Re: Filter, the pic posted by 69
Hurst Olds shows both the general physical description of the filter, where it ought to be found on the fire wall and who to score a new one from. My own Monte is an 85 with the factory gauges and came with one of these by default. All I can say is that the Tach needle seems to rise and fall smoothly according to whatever rpm is being generated by the engine and there is no bounce or stutter.
To troubleshoot this, were it me, I think I would investigate to learn whether that filter is there or not. If mia, it can be acquired and replaced into the tach harness. If there, it could still be damaged and need replacing so having a spare just in case is not a bad thing. Along the way the harness itself can be examined to make sure there is a good tight connection at the distributor head and no damage to any of the wire along its length.
Once that wire passes through the wirewall, it becomes kind of difficult to trace but ultimately it does end up at the plug in for the gauge cluster. The cluster, unlike previous iterations, is not wired on a gauge by gauge basis and no wires go direct to their respective gauges. They, the gauges, are all attached to a mylar strip that has thin ribbons of copper embedded in it to act as the contacts. This contact "sheet" can become damaged if the retainers that hold it to the cluster body are disturbed so taking it off is not recommended.
Once you do get to the tach, as described above, the existing unit can be accessed from the front of the gauge body by disassembling the front and just removing layer after layer until you get down to the gauge itself. Once there you should find that it sits on a plastic "pedestal" and is retained by two sheet metal nuts that you have to remove from behind. Those nuts are screwed down onto studs ,One of which is adjacent to a copper strip and from which the tach takes its feed. As for physically turning the tach proper into itty bitty components after that, have fun> Simpler as stated above to treat it as a plug and play and replace your old one with an intact new one if you believe it to be the culprit. Either way you will still have to dissect the dash cluster to get to the tach to deal with it.
(Or forego ithe factory unit altogether and run a huge honking Moroso Street Unit, attached to the dash by boiler size sheet metal screws and duct tape!!!! And yeah this is a dumb idea but you'd be surprised how many pristine dashes got butchered this this.)
Nick
