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 :thumbs:
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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.
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Ken, I'm just curious - How do you know that it's out 200 RPM's at highway speeds? Are you running a separate electronic tach to check it?
- Pat |
Originally Posted by C2Driver
(Post 1567826544)
Ken, I'm just curious - How do you know that it's out 200 RPM's at highway speeds? Are you running a separate electronic tach to check it?
- Pat |
Originally Posted by GCD1962
(Post 1567826542)
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.
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Originally Posted by Kensmith
(Post 1567826657)
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.
My 2 cents. - