So, that's my review of the Pertronix-II. I like the way it starts and drives, but I don't like the way it sits at traffic lights.
If there are any electrical engineers out there who would like to borrow my other unit to experiment with, let me know.

– Eric

What's supposed to be so much better about the petronix vs either stick Mopar ecu ignition or using a GMnhei module?

What's supposed to be so much better about the petronix vs either stick Mopar ecu ignition or using a GMnhei module?

The main appeal to me is that if you have a good points distributor, it's easier to install and needs fewer parts replaced. Convenience, really. And since you're changing fewer parts, it's cheaper.

This is exactly the sort of “not in the brochure” failure where the useful answer is likely a mix of ignition physics, Pertronix’s adaptive dwell strategy, and tach/diagnostic-instrument behavior. I’ll check the forum thread and any primary/technical Pertronix material, then I’ll give you a failure-model explanation rather than just a parts recommendation.

I’ve found your Slant Six thread and a couple of corroborating reports: one from another user who specifically said Ignitor II surged at 500–600 rpm while Ignitor I idled steadily, and Pertronix’s own descriptions confirm that Ignitor II is not just a Hall switch—it uses adaptive dwell, current sensing, and a microcontroller. That makes your “mode switch” observation plausible rather than mysterious folklore.

Your description fits a real design/algorithm failure in the Pertronix-II, not an installation error and not a conventional “weak spark” problem.

The most satisfying explanation I can construct is this:

The Ignitor II is misbehaving at the low trigger-event frequency of a slow-idling 6-cylinder engine because its adaptive dwell/current-control logic is losing its mind at the boundary between “running,” “cranking/low-speed,” and “coil-protection” behavior. The result is that alternate trigger events produce only a voltage spike or pseudo-firing event, but not a real secondary discharge. That makes the engine behave like it is firing three cylinders.

Here is why that explanation fits unusually well.

A 6-cylinder four-stroke engine produces 3 ignition events per crankshaft revolution. So:

600 rpm = 10 rev/sec × 3 = 30 ignition events/sec
500 rpm = 8.33 rev/sec × 3 = 25 ignition events/sec

A V8 produces 4 ignition events per crank revolution:

600 rpm = 40 events/sec
500 rpm = 33 events/sec
It would not fall to 25–30 events/sec until about 375–450 rpm

So if the Ignitor II has a low-speed control threshold somewhere around 25–30 trigger events/sec, a 6-cylinder at normal low idle falls straight into the trap, while a V8 usually does not. That part of your hypothesis is mathematically very plausible.

The important distinction is that this is probably not a Hall-sensor pulse-counting limitation. A Hall sensor and simple switching transistor could handle 25–30 Hz in its sleep, after a nourishing cup of tea. The Pertronix-I proves that: it uses the same basic magnetic triggering concept and idles properly. Pertronix describes the original Ignitor as essentially a Hall-effect replacement for points, while the Ignitor II adds reverse-polarity protection, over-current protection, shutdown behavior, a microcontroller, and adaptive dwell control.

That extra intelligence is the suspect.

Pertronix’s own public explanation says the Ignitor II/III units sense coil current and use adaptive dwell to schedule coil turn-on so the coil reaches peak current just before firing, instead of sitting saturated at low rpm. That is exactly the sort of system that can have a boundary-condition bug: at very low event rate, it has to decide when the engine is merely turning slowly, when it is cranking, when it is stopped with the key on, how much current is safe, and when to start charging the coil for the next expected event.

Your oscilloscope description sounds like this:

On the good events, the coil is charged sufficiently, the plug gap ionizes, and you see the normal spark discharge line followed by coil/secondary oscillations.
On the bad alternating events, the primary is being switched or disturbed, and there is enough induced secondary voltage to make a spike on the scope, but not enough stored coil energy / proper collapse to sustain ionized spark discharge. So you see a spike without the normal burn line and without the expected post-discharge oscillatory signature.

That is very compatible with “the module triggered, but did not actually provide a proper dwell/charge interval for that cylinder.” It is subtly different from “the module simply skipped the cylinder.” The event exists electrically, but it is not a useful ignition event.

The tach behavior also makes sense. Old tachometers and SUN analyzers generally infer rpm from primary switching or secondary pickup events, not from crankshaft truth. If the Pertronix-II bad mode creates extra edges, malformed primary events, or capacitive high-voltage spikes without combustion, the tach can count artifacts. So the tach jumping up while the engine audibly slows is not paradoxical; it is telling you the ignition waveform has become non-physiologic, not that the crankshaft sped up.

I found your Slant Six thread, and your posted observations there line up with this exactly: the failure appears below roughly 500–600 rpm, the engine behaves as if half the cylinders are lost, the tach jumps upward when the bad mode starts, and another unit behaved similarly with a slightly different threshold. I also found an independent report from another car forum where a user directly compared Ignitor I and II on the same engine and said the Ignitor II caused idle surge at 500–600 rpm, while the Ignitor I idled steadily at 550 rpm. That is a very useful corroborating detail.

