Long story short, I want to measure the ignition voltage inderectly by measuring the primary voltage. Since the ignition coil is an autotransformer with 100:1 turns ratio, the secondary voltage should be 100 x primary voltage, correct? As for how to measure, Im thinking of using a basic RC integrator ckt. (see pic) and a DVM. Think it'll work ok?

As for why I need to:

I have a vortec truck ignition with the crab style dist., and it LOVES to eat dist. caps. Some last a week or maybe a month, others blow as soon as I turn the key. Always shorts or leakage between two adjacent leads in the cap.

Just some quick back of the envelope calculations show the isolation in the cap between terminal leads should be good to about 50 kV (150 mils of, I think, polyester or something similar between leads). But, the best I get is leakage and intermittent cross-firing at low loads. It gradually gets worse until the cap is shorted internally. Ive run two different coils and modules... same result.

From what I can tell, the module doesnt have any voltage limiting on the primary like the older HEI ones with a bunch of zeners. It looks like the voltage is limited by restricting the max timing based on temperature and baro pressure since the large diameter of the cap and terminal spacing is the primary driver of how much voltage is developed. The module itself just looks like an amplifier (see pic) with a power transistor next to the main board.

 

For the voltage needed to jump to the terminal, using Paschen's law:

x = 293 * p * d / (760 * T)
Vbreakdown = 24.22 * x + 6.08 * SQRT(x)

p = pressure in Torr (mm Hg),
d = distance in cm,
T = Temperature in Kelvins
Vbreakdown in kV

with the distance between terminals and 30 degrees of advance, the predicted break-down voltage is 37.5 kV, which is quite a bit. It stands to reason that, at least for a few nanoseconds while the current pulse makes its way down and back along the wire, the cap is subjected to a lot of voltage.

 

Easier to measure primary voltage more directly by seeing how much gap it will jump, no?

 

What I want to see is what the voltage is in operation. What Im planning on doing is hooking this up and recording the output on my DVM while I drive the car. What I want to know is why it eats one cap after the other. I suspect its the large terminal base circle needed for the vortec distributor cap. Corona discharge voltage break-down has been a major issue on this ignition system. Its not the spark plug gap as thats set to .045".

 

As for that, I suspect that may not be the highest voltage the ignition system sees. If the gap in the dist. cap is the higher of the two voltages, that will be the peak voltage the cap and coil will be subjected to. In this case, the spark plug gap has no effect on the life of the cap and how much corona discharge there is at the coil. Closing the gap down saves the spark plug wires, but the coil and cap are still suffering from corona discharge when the engine is running.

 

I'm talking about a calibrated spark tester gap, not a plug gap.

 

Im aware thats what you where getting at, but would that be able to give any info on the voltage other than the max the system can put out? The gap would tell what the max the cap can sustain before it pukes, but in this case it would be an expensive $100 do once experiment. It would be nice to have a non-destructive way to just passively monitor it.

I do agree that it may still be worth it to lock it out at 0 advance and crank the gap up till it blows a hole in the cap.

 

I think the solution to this is actually just to move the distributor until its 20 degrees advanced to reduce the distance to the terminals. The PCM controls the coil directly, so the dist. can be put where ever. But it would be nice to have a way to quantitatively measure the voltage difference.

In a way, I already do have a calibrated spark gap in the distributor by measuring the distance between the terminals and the distance from the rotor, which gives a max voltage of 38 kV at the max allowed advance of 33 degrees. The distance there is .52" from the rotor tip to the terminal.