Tach
Thread Starter
Heel & Toe
Joined: Mar 2008
Posts: 20
Likes: 0
From: Georgia
Tach
I have a 1979 Coupe. The tach works but it's showing a progressively higher RPM than the engine is actually turning. For example at idle it seems to be correct, running about 800 RPM. At 60 MPH the tach is showing 5500 RPM, but I can obviously tell that the engine is not turning that fast. It sounds like it's turning about 2500 RPM (I'm guessing). This vette has an L-48, Turbo 350 trans and a 355 diff.
I changed out the tach's printed circuit board but it made no difference. Could it have anything to do with the tach filter at the distributor?
I changed out the tach's printed circuit board but it made no difference. Could it have anything to do with the tach filter at the distributor?
Le Mans Master
Joined: Sep 2002
Posts: 5,236
Likes: 898
From: Myrtle Beach SC
I have a 1979 Coupe. The tach works but it's showing a progressively higher RPM than the engine is actually turning. For example at idle it seems to be correct, running about 800 RPM. At 60 MPH the tach is showing 5500 RPM, but I can obviously tell that the engine is not turning that fast. It sounds like it's turning about 2500 RPM (I'm guessing). This vette has an L-48, Turbo 350 trans and a 355 diff.
I changed out the tach's printed circuit board but it made no difference. Could it have anything to do with the tach filter at the distributor?
I changed out the tach's printed circuit board but it made no difference. Could it have anything to do with the tach filter at the distributor?
Racer
Joined: Jul 2005
Posts: 420
Likes: 0
From: Yerington NV
I have a similar issue, but not as severe as yours .... when I had car at dyno, the tach reads 6000, but the dyno indicates 5700 ..... Irritating knowing that, but not enough to go back into dash ....
The tach filter should not be affecting your reading, so I would venture to guess that there is something tach-board related, is the stock HEI ignition still on car?
The tach filter should not be affecting your reading, so I would venture to guess that there is something tach-board related, is the stock HEI ignition still on car?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
It could be the filter, if it was 'passing' unwanted pulses. Try repairing [or replacing] the tach filter first, as it is easier to do. It probably needs it anyway. {search for old threads on "tach filter"...there is info detailing the parts in it; you can make new 'guts' for pocket change by buying the parts at Radio Shack}
Melting Slicks
Joined: Jun 2005
Posts: 2,239
Likes: 74
From: Palatine, Peoples Republic of Illinois
St. Jude Donor '09-'10-'11-12-'13-'14-'15-'16-'17-‘18
NCM Sinkhole Donor
My understanding is that the tach filter only filters out unwanted RF signals that can effect the radio. It has no effect on how the tach works. I have no filter on and the tach works great.
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
The problem is cheap foreign replacement citcuit boards that are not calibrated. You need to purchase a board that is calibrated. I bought mine from Mid America, and it's way off. Maybe someone will mention who sells the calibrated board.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The tach filter has nothing to do with the radio. It is intended to 'filter' out any spurious signals that do NOT come from the tach signal board. There are several radio filter capacitors in a C3 intended to prevent noise and pops from getting into the radio signal. Two entirely different things.
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
EGT1,
This problem existed before you changed the board I would suspect it’s an input problem or something else since the new board is doing the exact same thing.
There are a couple of different people making the board and I don’t know which one you purchased. We test ours before sending them out. It is easy to blame the board but if with your original tach board doing the same thing as the new one I would look for a different problem.
If you want to send in the Tach to us, I’ll check it for you for free. This would eliminate or pinpoint the tach/board as being the problem.
Again, our boards are tested and there are plenty of forum customers that have purchased it without any problems.
Willcox Inc.
This problem existed before you changed the board I would suspect it’s an input problem or something else since the new board is doing the exact same thing.
There are a couple of different people making the board and I don’t know which one you purchased. We test ours before sending them out. It is easy to blame the board but if with your original tach board doing the same thing as the new one I would look for a different problem.
If you want to send in the Tach to us, I’ll check it for you for free. This would eliminate or pinpoint the tach/board as being the problem.
Again, our boards are tested and there are plenty of forum customers that have purchased it without any problems.
Willcox Inc.
Last edited by Willcox Corvette; Feb 23, 2009 at 10:24 PM.
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Gone, but not Forgotten
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St. Jude '03-'04-'05-'06-'07-'08-'09
I'd check with our supporting vendors. Let's remember, they are the ones who pay the bills here. Please do NOT post links/info to non-supporting vendors.
I think Wilcox, for example, has given you some sound advice.
I think Wilcox, for example, has given you some sound advice.