Tach Filter
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From: Vinemont Al
Tach Filter
Tach won't work on 89 . Strobes up when switch is turned on but will not register rpm afterwards. Will tach filter fix? Need cluster repair?
Where can I find tach filter since it is discontinued by GM and parts suppliers? I seen a site on the web that has them but cannot find one now. Thanks!
Where can I find tach filter since it is discontinued by GM and parts suppliers? I seen a site on the web that has them but cannot find one now. Thanks!
Melting Slicks
Joined: May 2004
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From: Lufkin Texas
St. Jude Donor '05-'06
I'm not even sure what it's for as no one at GMC dealer could even tell me. I think it's to eliminate noise from tach signal on stereo. On my 90 it took a new instrument cluster to fix tach and I never even put the filter back on and it works great. But it is a different dash than your 89. But the filter still shouldn't be a big issue.
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From: Mexico Beach FL
The tach signal, on a stock system before about the '96 model, is derived from the primary of the ignition coil. The signal at the coil has a high-frequency (oscillation/ringing) component riding on a low-frequency square wave component. The tachometer only wants to see the square wave component. So, the tach filter is designed to filter out the "ringing" while leaving the square wave component to pass on to the tachometer.
After-market ignition systems, already have a "conditioned", or filtered, square wave tach signal. Therefore, they do not need the tach filter -- in fact, the tach filter will attenuate their signal and may cause problems. These problems may include a "rounding off" of the square wave.
The '96 model C4 has a dedicated, conditioned tach output signal from the ECM and does not need a tach filter.
Most CD ignition systems do not need a tach filter.
Most multi-coil ignition systems do not need a tach filter.
Tom Piper
After-market ignition systems, already have a "conditioned", or filtered, square wave tach signal. Therefore, they do not need the tach filter -- in fact, the tach filter will attenuate their signal and may cause problems. These problems may include a "rounding off" of the square wave.
The '96 model C4 has a dedicated, conditioned tach output signal from the ECM and does not need a tach filter.
Most CD ignition systems do not need a tach filter.
Most multi-coil ignition systems do not need a tach filter.
Tom Piper
Last edited by Tom Piper; Aug 12, 2004 at 07:20 AM.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
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From: seattle WA
Most tach problems according to Killebrew are caused by water in the main electrical bulkhead connector-where your wire cluster comes thru the firewall.Just unplug the connector and look for greenish or whiteish color-if you see it just blow it out, and leave the black grease in there-it's an anti corrosive grease.When you reconnect it seal the top and sides from further water intrusion.Hope this helps. 
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From: Vinemont Al
Thanks for all the help. I think I have read that Killebrew article. If he is speaking of the main plugs behind the battery I checked them before. They seemed ok.
I just ordered a Tach filter from Ecklers. Guess it will be a waste of money. I just thought it was strange the tach strobes when you crank the car but doesn't register afterwards. Looks like if the cluster was bad it wouldn't do this.
I will try the tach filter and see what happens.
Thanks again to all.
I just ordered a Tach filter from Ecklers. Guess it will be a waste of money. I just thought it was strange the tach strobes when you crank the car but doesn't register afterwards. Looks like if the cluster was bad it wouldn't do this.
I will try the tach filter and see what happens.
Thanks again to all.
Cruising
Joined: Aug 2004
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If your problem is not water tehn you will be glad you ordered it as the 90-91 is no longer available. My guess is that the module will solve your problem. Corvette team at my dealership said this part's failure is common and it does take your tach out
Melting Slicks
Joined: May 2004
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From: Lufkin Texas
St. Jude Donor '05-'06
Originally Posted by Todd90L98
If your problem is not water tehn you will be glad you ordered it as the 90-91 is no longer available. My guess is that the module will solve your problem. Corvette team at my dealership said this part's failure is common and it does take your tach out
http://www.pixfiles.net/pix/805555595973936840.jpg
My tach works flawless without it since I got the refurbished cluster from GM.
I really do hope it fixes his trouble though.
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From: Vinemont Al
Just replaced the tach filter today and now my tach works. I have read where many have problems finding one since GM doesn't handle them anymore. You find find some at various prices on the internet. I got one from Ecklers for about 29.00 I think. It's almost impossible to take the old one off since it is on the back of the motor below the distributor. Easiest way is to cut and splice the wires to the new one.
Thanks for all the ideas.
Thanks for all the ideas.
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From: Ohio
Originally Posted by Tom Piper
....
After-market ignition systems, already have a "conditioned", or filtered, square wave tach signal. Therefore, they do not need the tach filter -- in fact, the tach filter will attenuate their signal and may cause problems. These problems may include a "rounding off" of the square wave.
....
Most CD ignition systems do not need a tach filter.
Most multi-coil ignition systems do not need a tach filter.
Tom Piper
After-market ignition systems, already have a "conditioned", or filtered, square wave tach signal. Therefore, they do not need the tach filter -- in fact, the tach filter will attenuate their signal and may cause problems. These problems may include a "rounding off" of the square wave.
....
Most CD ignition systems do not need a tach filter.
Most multi-coil ignition systems do not need a tach filter.
Tom Piper
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From: Mexico Beach FL
Originally Posted by h rocks
Based on your knowledge of things voltage, I should be able to remove my tach filter, as I have an MSD? Right now I have an Autometer tach attached to the column. (in front of the stockie) They both work, although the OE ones are reportedly notorious for inaccurate readings. (Not that I look at it) So if I can pull the filter, anything to clean up the engine bay is a good thing. Your thoughts are appreciated, as that's one area that's voodoo to me!
I've read where GM has discontinued the tach filter, so you might want to keep it in case you change it back.
Tom Piper
