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Does anyone have a failed 4500 RPM module they are willing to send me so I can dissect it and figure out how it works? Black boxes drive me nuts. Something that hasn't let smoke out of the box would be nice. Hopefully these modules are not fully potted.

I will draw up and post a schematic.

I can send postage to you via PayPal.

Thanks!

I have one. replaced mine.

PM me.

edit: received your PM. sent answer.

They are not potted, just a PCB and discrete components. Good luck figuring out the transistor types!

From what I can tell, the box detects the current pulses to the coil when the points close, and use that frequency to detect RPM. If the RPM gets to ~4500 then it does two things: Lights the 4500 dash light, and either starts to decrease the coil current (or perhaps was intended to interrupt coil current) to weaken or kill spark to prevent revving any higher. One of the common failure modes is to start to reduce the voltage/current to the coil at all RPMs and the spark gets weak. Coil + terminal voltage values when running are normally about 17V, but when my 4500 box failed I only had 8-9 volts at the coil; the engine idled easily enough, but would sputter as the throttle was opened.

FYI, since the coil should get only about 17V, a Pertronix would not see even that much voltage; this is why it works with our "24 volt" vehicles reasonably reliably. But if the voltage goes higher then the Pertronix could be damaged or destroyed.