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Ignition Systems Analysls


Captain Obvious

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I've been poking around with the ignition electronics on and off and there seems to be some interest in getting into some of the details of how things work. So here's a place to start.

Here's a pic of the ignition module signals. The input signal from the VR distributor is in blue, and the output signal that drives the coil is in yellow. Note that when the yellow output signal is low, the coil is charging (dwell), and when that output signal goes high, the coil would fire a spark.

Spinning the distributor (by hand) the signals look like this:
CCW1.JPG

First think to note is that the ignition module (1977 style) fires the coil on the negative-to-positive (N-P) transition of the VR input signal.

Also noteworthy is that the triggering N-P transition VR signal is a steep, almost vertical, slope. This is important because the steeper the slope here, the more consistent the timing will be with less ambiguity and spark scatter.

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And for discussion, here's what happens if you get the distributor input to the module reversed somehow.

Note that the module still fires a spark on the negative to positive transition, but the slope of the input signal at that time is very gradual. Not good for spark scatter and consistency:
POLneg1.JPG

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Are you taking questions or will you just be adding things as you explore?  You had more in Av8's thread, with the Z wave form, compared to the ZX.  

Still wondering why this forum doesn't have Stickies.  So many good discussions are buried back in the past.

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8 hours ago, Zed Head said:

Are you taking questions or will you just be adding things as you explore?

Sure I'm taking questions. Admittedly I'm having a little trouble figuring out the best way to organize my thoughts for this thread. I've done a bunch of analysis of a couple different designs and I'm trying to do a knowledge dump. Problem is I'm not sure the most effective way of getting info out.  LOL

But absolutely questions would be most excellent.

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So here are a couple things about the 77 and 78 modules:

The cases are electrically isolated from everything inside. In other words, there is no electrical connection to the metal boxes that house either the 77 or 78 module electronics.

There is no current limiting in the 77s output stage. That is why a ballast resistor is required to limit the coil current.

The 78 module, however, DOES have current limiting built into the output stage which is how Datsun was able to get rid of the ballast resistor for 78.

Other than that current limiting and some temperature compensation (which the 77 also does not have), the electrical designs of the 77 and 78 modules are pretty much the same.

Since everyone likes pics, here's a pic of the 77 module guts:
P1110794.JPG

And here's a pic of the 78 guts:
P1170438.JPG

Edited by Captain Obvious
typo
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Could you set up a test bench to measure module performance against RPM?  Look for timing changes and/or reduced charging time.  I think that that is the issue that Vizard had with the GM HEI module.  As I recall he proposed it but never actually showed that it happened.  I think I've written about it in the past.

You could even blow up some modules by leaving them ungrounded or with poor cooling.  Only $25 / experiment!

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I hope this adds to the discussion. After watching a couple of videos that @Captain Obvious provided me links for, I bought a 20:1 attenuator for the voltage probe. I hooked up the spark induction probe on the coil wire and shot some video while monitoring my 240Z with a Pertronix Ignitor II. Maybe tomorrow I'll shoot some more video with the 260Z using the ZX distributor.

Here is one of the videos the good Capt sent me: 

 

I'm not sure in my video if I'm confusing voltage kick with firing line. I noticed a lot of variability in the voltage peak at the firing line. I also noticed as the engine speed increased, I couldn't see the voltage at the coil as easily on the scope.

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I thought this link was interesting: http://commons.princeton.edu/58-tiger-cub/wp-content/uploads/sites/75/2018/08/ignition_waveforms.pdf

It talks about how to use the analyzer to diagnose the engine. 

If you watched the video I made, you will see the voltage at the firing line vary. I have another video where I used the inductive probe on cylinder #1. From that, I can see which cylinders have the most of the anomalies happen and possibly optimize the spark performance.

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19 hours ago, Zed Head said:

Could you set up a test bench to measure module performance against RPM?  Look for timing changes and/or reduced charging time.

Working on it. I was originally driving different modules using a signal generator, but after messing around with that a bunch, I'm thinking that it's not a good representation of the real world situation, so I'm going to have to figure out a way to use a real VR signal.

First attempt at setting something like that up worked OK for speed control, but I was using a variable speed DC motor to drive it and I was getting huge amounts of noise on the system. I need to come up with something quieter.

