Tachometer filter needed with HEI ignition?

[kmoses]

I am install in a Marelli Plex 201 mechanical advance distributor, Accel 35361 trigger module and Fiat #9938064 coil. Because this is considered to be a HEI ignition, will I be needing a filter between the tachometer and coil to keep from frying the tachometer?

[bluespider262]

If its the stock tach and your spider had electronic ignition w/ the module already, no it is not needed, as it was already HEI.

The HEI filter's main job is actually to drop 1/3 of the voltage as some older tachs used on systems w/ ballast resistors couldn't cope with HEI (or other electronic ignitions) that no longer used the ballast. A 10-15K 1W potentiometer or a couple of resistors can accomplish the same at about $40 less cost.

They also have a capacitor or two to ground, which in conjunction with the resistors, does some high frequency filtering to clean up the signal (thus the filter part of it) but those aren't necessary if your needle isn't bouncing around without them.

[kmoses]

My '79 had a points distributor when purchased. Thanks for the information.

[bobplyler]

The distributor must have been replaced. The 79 came without points. And it used a GM control module.

[bluespider262]

Yeah that's weird. My 79 is also electronic w/ the HEI module. Your tach will work w/out anything extra.

[kmoses]

Yes, it seems a lot of things were "changed" on this '79 before I bought it. But very soon it again will travel the roads, just as it's creators envisioned!