Ignition Systems Analysls

[Captain Obvious]

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:

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.

[Captain Obvious]

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:

[Zed Head]

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.

[SteveJ]

I think this link will add to the discussion: http://www.atomic4.com/dwell.htm

The title is What is Dwell Management and Current Limiting?

[Captain Obvious]

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. 

But absolutely questions would be most excellent.

[Captain Obvious]

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:

And here's a pic of the 78 guts:

Edited September 23, 2022 by Captain Obvious
typo

[Zed Head]

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!

[SteveJ]

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.

[SteveJ]

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.

[Captain Obvious]

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?

[Captain Obvious]

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:

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

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:

[Captain Obvious]

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: