Electronic Ignition Waveform -- abnormal?

[FastWoman]

Hi Ron,

You wrote:

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.

Interestingly, I think there could be some similarities to electronic ignition. One possibility I've considered is that of inadequate current to the coil during the dwell phase, either from semiconductor breakdown or from crusty wiring. The weak tensioner spring might produce the same inadequate current. I'd love to have a peek at that waveform. Could you post it for me?

Thanks so much!

Peace,

Sarah

[geezer]

Hi Ron,

You wrote:

Interestingly, I think there could be some similarities to electronic ignition. One possibility I've considered is that of inadequate current to the coil during the dwell phase, either from semiconductor breakdown or from crusty wiring. The weak tensioner spring might produce the same inadequate current. I'd love to have a peek at that waveform. Could you post it for me?

Thanks so much!

Peace,

Sarah

I would think other than higher voltages and differing possible causes of an abnormal waveform the traces would be very similar between conventional & electronic ignition.

There isn't an example waveform for improper spring tension, only a mention concerning spikes above zero line. It says " If the breaker points bounce open slightly after they've closed, the small secondary induction which occurs at the point-close will be increased. This may impede full coil saturation at high speeds, resulting in low secondary voltage and consequent misfire."

I really think this like pulling at straws and a moot point if several others things have checked out such as reluctor gap, pickup coil resistance or an indication of the ignition module not operating correctly, which it seems to. Solely on the lack of the nice cone shaped pattern that you would see in a normal pattern, I am really leaning towards a defective coil as being the culprit, just as Steve figured from the getgo.

In the days of old we always had a spare of everything on the shelf to slave on, to speed the diagnosis of these problems.

[doradox]

I would think other than higher voltages and differing possible causes of an abnormal waveform the traces would be very similar between conventional & electronic ignition.

There isn't an example waveform for improper spring tension, only a mention concerning spikes above zero line. It says " If the breaker points bounce open slightly after they've closed, the small secondary induction which occurs at the point-close will be increased. This may impede full coil saturation at high speeds, resulting in low secondary voltage and consequent misfire."

I really think this like pulling at straws and a moot point if several others things have checked out such as reluctor gap, pickup coil resistance or an indication of the ignition module not operating correctly, which it seems to. Solely on the lack of the nice cone shaped pattern that you would see in a normal pattern, I am really leaning towards a defective coil as being the culprit, just as Steve figured from the getgo.

In the days of old we always had a spare of everything on the shelf to slave on, to speed the diagnosis of these problems.

You're right that point and electronic should, and do, look about the same. It's just electronics turning the current flow on and off instead of a mechanical switch.

The lack of the cone shape (healthy oscillations) is what strikes me most but the whole thing just looks off. Anyway, life was a lot easier when one could swap a component real quick to verify a diagnosis.

Steve

[FastWoman]

I was able to go through all the connections, and they were all beautiful. Not a bit of corrosion or dirt or any other indication of anything gone wrong. Taking measurements at the ignition module connector, I was able to confirm a healthy supply of power and good connectivity to the reluctor and the primary side of the coil. The coil's primary resistance did measure a bit on the high side, though -- about 1.3 ohms, vs. the .84-1.02 ohms that a new one should read, but still less than the 1.8 ohm reject level cited in the FSM. The secondary side of the coil measured about 13.5k, only slightly higher than the 8.2-12.4k specified on a new coil.

I also opened up the ignition module to have a peak inside. Very pretty vintage circuitry, obviously assembled by workers who would have dishonored their families and company (Hitachi) by failing to line every mylar cap up perfectly perpendicular to the board. Nothing jumped out as obvious (e.g. scorch marks on any of the parts). There was one power transistor and several low-power transistors of the same type. Neither these two transistors nor their data were available on the Internet. Sort of expected, but.... drat!

Anyway, it's not the wiring or connections. That's for sure. So I guess I'll start replacing parts -- first the coil. I also would like to have the NGK-type wires, so I'll go ahead and get them too. If I need to replace the ignition module, I've confirmed that an MSD-6A will fit the space on the passenger kick wall, and I can use all of the same wiring.

I'm going to install an MSD Blaster 2 coil to match an MSD ignition module -- based on my expectation that the ignition module is actually bad. (The trace really wreaks of semiconductor breakdown to me. The oil in those power transistor cans sometimes leaks out, leaving the contents to bake. I suspect that's what happened.) From what I've read, the MSD-6A, the Blaster 2 coil, and the NGK wires are all a good match for each other.

I guess this will be next weekend's project, depending on how fast the UPS truck gets here.

Again, thanks for helping me with my ignition. I'll post back when I find out what was giving me that funky waveform.

[zcarnut]

(post deleted)

Edited October 29, 2009 by zcarnut

[FastWoman]

Steve, I won't belabor my qualifications. I have them too -- not as good as yours, admittedly.

I recorded this trace through a passive attenuator network -- garden variety, 100% resistive T-attenuators. No capacitors. No AC. Only DC. Irrespective of the properties of the attenuator assembly, and irrespective of whether it works properly or not, it's still a resistive network, and it still passes DC, not AC.

Is it not possible that the clamping diode in the ignition unit has failed, making the negative swings not quite so impossible? That said, I don't remember seeing a diode in the circuit -- just resistors, transistors, and caps. Furthermore, negative voltage swings are indeed possible in oscillating circuits. I admit I'm stumped as to the magnitude of the swings, but I'm not willing to say they're impossible. A working oscilloscope and a pile of resistors say otherwise.

I do appreciate your input, but I have to say I'm a bit put off by the tone. I wish I had a more impressive test bench, so that I could look smarter and more authoritative. Unfortunately two long distance moves and a divorce leave me with very little in the way of test equipment. I'm slowly rebuilding, hence the cheapo Chinese USB oscilloscope. I'll cobble together a proper 100:1 probe too, but for this measurement I simply grabbed what I had sitting around.

Sarah,

Edited October 29, 2009 by FastWoman

[FastWoman]

Just wondering: Is it possible for the secondary to have shorted to the primary? That could result in some crazy-high-voltage ringing in the primary trace. Hmmmmm....

[kenz240z]

OK, just a few guesses here...have you checked the dizzy cap to make sure there is no carbon tracking inside that could cause a misfire? Have you verified that there are no issues with an injector that intermittently fails to fire? - disclaimer: I'm new to the world of fuel injection, so I'm grasping on this one...:stupid:

Have you considered replacing the dizzy and module with a ZX dizzy/module? Forgive me if the '78 uses the same one...hey, I'm a 240Z kinda guy! Anyway, my thought is that you might try making one change at a time to see if you can narrow down the culprit. Not the best method of troubleshooting, as it would be nice to pinpoint the trouble based on the scope trace; however, that doesn't seem to be yielding anything definite yet.

Oh, and, um, well it's been awhile since I put my EE hat on (No, not elementary education!!!), but it appears to me that a DC offset is present in the waveform...:ogre:

[doradox]

Just wondering: Is it possible for the secondary to have shorted to the primary? That could result in some crazy-high-voltage ringing in the primary trace. Hmmmmm....

Can't say I've ever seen that, but that doesn't mean it couldn't happen. Or that I didn't recognize it as such.

Steve

[zcarnut]

I apologize for my tone. I’m sorry my knowledge of something I work with everyday comes off as reeking of arrogance. I only stated my qualifications so as to give my response some substance. In the 1980’s the company I worked at (a division of Magnavox) was a supplier to Delco for the “six pack” coil that was used on the first production DIS. We also designed an ignition unit but Delco choose one from Motorola.

Yes, the clamp diode is there. It is even in the schematics of the ignition units. Even if the clamp diode is omitted (not a good practice) the collector-base junction would clamp the collector waveform.

If you’re happy with your test set up, then so be it.

Edited October 29, 2009 by zcarnut

[FastWoman]

Hi Kenny,

The cap looks clean and appears to have very little use on it. That said, it might be at least 5 yr old, as it doesn't show up in the PO's service records (which were printed off for me by his former Nissan (dealer) mechanic. Not many miles on it, though.

The distributor itself is new/rebuilt. I replaced it when I discovered the advance mechanisms were all frozen up and that there was a slight bit of play in the shaft. The reluctor got replaced with the distributor. I was hoping a new reluctor and tighter shaft would also resolve the tach double triggering (which I though might be attributable to double firing of the ignition system), but that didn't happen.

My injectors are very old and indeed might be sticking or spraying poorly. Of course that's an expensive issue too. I've been running Chemtool B-12 through my gas from time to time (every few tanks) to clean out deposits that might be there (and on the intake valves). Cleaned/refurbished injectors are on my "next" list, if new ignition components don't resolve the problem. BTW, fuel pressure is normal, the exhaust has a normal (slight hint of gas) smell, and the plugs look normal.

I'm continuing to work my way through the system. As I said, plug wires and coil first -- coil being a likely, not too expensive cluprit and the wires being rather old and not of a type I like. I already received and installed the new NGK wires. It runs a bit better, but the problem isn't resolved. I'll probably get the coil today or tomorrow and hope to see a miraculous recovery.

Doradox, I really should have checked for continuity between the primary and secondary on the coil. I was in too big a hurry, with nightfall upon me. Ah well.

Zcarnut, no problem. I'm not particularly "happy" with my test setup, but it's really all I have right now. As I said, I'm rebuilding. I haven't had much use for a test bench post-university, but there are a few specialized devices I would like to develop for my photographic work.

Just curious: Do you have the schematic specifically for the Hitachi control unit in my '78 280Z (different unit from the '77)? I did a quick search for the two transistor types I found in there. I found reference to the power transistor on the heat sink, but no technical specs. I didn't see any obvious place I could buy a replacement, nor did I find cross-reference info. I also found nothing on the handful of little transistors sprinkled throughout the circuit. I feel as though I might be able to rebuild the circuit if I can find compatible semis.

Thanks, everyone!

Peace,

Sarah

[240Z-Fan]

FastWoman

FWIW, I had issues with cross-firing with newish NGK wires some 14 years ago (these NGK's had the translucent blue insulation).

I could see the misfire on a timing light at idle by clipping the *pickup* to each wire and watching the strobe flash. The problem would go away when I separated the wires. #'s 5&6, I think.

I don't know whether it would show up on the scope (I seriously doubt it would cause the tach needle to bounce as you described), but I have been leary of NGK wires ever since.

If it were me, I would try a known good ignition module and see what happens.

FWIW, I am very interested in seeing what you learn; I am BSEE that has been away from it far too long and would love to put my old tektronics scope to use on my car hobby. I have learned a lot already from following this thread.

Al

Edited October 30, 2009 by 240Z-Fan