75 corvette tach filter diagnostics
Former Vendor
Joined: Aug 2006
Posts: 76,656
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Not without a scope... Pull the wire from the tach filter and plug A into the distributor as shown below. If the tach starts working the filter is the issue. If the tach doesn't work and you have continuity from the connector to the gauge.... then you have a board issue and then I'd review our page on how to pull the left side dash pad the easy way.. listed at the link below the pictures.
http://repairs.willcoxcorvette.com/w...ad-removal.pdf
Willcox
http://repairs.willcoxcorvette.com/w...ad-removal.pdf
Willcox
Last edited by Willcox Corvette; Jul 29, 2017 at 10:11 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
The tach filter eliminates extraneous pulses (ignition 'junk') from being read as engine revolutions. If the filter is not working properly, the tach will read higher than it should and it will be a bit jumpy when the engine is at a fixed elevated speed (ie, 2000 rpm). If you have another way to measure engine rpm (dwell meter/tach meter), hook it up and compare with each other at various engine speeds.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The main purpose of the filter is to knock the edge off the square signal before it hits the board (converting it to a wave)... Why GM had this in place I have no idea because the LM1819 (just like the chip we use in the manufacturing of the reproduction boards) chip can read both signals square and wave, so the only logical explanation would be to prevent spikes and with this prevention the square is knocked off. Once the signal is past the filter, it's a wave. Then it hits the tach board chip and the square is added back to it....
But.. the board will read correctly with either signal as a base input if all is well with the board there should not be any discrepancy in the reading.
The crazy thing is that GM designed two different boards, one for the Corvettes that used a single common and one that was swapped around for the trucks and other cars that used a double common of which both used the same chip just a different board design.
I used to make the boards for the trucks and sell them but this became an issue when we didn't have any clue before hand if the board was a dual common or single so you can imagine the issues this would create. So now we only do the truck (and nova, Camaro and other boards) if the customer sends us the tach.
Ernie
But.. the board will read correctly with either signal as a base input if all is well with the board there should not be any discrepancy in the reading.
The crazy thing is that GM designed two different boards, one for the Corvettes that used a single common and one that was swapped around for the trucks and other cars that used a double common of which both used the same chip just a different board design.
I used to make the boards for the trucks and sell them but this became an issue when we didn't have any clue before hand if the board was a dual common or single so you can imagine the issues this would create. So now we only do the truck (and nova, Camaro and other boards) if the customer sends us the tach.
Ernie
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
The original tach filters from the factory were nothing but resistors and capacitors. It got rid of "noise" in the tach signal. Modern 'signal conditioning' systems are better, I'm sure; but I don't know how necessary they are.
Le Mans Master
Joined: Jan 2001
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If you're not sure after the above testing, then just make a filter.(can you solder?)....should cost about 2 or 3 bucks in parts...and replace the one you have. If it works then you know what you need. If it doesn't them you'll know before you tear the dash apart. Another thing you can do is use an aftermarket tach to see if the leads/signals going to the tach are good....that's what I did before tearing my dash apart to install an electric tach.
