemble the waveform they show of a dying ignition module.

I'm left wondering whether the abnormal waveform is because of my ignition wires (which are conventional carbon core) or because of the ignition module. The false triggers, if that's what they are, would seem to indicate a bad ignition module. Anyway, I would greatly appreciate input from anyone who knows what real-world ignition traces look like.

Thanks bunches!

Peace,

Sarah

Edited October 24, 2009 by FastWoman

[geezer]

One thing I notice is the alternator pattern in the dwell section of the secondary waveform, indicative of loose or highly corroded alternator connections, or one or more defective diodes.

Here's a few pages from the Engine Analysis using the Automotive Oscilloscope manual to explain that. Pretty big manual but if there is anything specific you would like me to look up, I'd be happy to do so.

[FastWoman]

Hmmmmm.... That MIGHT just be noise in the data stream of the digital oscilloscope, but then again, it MIGHT be the problem you describe. I'll need to scope the +12VDC for a better look.

Hrumph! I just replaced the alternator last week. The previous one was a lifetime warranted unit that lasted a month before it quit. It's definitely connected VERY well from battery to block. Quality parts, eh?!

Any thoughts about the ignition, though?

[doradox]

Your trace is abnormal. It should look a lot like the ones in the FSM.

These links may be helpful.

http://www.picoauto.com/automotivetopics/primary.html

http://www.motor.com/magazine/pdfs/052005_04.pdf

It's been many years since I was scoping engines regularly so it's a little fuzzy but I'd try a coil first.

Steve

[FastWoman]

Thanks, Steve! The links were extremely useful. The article is a bit dense, but a very informative read. Now I think I have a better clue what I'm looking at.

Missing in my screen shot is the time scale, which was 0.4 ms/div. The "false triggers" I thought I was seeing were simply the ringing at the end of the burn time. I hadn't included the time scale, but it'w 4ms/div. My burn time seems to be about 1.5ms, give or take, measuring by eyeball. The engine is spinning at about 2kRPM. The glitch maybe 3.5ms prior to the spark seems to be the beginning of the dwell period.

I'm still not completely sure what I'm seeing; however, the burn time (with normally gapped plugs) is undeniably about 1.5 ms, which says something good about the total energy the coil is delivering through the plug. Perhaps the peculiarities with the waveform have more to do with my testing apparatus than the ignition circuit. I cobbled together a high voltage probe, with overvoltage protection, that would allow me to hook up my cheap USB digital oscilloscope to a high voltage source without blowing it up (along with the computer). I might start with a repeat of my diagnostics, after cleaning up terminals, establishing better probe connections, examining different voltage scales, and lastly, figuring out how to digitize/record maybe 10 or 15 revolutions in enough detail that I can see what's going on. That will let me look at what's going on in the individual cylinders. (Very cool.)

Steve, if I do need a new coil, are the flamethrower coils compatible with OEM ignition? If so, it might be good to have the flamethrower coil, in case I needed to retrofit with a Pertronix Ignitor module. How are the Ignitor systems, BTW? It seems like a lot of high power circuitry in a little module, stuffed into a tiny enclosure (the distributor). It seems like it would be prone to overheating. Do these modules have a good service life?

I really appreciate your help!

Thanks!

Sarah

[FastWoman]

Thinking about the article a bit more, some of what I've been experiencing makes more sense. My engine still misses quite a lot. The idle is somewhat unsteady, as though it had a very mild cam. Acceleration is somewhat weak, and it feels as though the engine is missing. Not having a very loud exhaust, it's hard for me to hear what the engine is really doing.

Anyway, when a cylinder misses, the "burn time" of the spark is shortened, due to lack of combustion products. That results in an exaggerated ringing at the end of the burn time period, thereby double-triggering my tach. I get somewhat less than 1kRPM swings of the tach needle at maybe 2500 RPM on moderate acceleration, so that would mean maybe a third of the cycles fail to detonate -- essentially like running on 4 cyl, instead of 6. It does feel a bit like that.

Question: When the ignition is weak, do the misses tend to occur more frequently under load? That's when my tach needle jumps around the most. At idle, it twitches occasionally. At higher RPM when not under load, the needle is pretty steady.

[doradox]

High load would be more apt to cause a miss all other things being equal. Are you able to get a trace of the secondary circuit? Also what is the voltage scale? The induced voltage part of the trace is odd in that it goes so negative before settling into the burn time.

Steve

[FastWoman]

Hi Steve,

Oddly, I can't give an easy answer ot the voltage of the primary trace, but it's quite high -- I think I would say "hundreds of volts." Unfortunately the attenuator I used was something I cobbled together long ago, and I've forgotten its design parameters, other than the fact it has an overvoltage clamp to protect the oscilloscope. Also unfortunately, I have no way to test the secondary circuit. The voltages are WAY too high. I think I have some glass-encased resistors I might be able to use for that. Dunno.

Anyway, I seem to have at least four things loosely corroborating faulty ignition:

(1) an abnormal waveform

(2) engine missing, especially under load

(3) massive black sludge accumulation (previous post)

(4) double-triggering of the tach during misfire. (Note: This problem occurred both with the OEM tach and with a modern retrofit tach board.)

You suggest starting with the ignition coil, probably because it's the cheapest suspect. Plug wires would also be likely suspects, except that I've tested them, and they're within normal limits. My car did come with the advance mechanisms of the distributor seized up, so I have a new/reman distributor now. Rather than trying to track down THE compenent that's giving me trouble, would this be a good time to upgrade to a more modern ignition system? What are my best options for a '78 Z?

Thanks so much!

Peace,

Sarah

[doradox]

I don't think I'd recommend an aftermarket ignition. The stock system is very reliable and for a stock or near stock engine is more than adequate. Some others here have "upgraded" and maybe someone could recommend a system for you.

I've seen coils cause some funky traces and without seeing the secondary it's the cheapest/safest bet. Probably a good idea to check all you connections to the scope and ignition/grounds first like you mentioned earlier.

Steve

Steve

[FastWoman]

Hi Steve,

I'll be checking/cleaning my connections tomorrow. You're right that that's the obvious first step. I'll be following your advice also about the coil. I think I might do the wires too. Any recommendations as to make/type/sources?

If it does become necessary to replace the ignition module, I notice Black Dragon has some rebuilt ones. How exactly would an IM be rebuilt? Would they replace all the electronics with new and simply re-use the old shell and connector? I feel the same way about old semiconductors as I do about (non-rebuilt) junk yard parts. I think I would far prefer a new, non-OEM module to a partially refurbished (i.e. repaired) OEM module.

BTW, I'm asking all these questions up-front to flesh out a game plan. These threads seem to die out fast, and I'm often lucky to get any answers to my questions. I'm pumping you for info while I still have you on the line. (You seem to have a good grasp of ignition issues.) I hope you don't mind too much!

Thanks again!

Peace,

Sarah

[geezer]

I looked at all the examples of abnormal patterns and found nothing resembling the one you posted. The only thing that came close was an example of weakened spring tension in a points set on a conventional system. Makes one wonder if that can be compared/related to some kind of reluctor or pickup coil abnomaly. I mistook your trace to be a secondary pattern (they look very similar to me). Since you are checking connections today and I don't know what information is included in your FSM I thought maybe these pages may help. In this manual they seem to present the '78 ignition module and 6 wire harness to be the same as the '77.

[doradox]

I agree on the rebuilt module.

I have always liked the Standard brand of ignition parts and Napa's premium brand.

Steve