Basically, you're asking if I can illustrate the timing changes @Av8ferg saw when he swapped from one module to another, right?

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So here's some info on the variable reluctance pickup (used in the Z starting in 74):

The only time a VR pickup puts out a signal voltage is when the magnetic field CHANGES. No magnet change - no output signal.

And as part of this concept, the FASTER the field changes, the higher the output signal will be. Rapid transitions in the field will result in higher output signals, while more gradual slow changes in the field will put out lower signals.

No motion at all, no output at all. Regardless of where the target is in relation to the pickup... No motion, no output.
 

Now let's talk about "polarity":

So you've got this VR pickup coil. You put a voltmeter on that sensor coil and it reads zero. Then you bring something magnetic (I'm calling it a "target") towards the sensor and you will see a voltage on the meter. Stop the target close to the sensor, and the meter will drop back to zero (remember... no motion - no change in field - and no output). And then when you move that target AWAY from the sensor, you will again see a voltage on the meter, but that voltage will be in the opposite polarity from when the target approached the sensor.

Which direction will the voltage go as the target approaches the magnet? The polarity is determined by the North-South orientation of the magnet and how you have the meter connected. Michael Faraday and all that.

On the Datsun ignition systems, the pickup coils have a red wire and a green wire. As a target approaches the sensor, the red wire will produce a negative voltage with respect to the green. And when a target moves away from the sensor, the red wire will produce a positive voltage with respect to the green.

Z car polarity:
Target moves towards sensor - Red wire goes negative
Target moves away from sensor - Red wire goes positive

Here's a snippet from the FSM that talks about the polarity and using a scope to check the VR pickup coil. The test procedure is to connect the scope (+) lead to the red wire and connect the (-) lead to the green wire, and then spin the distributor. You should see a waveform that looks like the solid line in the pic:
fsm polarity.JPG

And here's what it looks like on the bench test. Positive input to the scope is red. Negative input connected to green:
P1200879.JPG

And here's the waveform out of the sensor when I spin it. Just a short burst spinning by hand. Note that this is an 83 sensor so the shape of the waveform may be a little different than earlier years, but basically the same:
P1200880.JPG

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I whipped this up to try to show how the motion of the VR target corresponds to sensor output.

As the target approaches, the red wire goes negative. And then as the target recedes, it goes positive. And the sharp transition point in the middle when the target reaches TDC and changes from approaching to receding:
fsm polarity2.JPG

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3 hours ago, Captain Obvious said:

Working on it. I was originally driving different modules using a signal generator, but after messing around with that a bunch, I'm thinking that it's not a good representation of the real world situation, so I'm going to have to figure out a way to use a real VR signal.

...

Basically, you're asking if I can illustrate the timing changes @Av8ferg saw when he swapped from one module to another, right?

That would be one question.  Then there's the supposed 4500 (V8) RPM limit for the HEI module (which would be 6000 RPM for a 6 cylinder).  Others might be "does the broken magnet in a ZX distributor cause it to stop working?", "what difference is seen between a 4.5 amp HEI module with a 0.6 ohm coil (factory setup) versus a 7 amp HEI module (Pertronix and others) and 3.0 ohm coil?", ... and on and on.

Ideally, I think, you'd be driving a real distributor at a known high RPM.  

Then there are the other odd questions, like "why does a GM HEI module work as a replacement for a 280ZX turbo ECCS "ignitor" (power transistor) when there is no zero-crossing?".  Is the 123 ignition module current-limiting?  Is an old Ford ignition module similar in function to the GM HEI module even though it's designed for a Hall Effect signal?

 

You could also get in to the heads of the engineers and explain why they changed the triggering mechanism from the single reluctor to the six point reluctor.  You touched on it in a past post as giving a higher cranking voltage.  I thought that it might also give more consistent timing cylinder to cylinder, because it averages all of the points in to one voltage pulse.  People have found that the 280Z type reluctor wheels have enough variation between the points that the timing varies between cylinders.  On the high performance engines it can cause problems.  

 

It's all interesting.  

 

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I'd be really surprised to find there was a rev limiter in the HEI module set for 4500 RPM.

The Datsun modules I've looked at in depth (77 and 78) have no built in limit and eventually crap out simply because they can't react fast enough. But that point is way above normal operating frequency. Way above.

