Tachometer calibration????
Thread Starter
Le Mans Master
Joined: Mar 2005
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From: Rocklin California
Tachometer calibration????
My tachometer reads about 200 RPM less than it should at highway speeds. I have checked the speedometer, calibrated the transmission, rear end, and tire diameter and everything points to the tach. Is there an easy way to calibrate or should I leave it alone as 200 RPM is no big deal. At idle it's dead on, only when I get up to highway speeds is it off. Thanks 
Race Director
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From: CT
There is an adjustment on the tach head, but ithe unit has to come out of the car. You could probably calibrate it yourself if you also install a temporary electric tach. The adjustment on the tach is quite simple.
Thread Starter
Le Mans Master
Joined: Mar 2005
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From: Rocklin California
No, I have a couple different rear ends I have tried along with gear calculators and no matter what rear, it's 200 RPM lower. Only at higher speeds as I have checked it at idle with a dwell tach.
Thread Starter
Le Mans Master
Joined: Mar 2005
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From: Rocklin California
I'm interested in this, any way it can be reached from under the dash?
Le Mans Master
Joined: Feb 2008
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From: Toronto Ontario
My 2 cents.
- Pat
Race Director
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From: CT
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Drifting
Joined: May 2006
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From: Southold New York
A very small mismeasurement in tire diameter can easily account for a 200 rpm perceived variation at speed. It is much easier to try several electronic tachs via temporary mountings to check the car's unit than it is to swap rear axle ratios.
Le Mans Master
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From: MA
larry
Race Director
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From: CT
John - On the C-1 tachs I remember being able to turn the head one way or another (but don't trust my memory!) to adjust the reading? or is that just to zero it out?
Burning Brakes
Joined: Dec 2002
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From: Hasbrouck Heights NJ
Why not verify with the dwell tach? 3 or 4K RPM should be the same in the garage as driving, set the dwell tach so you can see it with the cluster tach and hit the gas. That and I would not expect 40 year old technology to be spot on, the cops used to give you 10MPH grace for speedometer error.
Race Director
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From: Beverly Hills/Pine Ridge Florida
Brian Tilles, the owner, and one of our Chapter members, demonstrated the gauss/degauss machine he has. Actually calibrated at few tachs and speedos for us as a demo.
You spin the tach (or speedo) at a dead on known RPM, then "zap" the magnetic cup either imparting or removing magnetism from it.
THAT'S the way it's done. NO external adjustments at all.
Chuck
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Le Mans Master
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From: Rocklin California
Instructor
Joined: Oct 2006
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From: Jupiter FL
The adjustment of the hair spring on the tach head has to be done in conjunction with the proper gaussing/de-gaussing. You can tweak the spring to read what you want at highway speeds, but that may throw it out of whack at lower speeds.(or higher speeds.)
Former Vendor
Joined: Oct 2008
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From: California
Absolutely correct! There is a small "arm" called a regulator that adjusts hair spring tension to "fine tune" the unit after the proper magnetizing, and demagnetizing procedure is followed. The unit must come out of the cluster to be serviced. Your tach is probably closer than you think, I would leave it alone. Another thing to think about, do you know that the speedo is "on the money" ? It may be off just a few mph and is compounding the problem.
Race Director
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From: CT
Absolutely correct! There is a small "arm" called a regulator that adjusts hair spring tension to "fine tune" the unit after the proper magnetizing, and demagnetizing procedure is followed. The unit must come out of the cluster to be serviced. Your tach is probably closer than you think, I would leave it alone. Another thing to think about, do you know that the speedo is "on the money" ? It may be off just a few mph and is compounding the problem.
Thread Starter
Le Mans Master
Joined: Mar 2005
Posts: 7,631
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From: Rocklin California
Absolutely correct! There is a small "arm" called a regulator that adjusts hair spring tension to "fine tune" the unit after the proper magnetizing, and demagnetizing procedure is followed. The unit must come out of the cluster to be serviced. Your tach is probably closer than you think, I would leave it alone. Another thing to think about, do you know that the speedo is "on the money" ? It may be off just a few mph and is compounding the problem.
Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
This was not an instrument of accuracy even when new! The tachometer calibration allowable variance is +/- 100 rpms so you are not far from that with what you have.
Willcox Inc.
Willcox Inc.
Last edited by Willcox Corvette; Nov 15, 2008 at 11:15 AM.