My tach was a bit jumpy, and low RPM was a bit shakey (weak) so I unpluged it......and all my problems were solved. Pluged it back in about 100 miles later. No more jumpy tach, not so shakey in the lower RPMs......but acceleration was noticeably weaker! I left it pluged in for about a month more, then I just unpluged it. WOW, I havent felt acceleration like this in a while!!

Now my question is, how can a simple electronic device that filters the electrical current so I can feed the tach with just enough energy to read the RPMs accurately, throw off the entire engine, greatly hindering engine performance?

This one stumps me........

Thats a good question, wish I could help. The only thing I know is Ive heard some people say that theres cracks and if it gets wet, it causes iratic idle and poor performance. Cooky as it sounds, apparently it does have an affect. Might just want to replace it.

Well, since the tach filter's connected to the primary of the ignition coil (at the same points as the ignition module which controls the spark), I can imagine it doing all kinds of mischief. Particularly when you think about the fact that the filter's case is connected to ground. Especially if moisture gets inside and breaks down the resistors in there.

Actually, I'm just making all this up, but it sounds good, doesn't it?

[This message has been edited by StuGood (edited 06-19-2000).]

The tach filter is just a plain capacitor. Capacitors, when functioning normally will not allow DC to pass, but will allow AC to Pass. In this case, the capacitor will allow the "pulse train" from the distributor to pass through to the ECM and tach, but does not allow any other DC electrical signal through.

When it fails, it in effect shorts out part of the ignition system and you end up with a lower amount of "spark" to the plugs, which will kill some efficiency and power.
You also will end up with transient signals bouncing around in your ECM and gauges causing various other things to south.

Agree that the tach filter LOOKS like a plain old condenser. And you're right - it seems to be a plain old capacitor. Plus 2 plain old resistors! It has a wire coming out of both ends, and the case is grounded. It's a 3-terminal device. Look at:
http://www.franken.de/users/dagobah/luke/fieroidx.html Click on "Fiero Electronics", then scroll down to where it talks about the tach filter. Click on the word "schematic," and there you go.

Notice also that the photo of Rodney Dickman's replacement tach filter has what appears to be a grounding lug on it. Three-terminal device, I think.

Of course, I just couldn't stand not knowing! I checked the filter on two of my cars with an ohmmeter. It showed 13k-ohms across the terminals. On both cars. That tends to agree with the circuit shown on the above website.

Made me look-!

In any case, I think you are absolutely right that a bad tach filter could wreak havoc on the ignition.

[This message has been edited by StuGood (edited 06-19-2000).]

The tachfilter is actually two low-pass filters in series. It consists of 2 capacitors and two resistors:

Thank goodness for good administrators! You barely saved my perfectly good filter. Was just now reaching for the can opener...

OK - now I'm ready to let this... pass .

[This message has been edited by StuGood (edited 06-19-2000).]