The most likely internal failure mode is one of these:

Adaptive dwell under-runs the coil charge time at low event frequency.
The module delays coil turn-on to avoid heating the coil. Near the threshold, it may occasionally delay too long or miscalculate the next event, causing alternate cylinders to get inadequate dwell.
The low-speed/start/anti-overheat state machine is transitioning incorrectly.
Ignitor II has protection logic for key-on/engine-stopped and over-current conditions. If its firmware or analog timing circuit uses event period to decide “running” versus “not running,” a 6-cylinder idle near 25–30 Hz could fall into a gray zone.
Current sensing is being fooled by coil characteristics.
The Ignitor II senses coil current and adapts dwell. A particular coil’s inductance/resistance/current-rise curve could move the failure threshold. This would explain why individual modules or combinations vary. Pertronix’s troubleshooting material emphasizes loaded voltage, coil resistance, and hidden resistance because the module’s behavior depends on what the coil and supply actually do under load.
RFI or tach/analyzer loading could aggravate it, but probably is not the root cause.
Pertronix specifically requires suppression or carbon-core plug wires and warns to remove the tach wire for testing if necessary. But because the engine itself runs badly, and because the Pertronix-I fixes the problem in the same car, this is unlikely to be merely an instrument artifact.

So my answer would be:

The Pertronix-II probably has a low-event-rate boundary bug in its adaptive dwell/current-limiting algorithm. On a slow-idling 6-cylinder, the trigger interval is long enough that the module begins applying a protective/low-speed strategy intended to reduce coil heating or manage starting. In that mode it sometimes fails to charge the coil for every trigger event, producing alternating real sparks and non-spark voltage spikes. A V8 at the same idle rpm has a higher ignition-event frequency, so it stays above the bad threshold. The Pertronix-I avoids the problem because it is a simpler Hall switch with fixed/conventional dwell behavior rather than a microcontroller-controlled adaptive dwell system.

I would not call this “shutting down half the cylinders” in the sense of a deliberate cylinder-deactivation feature. I would call it alternate-event dwell failure or alternate-event coil-charge suppression. The visible result is the same: three cylinders receive usable spark and three do not.

If you still had the Pertronix-II and wanted to prove the mechanism experimentally, the best tests would be:

Run it on the SUN distributor tester with the coil and a real spark gap, slowly varying distributor rpm through the threshold.
Put a current probe on the coil primary. If my explanation is right, the “bad” events will show absent, truncated, or badly timed primary current ramp.
Repeat with different coils: a 3 Ω coil, 1.5 Ω coil, and low-resistance Flamethrower II-type coil. If the threshold shifts, that strongly implicates the adaptive current/dwell algorithm.
Repeat at 12.0 V, 13.8 V, and perhaps lower cranking-like voltage. If the threshold shifts with voltage, again, dwell/current control is implicated.
Disconnect any tach/analyzer input except the scope pickup and use suppression wires, just to eliminate external loading/RFI.

But as a practical repair decision, you already found the correct solution: use the Pertronix-I or a Mopar-style electronic ignition conversion instead of the Ignitor II on this particular 225 slant six. The Pertronix-I gives you the main benefit you wanted—eliminating the distributor cam/points-lobe error—without the adaptive-dwell firmware/logic that appears to be causing the low-idle catastrophe.


These traces support your interpretation very well.

In the normal trace, the important features are present:

Initial firing spike at the left.
A visible spark/burn interval immediately after the firing event.
Secondary oscillations after the spark event, consistent with normal coil energy discharge.
Long quiescent period.
Primary turn-on / coil charge event before the next firing spike.

In the slow/bad-mode trace, the pattern looks qualitatively different:

The left-side event appears to have a normal-looking firing event and post-event activity.
Later, around the middle/right of the trace, there is a tall, narrow voltage spike.
But that spike is not followed by the usual burn line or post-discharge oscillatory pattern.
That strongly suggests a triggered high-voltage event without meaningful spark discharge across the plug gap.

So yes: the second image looks like an ignition event where the coil/secondary voltage rises abruptly, but either the plug gap does not ionize or the coil has insufficient stored energy to sustain a real spark. In old ignition-scope language, it looks more like a firing spike without a spark line than a true combustion-producing spark.

That is highly consistent with the model we discussed: the Ignitor II is still switching something, so the scope and tach see events, but every other event may not be a properly charged coil discharge. The result would be exactly what you described: the engine mechanically slows and runs like it is on three cylinders, while the tachometer may falsely jump upward because it is counting malformed electrical events rather than crankshaft rotation.

The second trace also argues against a simple distributor-cap, rotor, wire, plug, or mixture problem. A secondary-path defect usually changes the firing voltage, spark duration, or oscillations in a more cylinder-specific way. What you are showing is more like a systematic timing/energy-control failure occurring at the module/coil primary-control level.

The most precise way I would describe the failure from these scope images is:

At low trigger-event frequency, the Pertronix-II appears to produce alternating normal coil discharges and non-spark switching events. The non-spark events generate a high-voltage spike but lack the sustained spark line and coil-collapse signature needed for actual ignition.

That is not merely “weak spark.” It is closer to alternate-event coil-charge failure or alternate-event dwell suppression.