Mod75

Doing some tune up stuff and notice that there is no tack filter on my 75.
The tack line goes straight from the dist into the firewall along with the bat wire. Not really sure when or why i removed the filter.
My tack sits at 1700 and doesn't move much, maybe 200 rpms up or down.

Looking at the Willcox tack filter instruction for 75-89 it say the wire can be removed from the filter for testing. My question is what does the tack filter out? static? Is it cleaning up the signal? And would a filter get the tack operating again. Are there any tests i can try with out pulling the tack out?

KenSny

The tach will function without the filter.

It was only used to filter the signal to smooth out the tach needle. Sounds like you need the circuit board is fried.

0Willcox Corvette

It smooths the square signal knocks off the edges...

bluthundr

Mine did the same thing. A new board from Willcox did the trick. I added a tach filter too at the time, I did not have one installed before. It isn't too hard of a project, the most time consuming part is getting the column in and out.

See http://www.youtube.com/watch?v=bA3khKLw2Ys

mrvette

Surely there are still copies of the electrical schematic equivalent of that tach filter here or elsewhere online,...easy build if Radio Shack has the parts anymore.....

we used to call them pulse integrators back 30 years ago in the electronic engineering test lab.....doing amazingly similar work to ignitions systems.....been a while....

the configuration is of a PI network....IN/resistor/cap to ground/resistor/another cap to ground, but I forget the values....damnit....

and no way to post it even IF I can find the schematic now....

bluthundr

bluthundr

If I get bored here in the next few days, I'll drag my oscope out to the garage and take pictures of the waveforms before and after the filter.

George Ries

should i use a tach filter with my pro billet Msd HEI on my 76 sbc i just install it today

KenSny

Does the tach needle bounce alot?

George Ries

it does a little and the filter is hooked up just not sure if i need it when i drove it yesterday it seemed to be off at higher rpms and moves around

mrvette

With MSD you have to use the MSD box tach output, not the stock one on the HEI.....that is why your tach is acting up.....I don't think you need the filter at that point.....

George Ries

put the msd in yesterday and it ran fine used the two blue springs that brings advance in at 2800 and the 21 degree stop bushing. now tonight get in the car to go for a drive and the tach is jumping around and the engine is pinging and popping out the pipes when you get on it hard it didnt do this last night any advice

SteveG75

When I installed my MSD E-Curve distributor and 6A box, I talked to the MSD tech and they said remove the filter. Tach works fine. Nice and smooth.