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Author Topic: Tachometer  (Read 2237 times)
1967RS
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« on: December 18, 2009, 11:30:17 PM »

My factory in-dash tachometer reads 200 rpm higher at idle and almost 400rpm higher at freeway speeds. Since you guys have helped solve all my other problems,I thought I would throw this one your way. By the way, I have a GM HEI distributor. a friend told me this may be the problem and I need to install a "electronic signal filter" ??
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JohnZ
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« Reply #1 on: December 19, 2009, 11:52:29 AM »

HEI started in 1975 in Corvettes, and they had a "tach filter" between the distributor and the tach.
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'69 Z/28
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BlackoutSteve
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« Reply #2 on: December 19, 2009, 04:56:44 PM »

Hmmm, now I'm wondering be if I have an MSD (85551) with magnetic pickup, am I'm going to need the same thing?
Will a 6AL make a difference as well?

I just found this..
http://www.msdignition.com/page.aspx?id=3300&terms=tach+filter
..and this..
http://www.msdignition.com/info.aspx?taxid=256&taxid2=267
« Last Edit: December 19, 2009, 05:25:12 PM by BlackoutSteve » Logged

Restoring my RHD 69 Jane in Melbourne, Australia.
http://www.usmuscle.com.au/Forum/showthread.php?t=2840
melav8r
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« Reply #3 on: December 20, 2009, 11:51:50 AM »

Heres a link for a home made filter. I had to make one when I switched to HEI, works great, best $2.00 you'll ever spend.

http://www.camaros.net/techref/electrical/tachfilter/

http://www.camaros.net/forums/showthread.php?t=115010
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dutch
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« Reply #4 on: December 20, 2009, 12:43:13 PM »

HEI started in 1975 in Corvettes, and they had a "tach filter" between the distributor and the tach.

John..
  I installed a Pertronix setup in my '68 Zed a couple of years ago and didn't notice if it was a corresponding problem time-wise, but my tach (Sun Super Tach II) started pulsating on a non regular basis about or close to the time of the Pertronix installation. I read to check for wires being in proximity of a plug wire or similar logical stuff that might cause such a problem - but nothing is obvious or has been found to cure the problem.
  Would or can a Pertronix unit (its the newer version - sorry the exact model name escapes me right now) cause the same type of problems as a HEI installation or something similar to this?
  My tach seems to read in the correct RPM range it should when at idle or driving - it just twitches up the scale every secong or two like its getting a signal from somewhere jolting it... or maybe it just needs replacement or a get a rebuild if that is possible somewhere.
  Sorry of this sort of highjacks this thread but it seemed somewhat related...
   Thanks - Randy
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JohnZ
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« Reply #5 on: December 21, 2009, 11:55:43 AM »

John..
  I installed a Pertronix setup in my '68 Zed a couple of years ago and didn't notice if it was a corresponding problem time-wise, but my tach (Sun Super Tach II) started pulsating on a non regular basis about or close to the time of the Pertronix installation. I read to check for wires being in proximity of a plug wire or similar logical stuff that might cause such a problem - but nothing is obvious or has been found to cure the problem.
  Would or can a Pertronix unit (its the newer version - sorry the exact model name escapes me right now) cause the same type of problems as a HEI installation or something similar to this?
  My tach seems to read in the correct RPM range it should when at idle or driving - it just twitches up the scale every secong or two like its getting a signal from somewhere jolting it... or maybe it just needs replacement or a get a rebuild if that is possible somewhere.
  Sorry of this sort of highjacks this thread but it seemed somewhat related...
   Thanks - Randy

I don't have any experience with Pertronix units and tachometers - you might want to contact Pertronix and ask them.
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'69 Z/28
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BILL69
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« Reply #6 on: December 21, 2009, 02:11:21 PM »

I have had this type of problem with a repop tach and a MSD distributor.   MSD said to use a tach adapter module between the coil and tach,  it did not cure the problem.   MSD then sent me the filter free of charge, it worked.  I don't know if the same type of signal mis-match is occuring with the HEI but it seems to be so.
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rich69rs
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« Reply #7 on: December 21, 2009, 05:13:05 PM »

Pertronix conversion and my dash tach worked just fine - no issues.
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Richard Thomas
1969 RS
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