I think you are just a little off the tangent here.
The old Kettering systems (points and condensor) had to approach saturation, and that TIME was pretty much constant regardless of RPM. What I think you meant to say was that with higher RPM, these old systems did not have enough time to satisfy the RL constant and ignition performance dropped off.
Some of the newer systems dont use this principle at all. F. eks. a capacitive discharge system charges the capacitor under high voltage conditions, so the time function is not as important. It fires a coil as a stepup transformer (approximately).. There are other variations.
Turned out it was a bad ignition coil with a carbon track. It would occasionally arc at the track and mis fire. The clue was that it was getting worse and worse, then I decided to pull the plugs and I reduced the gap and it then ran MUCH better but was not really fixed. That was a definite indication of a spark strength problem (not timing or fuel). I opened the distributor and inspected the coil (it's inside the distributor on this car) and saw a crack in the potting insulation and what looked like a carbon track. I got an new coil, put it in and the car has been perfect. I hooked up the old coil on the bench with a 12 volt 5 amp current limited power supply and pulsed the primary and saw the secondary arc to the crack. QED.
FYI If you ever test a coil on the bench like this by pulsing 12V into the primary, it was interesting to see that it did not develop a good spark unless I also connected a capacitor on the primary.
This is the coolest damn thing. If you actually look at the waveform on a scope, it's a huge damped sine. The coil has got a ferrite core, so it really is only effective at high frequencies. You pulse it, and only some of the high frequency components of that square wave actually make it through the coil.
BUT... The coil inductance combined with the shunt cap make a resonant circuit and the resulting damped sine wave makes it through the coil very effectively.
Kettering figured this whole thing out and it's a hell of an ingenious idea.
It sounds like he was using a power supply and not a battery for his bench test. It should work a little different with a battery.
Well no not exactly. When its working correctly the spark plug will fire before it gets 1/4 of the way through the first period of the sinusoid. If you are seeing something that looks like a sine wave that indicates the high voltage is not making it to the spark plug. At any rate if he was looking at the wave form on an o-scope he should have seen the misfires. It's possible he may have been interpreting the waveform produced by misfires as a bad dwell or timing issue.
If there is one single thing and O-scope is good for finding, it is the very problem he says he had.
Pulling a plug wire, sticking a spare plug into it, holding the plug body to a handy ground, and operating the starter will almost never reveal this problem. That is because it takes much more voltage to fire the plug inside the engine when cranking then it does to fire the plug when it is outside the engine resting against the block or other ground.
Well I'm happy the car is working but I have been wondering about this very question.
My conclusion (as Tegger is correctly needeling me about) is I don't think a scope is very good at detecting this particular type of mis- fire.
When the mis fires happened casued by the carbon track, there was still a spark it's just that the spark happens at the coil instead of at the plug. The scope is coupled to the wires which still sees the almost the same volatge waveform. The scope can't tell which end of he wire is sparking. My thoughts are that the waveform isn't really that different, there is still a spark in either case.
Remember, the carbon track didn't actually short or resistivly load the secondary, it just provied another spark gap and the spark would jump at that gap instead of at the the plug gap, occasionaly.
But Tegger is right, this is a lesson in starting with the basics..
Yes but it is going to appear as a different waveform on the scope. Heck just changing the spark plug gap by a couple thou will change the waveform. For one thing when it fires at the coil instead of the spark plug it doesn't have the resistance of the spark plug wires and spark plug.
You are looking at the primary waveform. that is a little different than the secondary waveform but you should still be able to see everything you need to diagnose.
But you need to know what the wave form is supposed to look like.
But it only jumps that gap when the voltage exceeds some high threshold. If it always arced at that breach in the coil you would always have no spark to any of the plugs.
There are ways to find that fault without an O-scope. But the procedure as he described is not going to work, because it requires much less voltage to fire the plug when it is sitting on the block at atmospheric pressure. The voltage will never exceed the threshold needed to take that path.
The scope did show a small difference in the waveform when there was a mis-fire, but it was a subtle difference and at the time (not knowing what the answer is) I couldn't tell if that differnce was the cause or an effect.
If for example, the misfire were due to a fuel problem (or timing which is what I was stuck on), I would also expect it to effect the waveform. So just becasue you see a subtle change in the waveform during a mis-fire that doesn't point to the casue of the change. The waveform durig the mis fire did NOT look like a weak spark.
You should look at the secondary for this sort of diagnosis (probably best to get or make a inductive pick-up). What you would have seen looking at the secondary voltage would be a lot less subtle.
The whole purpose of using a o-scope is to view and interpret the subtle changes in waveform. From what I gathered you were focusing on the dwell part of the waveform. The type of problem you had should have caused no change in that part of the waveform.
From what you described it wasn't a weak spark. It was a spark that followed a different path when the secondary voltage exceeded a certain level. I would expect it would misfire when you punched the gas pedal. Usually the spark following a carbon trail gets worst when its damp and humid and under heavy load and not when the engine is hot, but I suppose if there was a crack in the coil insulation it could open up when its hot.
If your spark plug wires or spark plugs are old you might want to make sure they weren't the root cause of the problem. If the coil is constantly forced to produce more voltage than it was designed for it can lead to breakdown of the insulation.
To test the coil voltage capability you need to force it to jump a wider gap than usual. As a rule of thumb the coil needs to be able to produce enough voltage to jump a gap equal to the ordinary spark plug gap times the compression ratio. So if the spark plug gap is .04" and the CR is
10 the coil need to be able to produce enough voltage to jump a gap (under normal atmospheric pressure) of 0.4" However don't force the coil voltage to go any higher than that (by jumping a larger gap) or you may cause the same problem again.
I did play with the bad coil on the bench, with a capacitor on the primary and pulsing 12V current limited to 5A into the primary, it would easily arc to the carbon track with no gap connected. If i connected a 0.04 gap, it would never jump to the carbon track,... but you just gave me the reason, under compression the gap is in effect
0.4"
So coils should be tested on the bench with a 0.4 to 0.5 inch gap..
thanks
I did check and regap the plugs and check the wires. car has been running perfectly with the new coil, even in the rain..
Bear in mind that with no gap connected you are pushing the voltage higher than you really need to test and that has the potential of creating the very problem you had.
Of course a closed throttle means lower cylinder pressures. It is a little surprising to me that you never mentioned that the car had a tendency to stumble or stall when you tried to accelerate hard from full stop. Or maybe you never do that.
Sure. But driving the coil to excessively high voltages may cause damage to the insulation. There is really not much point in testing for a lot more voltage then the car needs.
yeah I was thinking about that too. I didn't notice any miss under power at first, after a day or two the miss at idle got worse and the miss a power became very obvious..
Lesson leaned is that spark problems can show at start, idle and under power.
also I was careful not to pulse the new coil on the bench without a gap connected the secondary.
I didn't care about more damage to the old coil, it was already defective, and I wanted to SEE the arc jump the carbon track to assure myself this was indeed the problem.
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