I am trying to eliminate the need for MSD tach adapters and R109 bypass by putting together a table of the 255996-018 tachometer capacitors at C3 and C5. I do not yet know if they differ by range of years as well as model. What I have so far is:
SC400 C3 is 35v 4.7uF & C5 is 50v 2.2uF
SC300 C3 is 25v 10uF & C5 TBD

Since these capacitors are in parallel with R4, C(Total) = C3+C5. However, parallel capacitors charge to the same voltage so why is one on the SC400 rated at 50v and the other rated at 35v while the one I know about on the SC300 is rated at 25v?

 

But what's wrong with R109 bypass ? i am confuse .

 

I read several threads and members reported mixed results. It is unclear to me it offers a solution for both the SC400 cluster going into the SC300 and the SC300 cluster going into the SC400 whereas the proper capacitors would allow full portability.

 

That was interesting...I soldered a 50v 2.2uF capacitor onto the damaged SC300 at C5 and the tachometer came back to life. Better, it displays the correct RPM for my SC300. I did not receive any feedback from the Forum on the correct capacitor but this one seems to have done the trick for my immediate problem.

Does anyone have input on how to pair up the capacitors at C3 & C5 so the tachometers can be modified for either six or eight cylinder operation?