ok, here's a thought. all you guys with high reading tachs..
start motor, observe tach reading. put on headlights, and pull the stalk to flick on the high beams. observe the tach. if it fluctuates or goes negative, go to the fix part.
if no change, click on a turn signal, try each way and see if tach changes.
if either of these cause the tach to change:
on the passenger side of the instrument cluster, there is a connector that plugs in, if the ground point on that has a bad connection, the tach goes whacko, and turning on those lights only used occasionally will also use that ground connection. you can try to unplug that and replug it, if the porblem gets better or goes away, you know what the problem was. the contacts on that plug are formed by the traces on that flexible printed circuit board, and they can peel up and either break off or not contact the proper pins on the plug.
the tach is the one that seems to see the effect of this the most, but sometimes you errant lights like one turn signal arrow glowing very dimly. just a thought to check for.
the tach filter usually causes erratic readings, not way off readings. there are a few people who run without them wiht no ill effect.
if you search, it is a simple device, you can make your own for a few bucks with radio shack parts. there are a couple of schematics for it floating around the net.
here is alink to one:
http://www.fieros.de/gif/tachfilter.gif
