Here is the second part using my 61 fuelie with points:


This is a typical parade or display pattern. Here you can see all the cylinder’s secondary patterns. Look for uniformity in firing voltage and spark lines. I am using the 25KV scale.
Notice the firing voltage is between 8 – 11 KV which is normal. This is how much voltage it takes to over come all resistances in the secondary which includes the plug gap.
Lean or rich mixtures and other factors will affect the firing voltage.


Here is a superimposed secondary pattern. It is a little blurry due to the camera setting.
1- This is the 8 cylinder scale, 45 degrees maximum.
2- Point closing
3- Dwell is approximately 30 degrees.
4- Look for sufficient oscillations here. If you only see one or two oscillations it could be a bad coil. It could also indicate low voltage in the primary.


Here two bad patterns that I created. The first pattern on the left is cylinder one, then 8,4.3,6,5,7 and the last one on the right is 2.
1- I am still on the 25KV scale and the red line is 10KV
2- This is number 4 cylinder. This indicates a possible shorted wire or fouled spark plug. Notice low firing voltage, 6 KV
3- This is number 6 cylinder. There is too much resistance in the secondary. Firing voltage over 15KV. Possible bad wire, open spark plug or very wide gap.
If all firing voltages are very high it could indicate excessive rotor to cap segment distance.
I checked a 58 Vette this year and all the firing voltages were excessive (25KV). He said anytime he accelerated it would backfire.
He changed everything electrically except the wires, rebuilt the carburetor, adjusted the valves twice and still couldn’t find the problem. He didn’t change the wires because they weren’t that old. (repros)
Since he had very high firing voltage I removed a wire from the spark plug and grounded it, very little change in voltage. Then I removed the wire from the cap and grounded the cap terminal and the voltage went down to 5KV. We changed the wires and the car ran perfect.



Here is cylinder 4 expanded. I closed the plug gap to create this pattern.
1- Very low firing voltage
2- Long firing line sloping downward.


This is number 6 cylinder. I opened the plug gap to simulate high resistance.
1- Higher than normal firing voltage.
2- Short firing line with slight upward slope.


If I see a very high firing voltage spike as in cylinder 6 I then remove the plug wire from the plug and using a ground probe I ground the end of the wire. If the spike lowers you have a bad plug. If the spike remains high I then ground the distributor cap terminal of the affected cylinder. If the spike lowers (1), as in this screen shot, then you have a bad wire.
Note the length and straightness of the firing line (2) without any plug resistance.




In this screen shot, I removed a spark plug wire from the distributor cap momentarily to simulate an open wire or spark plug. This is also a test to see the max output of the coil. It went off the 25KV scale which tells me the coil has enough reserve to fire the plugs under load.
Do not take the wire completely out of the cap because the spark will seek a path of least resistance to ground. On a point type system this does not pose much of a problem, but with electronic ignitions, especially HEI which can produce over 40KV, it could damage the control module.

These are the basic patterns and will be the most common ones you will see. Recreate the problems I have show you here so you can see what it will look like on your scope.
An oscilloscope is a very valuable diagnostic tool. Coupled with a cylinder balance tester and a vacuum gauge most problems can be diagnosed easily. Even if you don’t see anything wrong with the patterns it just told you to look elsewhere, the problem is not electrical.

Joe

 

Great information Joe!! Thanks for the lesson, makes me want to go out and find a scope. Please keep the lessons coming.

 

Joe,
Does this post still use the same connections as in your thread http://forums.corvetteforum.com/c1-a...e-pattern.html

I've got a Sears Diagnostic Analyzer my dad just handed down to me, and using it to measure the secondary ignition circuit is showing less than 1 KV per wire, so I'm thinking I need to connect the green wire to my coil as you have in the first picture of the referenced post.

In any event, clarification of connections associated with your Scope Patterns 2 photos would be helpful in case I buy a scope.

Jeff

 

Yes, if you look at the connections in the pic you can see the top of my FI unit. All these pics are from this car.



In order to get a proper pattern you need:
1- number one wire inductive clamp
2- Coil primary negative connection
3- Inductive coil clamp
4- Ground
5- Since my scope is powered by the car battery I need a 12v positive connection.
Joe

 

Great job Joe! Thanks!

