Pertronix failure modes -- high RPM issues?

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ChickenPinz
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Pertronix failure modes -- high RPM issues?

Post by ChickenPinz »

For a long time at higher RPMs (>3,000) my tach (Sun Super Tach II) sometimes bounces around a lot, but it's super stable at lower RPMs. I've largely dismissed it as a tach issue, but today I noticed a cutting out of engine power on the highway and the bouncing needle was more dramatic so the issue seems to be real.

Are there any known failures of the Pertronix setup (with original cap) that show these symptoms?

Just looking to leverage the wisdom of those who've gone before me before digging into the wires/distributor/coil.
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ChickenPinz
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Re: Pertronix failure modes -- high RPM issues?

Post by ChickenPinz »

I've wrestled this for a long time, but after instrumenting the engine, I narrowed down the issue. For some reason, I see 130us (approx) interruptions in the power to the Pertronix, as picked by a siamese splice into the green wire at the 6-pin connector into the heat shroud. This short interruption in power seems to occur at consistent timing relative to the "points" opening/closing, but off-cycle. This causes a spike on the TACH signal (low-side of the coil), and causes the tachometer to get excited (bounce).

Adding a BFC (2200uF 50V Al cap) on the power wire appears to remedy the issue.


I can see no clear reason why this brief interruption in power occurs, especially since it's consistent in timing to the spark, relatively consistent duration, and only seems to occur at high RPM, and *seems* more common when the engine is warm-versus-cold. I'm open to ideas. Remember, this is the 24V bus power from the 6-pin connector, so not affected by the rev limiter which is on the coil power side.

PS: Through the debugging process, I now have a spare coil and spare Pertronix module.
PPS: I also found a loose-wire connection into the suppressor, which I believe was the cause of my loss of engine power randomly. So there were two issues, both causing a bouncy tach, but only one actually causing a power loss (the loose suppressor wire).
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rmel
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Re: Pertronix failure modes -- high RPM issues?

Post by rmel »

Hmmmm, boy, that's mighty perplexing?

Such a clean negative going pulse with little signs of undershoot or ringing.
Even with 2.2Mf of capacitance? Yikes, that is going to be a HUGE amount of
current looking at those edge rates, unless the ESR is outrageous. And it
leads not lags outer events -- WTF ??

Only thing I can think of which doesn't explain the timing relationship is the
Probe and Scope ground reference ?

A head scratcher for sure ://
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712MK

Driver: Ron // KO0Q
ChickenPinz
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Re: Pertronix failure modes -- high RPM issues?

Post by ChickenPinz »

Some of the ringing is inherent in the probing challenges. There's a fair bit of electrical noise, and most grounds gave similar results.

The 24->1->24V pulse is only present without the large cap -- adding the cap prevents it, and there isn't any obvious sign of it then. FWIW, I chose 2.2MF because that's what was on top of the bin with large caps.

The pulse width is the real head-scratcher, as it's pretty consistent (though not exact) at around 130uS. And the edges are pretty clean, not like a wire coming loose. And it seems pretty legit because the low-side of the coil bounces up, and the output of the suppressor bounces up as would be expected when the "points" open. The Pertronix module doesn't find the loss of power very funny, and as expected, opens the output.

I also tried with the blower fan on since I believe that's on the same circuit, thinking that maybe the back-EMF from the blower might provide power it if was an interruption, but it had no impact.

I have to rule out the alternator as a source since it's not 1:1 with the firing cycle, and that really only leave the Pertronix itself.
I have to rule out my Sun Supertach II since it's only connected to the tach signal, and not the distributor power wire. But it's clear why it bounces now!

For now, if the BFC fixes the issue, I can move on with life until I have lots of spare time and decide to dig into this science project again. My next test will be to power the Pertronix with an outside power supply to see if the pulse is still there.
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rmel
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Re: Pertronix failure modes -- high RPM issues?

Post by rmel »

Oh, OK, although a different time-base I thought the
2'nd panel was W/ the capacitor -- that's why my
comment w.r.t. potential high current and no reason
for it. So, the cap did the trick good. Still, that pulse
makes no sense.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712MK

Driver: Ron // KO0Q
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