I don't have any of the aftermarket stuff here for analysis, but I'm hoping to get my hands on a couple of the GM modules to poke around with.

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It's not a "rev limiter" it's a deficiency in the quality of the spark, according to Vizard.  I remembered that I had found and copied some pages from an old article by Vizard and managed to find them again.  His whole issue was that the stock HEI system's spark quality, aka voltage, dropped off too soon (for his desires).  As I recall now, the reason it bothered me was that he was bashing the system based on race engine needs.  Typical bench racer stuff.  (p.s. actually, in looking at what he wrote, I realize that what's bothered me over the years is how this limitation shown by Vizard has been spread around the gear head world as gospel, even though it's basically nonsense for a typical car hobbyist.)

In looking at the pages I even see that his "limit" was actually 5,500 RPM.  His recommendations are for ways to get the RPM spark quality up to 7, 8, or 9,000 RPM.  

Here are a few excerpts from those pages.

image.png

image.png

image.png

Edited by Zed Head
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The "energy starts to fall off" mention is kinda interesting. At low RPM there is plenty of time for dwelll and plenty of time for spark. At high RPM however this is not so true. But the issue is it doesn't matter which ignition system you are running, you are still bound by the same cycle times.

By that, I mean... On a six cylinder engine spinning at 6000 RPM, there are 3.33 ms between firing events. In that 3.3 ms you have to charge the coil and then spark that coil. If you want enough spark time, you have to be able to charge the coil fast. And that's where a no-ballast system that uses a current limit in the module would have an advantage. You can charge it faster so you have enough time for adequate spark time.

I bet all the systems "fall off at higher RPM's". Unless they are using the same dwell across the entire RPM range, then they would all fall off as dwell decreased at higher RPMS. I haven't measured any of the aftermarket stuff with built in dwell control to see if they use the same dwell time at idle and 6000 RPM. I suspect not.

I had all those measurements done for the 77 and 78 modules, but I was just using a signal generator as a simulated input. But after seeing how the modules can react different using the real VR pickup, I'm rethinking the validity of those measurements. I'm not sure using a sig-gen is valid.

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16 hours ago, jonbill said:

I guess the focus here is the std distributor/single coil setup? multiple coils of course allows longer dwell per coil and stronger sparks. 

 

It's a good starting point since that's what most people here are running, but it would be interesting to learn more about ignition for the coil on plug systems.

Just from talking with @Captain Obvious on the phone and looking at some videos he suggested, I was able to detect a potential issue with my 240Z by using my oscilloscope. The scope indicated there could be some extra arcing (having to jump more than the gap between the rotor & cap and spark gap) at the firing line of the spark at different cylinders. When I looked at the cap, wires, and coil, I found significant corrosion at the coil. 

Before changing the coil and wires (yellow wire is the voltage at the negative post of the coil):

image.png

After the coil and wires were replaced (again, yellow trace):

image.png

Next up is for me to get a current probe for my scope so I can chart the current limiting effects of some of the ignition systems.

Edited by SteveJ
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5 hours ago, jonbill said:

I guess the focus here is the std distributor/single coil setup? multiple coils of course allows longer dwell per coil and stronger sparks. 

For me, it is. Everything I have is stock parts. Spectrum of different years, but all stock. I'd be happy to look into other systems, but I don't have anything here.

It all started for me wondering what they did in 78 to allow the removal of the ballast resistor. And the answer is they started including current limiting inside the module in 78.

Then av8ferg wanted to be able to mount two different style modules (ZX matchbox and GM HEI) on his car and be able to swap between the two if one of them failed. But when he tried it, he noticed a timing difference between the two. So I thought it might be interesting to investigate that.

That's how I got here. I've been poking around inside the stock modules for some time now but figured nobody (other than me) really cared, so I didn't post about it.  LOL

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1 hour ago, kickstand80 said:

It would be interesting to see how a broken magnet vs an intact magnet affects the performance of the induction pulse. Does your 83 distributor have an unbroken magnet?

I think the ZX distributor I've been messing with has an unbroken magnet. I think I have one here with a magnet in pieces too, but it may be in too many pieces. LOL

I'll look to see what I have.

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