Who woulda' guessed?

 

I absolutely agree. A scope is only needed when trying to debug an especially tricky ignition problem, designing an ignition system, or trying to explain, on an engineering level, how the system actually works.[/COLOR]

 

In order to get accurate [quiescent] readings, you MUST be sure the points are OPEN - i.e. matchbook cover, etc. stuck between the contacts to be sure they are.

** Edit ** Sorry 69427 - you got in there while I was typing - and I agree!!

 

OK Gentlemen:
I went back and tested both: points closed & points open

OE Points Ignition: key on but motor not running
Coil + = 7.2 vdc (points closed)
Coil + = 12.5 vdc (points open)
Battery = 12.6 vdc

That's the facts ... and until I can determine otherwise, I defer to you. And, until I can determine otherwise, it seems myself & countless others are dead wrong. For now, I'm mistaken on this ... I still don't know why ... but I'm mistaken.

Perhaps someone can explain this:
Why is it, when a good HEI is installed in a older chevy that has good viable OE breaker-point wiring, it often does not perform well? And ... why is it, when that good viable OE breaker-point wiring is is replaced by a NON-resistance wire, that same good HEI performs better & as it should?

*edit*
Is it simply because the heavier gauge replacement wire can supply more current (not necessarily more voltage) than the resistance wire? ... as was indicated a previous post?

 

[QUOTE=jackson]OK Gentlemen:
I went back and tested both: points closed & points open

OE Points Ignition: key on but motor not running
Coil + = 7.2 vdc (points closed)
Coil + = 12.5 vdc (points open)
Battery = 12.6 vdc


The reason you measured 12v when the points were open is because the circuit is "open" and there is no current flow. You can measure any spot starting at the plus side of the coil all the way to the terminal screw on the point set.... including either resistor lead, and you will measur 12V when the points are open. Try it.

In order for there to be any voltage differences in the "circuit", current has to flow. Except for the initial charging of the capacitor (condensor), there will be no current flow when the points are open.

What you observed all makes sense.

 

pws69,

I greatly appreciate your contribution. I'm getting tired trying to do all the typing on this side of the thread.

thanks,
Mike

ps: My compliments on the color of your '69. Very good choice.

 

The original question was about control modules. I have a 75 and do not have points. Do the points style distributors have the boomerang control module? I know on mine which is an HEI there is only one wire that goes into it, and it’s 12V.

 

I think you have correctly answered your own question.
Most points systems run about 2-3 amps (it depends on the total resistance of the ballast wire and the coil winding resistance). The HEI distributor module, by contrast, will automatically vary its dwell period to try to force about 5.5 amps through the coil. The lower inductance value of the HEI coil needs this extra current to deliver the desired energy to the plugs. (I have a formula to explain this, but I've caught too much crap previously when I do that. ) If the ballast resistor/wire is too high in resistance then the coil "charging" current is insufficient to deliver enough energy into the coil, and then into the plug, to guarantee a good light-off of the fuel. This is the reason why, unless you actually measure (or read) the specs of an ignition coil, you're just flipping a coin to determine if you gained or lost performance when you change ignition coils. It might be bright and shiney, but that means nothing to the spark plugs.
Additionally, this HEI 5.5 amp primary current will cause an horrendous voltage drop across the ballast resistor line, assuming it can reach anywhere near this current. The HEI module, which would be powered off this line, really prefers that its supply voltage remain above 6 volts, preferably above 10.
It may not have been mentioned in prior posts, but I do enjoy our back and forth questions and comments. Ninety eight percent of the Forum members are quite polite. (I'm hoping the other two percent were just having a bad day.) So far the Forum has been a very interesting place to share knowledge and experiences.

Thanks,
Mike

 

Mike:
I have no problem w/ a disagreement ... so long as we all agree that we can disagree ... without getting into personal assualt. I also have no problem admitting a mistake or to ignorance ... I didn't know crap when I was born & what I do know now, I've learned ... it wasn't automatic. There was no mudslinging from you ... thanks! You understood this issue where I was ignorant & ill-informed ... plain & simple.

I would really like to know how to both measure and calculate coil performance ... formula not a problem with me. Neither is email.
Jack

 

pooh on them, can't be worst than sqrt(3), right?
i say throw in formulas too if it helps. . .

 

Comparison of commercial HEI coils for energy and saturation time was made here:
http://www.corvetteforum.net/c4/doct...ilcomparo2.jpg

Coil evaluation tool is here:
http://www.bgsoflex.com/igncoil.html

For HEI evaluation you need to observe the current limits built into the GM module -
(~5 amps for a 4-pin, 6 amps for a 7-pin, 7 amps for an LT4) -
and the 'off-time' of the ECM control - which I seem to think was about 1 ms per
spark event (use that when considering the 'dwell angle' of an HEI system) with the
P4 computers.

Beyond that you need to know the coil primary inductance and resistance (if you
can find them published anywhere).


This was all covered in a discussion here, back in the day:
http://forums.corvetteforum.com/showthread.php?t=419893



Most of those links are dead, but the Motorola 4-pin module is still shown here:
http://www1.jaycar.com.au/images_uploaded/MC3334R0.PDF
Note the current-limit feature, and the slope of the current output (V batt /
primary inductance ).


If HEI modules are dying on a regular basis I'd look for a short in the coil primary
wires - the modules don't live long if the output is shorted directly to ground (infinite
slope on the current line).

HTH
(sorry to intrude, I ran across this thread while I was looking for something else )
DrJ

 

Looked over DrJ's stuff. Lots of interesting reading for those of us who get into this sort of detail (or minutia). Thanks!