Interference for the tach or radio? I understand the radio interference.

[QUOTE=mkapp7879]Interference for the tach or radio? I understand the radio interference.[/QUOTE]As mentioned earlier, it stabilizes the fluctuations of the electronics to keep the tach needle from bouncing erratically. 

Because of the fiberglass firewall??

[QUOTE=mkapp7879]Because of the fiberglass firewall??[/QUOTE] Yes

Yes, cars made of steel absorb the RFI, Radio Frequency Interferance, and stops the problems.   But Fiberglass does not do that, so it has to be stopped by other means.

Why is the tach affected by radio frequennce interference?

Remember most of all this whas to keep this noise out of AM radio, most of this will not interere on the FM, with the lack of metal, poor grounding. The filters kept this out. and in the case of the Tach the same static that would be herd in the radio, would be efffect the Tach. Rember resistor and no resister plugs?

I understand the filter taking noise out of the radio, but why is there "noise" in the tach when the signal comes through a wire?

Any disruption in an electrical signal is considered electrical noise.  It creates false signals.  In the case of a radio, you actually get noise from the speakers, but in the case of anything with electronics the electrical "noise" caused functional problems due to confusing the computer circuit.  The tach board is a simple computer electronic circuit.

Well, I've replaced 2 tach boards in the last three years so it must not do a very good job protecting em!(2 dif Vettes)