Tach Filter

[bigben5054]

I picked up a used tach cluster to replace my regular gauge cluster.  Not sure of the year, but it looked the same as the gauges in my 1979 with the exception of it had a mechanical oil gauge and mine was electric.  My question relates to the tach.  It had the factory three wire harness still on it, so I ran one wire to the tach, one wire to ignition 12 volts and one to ground.  It works, but it is horribly inaccurate and bounces all over the place.  I figured it must be shot and was ready to buy a new one.  But I have since read that sometimes when you put a factory tach into a vehicle that has HEI, you need a tach filter in the wire that goes to the tach.  Can anyone shed some light on whether my problem is a junk tach versus needing a tach filter?  If I need a filter, any leads on where to find?  The chances of finding one in a junkyard around me are absolutely zero.  Thanks, as always.

[FlatBlack77]

I had that issue with an aftermarket Sunpro tach several years ago. It turned out to be a grounding issue. I had it grounded to the dash but the dash wasnt grounded to the body properly. Theres a point down near the Ebrake pedal where there should be some type of grounding fixture. I used that point and I ran an additional ground from that point to the metal on the dash below the headlight switch and haven't had a problem since.

That was kind of worded badly lol I hope you get what I mean

[bigben5054]

Update:  The tach filter did not solve the problem.  It reads 4,000rpm when its 1,000 - but at least it is a steady signal now.  I try the grounding tip above later today, but I suspect it's just shot.