Now I am waiting for my "new" scope to arrive.

Plasticman

 

Great, thanks Joe

 

I've got a newbie question for sure.

With the #1 wire inductive clamp on the #1 plug wire, and the coil inductive clamp connected to the coil wire, and the green wire on the negative pole of the coil.....

How does the scope lay out 8 patterns? Is it sensing the ignition circuit for the plug currently being fired, using the #1 wire clamp to set a basis for the start of the 8 cylinder firing cycle, and deriving whether or the "current" plug being fired is #5 or #3, etc?

 

Good question Jeff. I was wondering how each cylinder was identified on the scope. Does it display the cylinders uniformly by firing order? Great job again Joe!

 

Based on the 3rd photo in the opening post, yes it does. And that's why I'm curious to learn how the scope knows how to lay out the 8 patterns when it only has a connection to the coil wire, the #1 plug wire, and the + coil terminal, aside from the power hookup.

 

Thanks for doing this Joe. It's interesting enough to have made me drag out the machine I bought about 10 years ago and could never figure out how to use. Maybe this time!
Fast Jimmy

 

The #1 clamp is the starting point for the firing order- it doesn't know it, it just knows how many cylinders it has. If you put that for example on #4, your pattern would be 4-3-6-5-7-2-1-8. I you screwed up and told the scope it was a 4 cylinder, you would see 1-8-4-3
The coil pickup is measuring the secondary , the negative coil lead is measuring primary, and these leads are where the patterns come from.

 

Does the gas analyzer still work in the Sun Machine Jimmy? Thats another real interesting subject and a great tool.

 

Fast Jimmy,
That brings back memories. As I have said I was trained on this machine. A friend of mine who owned a shop had two of these!
When I got my shop I decided to go with Allen Test Products.
With that machine you should be able to diagnosis any ignition/engine problem.
If you want to take some screen shots you have to slow the camera shutter speed down. It took me a lot of tries to get the right shutter speed.
Joe

 

sure glad you are within reach LOL enjoy the winter season see you in springtime joe !

 

I will try and explain.
Since all the firing voltages goes through the coil wire for every cylinder, all eight patterns you see are taken from the coil wire inductive pick-up.
Now the scope needs to know which cylinder to display as the first cylinder in the firing order at the left of the screen.
That is why you have a number 1 inductive clamp. It doesn’t matter which cylinder wire the clamp goes on, the scope will just start displaying that cylinder and then the rest of the cylinders in the firing order of the engine from left to right.
Now that the scope sees number 1, it has to know what to do next. This is where the coil negative primary coil lead comes in. If you look at the pattern you can see that whenever the points open it displays the firing voltage of the next cylinder in the firing order.

Here is the chain of events:


The scope sees one firing voltage at the coil wire and at the same time it sees the voltage from the inductive clamp on number 1 cylinder. It now knows to put this pattern first on the left, cylinder 1. Now the points close and when the points open again, the scope now displays the firing voltage of cylinder 8 from the coil wire, then the next cylinder, 4, and so on until it comes back to cylinder 1, which it puts back on the left.
‘A’ is the firing voltage for cylinder 1. ‘C’ is a continuation of ‘B’ and the cycle starts over again.
I hope this makes sense. I haven’t had to explain this for over 30 years and my brain hurts.
Joe

 

Yes Joe, that is perfect, thanks!

I'm trying to relate what your scope is showing, to what my Analog diagnostic tool is showing (1.9 KV at the coil wire) so understanding how your scope works, helps me now as well as later.

 

Jeff,
1.9KV is very low firing voltage. Does you tester use an inductive clip?
Is there more than one KV scale?
Joe

 

I'll PM you so as to not take your excellent thread on a tangent.

 

Gas analyzer? I admitted I don't know anything about these, but I am willing to learn. There's a whole bunch of hook-up stuff in the cabinet so I'll have to try it and see if the analyzer works, if I can figure out how to hook it up. The one thing I didn't get with it is a good manual to tell me how to work things.

 

Post a pic of what you have and if I remember I will tell you how to hook it up.
Joe