Does the Tach in a 1980 require a filter?
 

Hi Everyone,

I was wondering if anyone knew whether or not a filter is need on a 1980 and up tach cluster on a 1980 pick-up.

Thanks!

David

Re: Does the Tach in a 1980 require a filter?
 

I have never seen any kind of filter on a tach harness for the 79-80

Re: Does the Tach in a 1980 require a filter?
 

yea, a fram is the way to go!

Re: Does the Tach in a 1980 require a filter?
 

Come on now, I need to use a good K&N filter for that tach.... LOL hahaha...

Re: Does the Tach in a 1980 require a filter?
 

I didn't think I remember seeing any filter on a 79' - 87' Tach, but I just wanted to be sure.


So on the 73' to 78' Tach clusters a Filter IS Required???

And if you don't use one, will it wreck the tach? The reason why I ask is I have a quite a few Tach clusters I was going to check but I want to know ahead of time if I need to test them using a filter. Accually one of the clusters I've had stored forever I think is a NOS 77' Tach cluster, if not it must be a reman. or something because it looks brand new.

Re: Does the Tach in a 1980 require a filter?
 

I believe the tach filters were used simply to prevent radio interference. So you won't damage the tach by not using a filter.

By the way, the tach filter's internal circuit is shown in some of the factory wiring diagrams and a functional equivalent could easily be built using parts from RadioShack (or other electronics stores).

The circuit consists of two resistors in series, and a couple capacitors to ground. The "filter input" wire coming from the negative side of the coil (TACH terminal on HEI) goes to a 5.6K resistor, followed by a 10K resistor (in series), which then goes to the "filter output" wire that plugs into the tach harness. A .15MFD capacitor connects from ground to the junction point between the two resistors. And a 100pF capacitor connects from ground to the junction point between the 10K resistor and the "filter output" wire.

Re: Does the Tach in a 1980 require a filter?
 

I didn't use one on mine when I installed it. However, they used to be pretty easy to find in 70's & 80's cars factory equipped with a tach. May be worth statching one if you see it.