HEI Distributor Tach issues, Issue has been resolved

I thought the extra capacitor in an HEI setup was a radio supression cap
stops your AM radio buzzing.

i.e its not used as a condenser like with points

presume it will take out a small part of the spike cuased by switch coil off which may or may not avoid overload of the timing circuitry in a basic tacho.
I belive there is some benefit in using one. although i don't on mine.....too much clutter around my coil already

I had the Tach problem, and overcame it mostly with a clamp, a Zener diode between coil - and earth. it probably achieves much the same impact

i have some idea how it works but not enough to fully explain
it limits a volatge spike from the coil primary while its creating a spark via the secondary, to 400volts, at one of the transistor junctions in the module
and of couse you tacho has to deal with this back emf as well.

basically HEI changed a bit over the years when it first came out it had the capcitor for radio noise superssion, and the diode for protection, The modules could handle 350V back emf on the coil primary side, so those 2 components had multiple jobs one of which was keeping the module alive, the other making AM radio and CB usage possible.

these days the modules are good for 500V and nobody uses AM radio so the diode and the capacitor are not necessarliy used when retro fitting this stuff to another car

i use a diode and my tacho works 99% of the time, previously it was 50% of the time.

presume i don't spike my tacho with some horrible blast of EMF when i switch on now, and i did before.

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potentially total overkill but i like robust....

Bolted into the body or bracket with a wire from it to the coil negative may help. IT ALSO MAY NOT... but its a feature of the factory HEI set up used on 6 and V12 jaguars from the 80s and 90s... they persisted with the origianl specification of capacitor and diode well after GM stopped...lazy design or sensible move i don't know but the jag used a small head distributor in a proper OEM set up..and that small dizzy cap set up may well have dicated 2 components that act as a limiter at a level obove what is needed to make a very good spark.

I'd suggest either capacitor or diode or both will provide some benefit if your tacho is basic hook up to coil negative tacho, with 50 year old capacitors in it, surely part of my problem.

350V one may be better but finding one is hard it needs to be an R diode so that the correct end of it has the threaded bolt like connector for its body/earth mounting

Dave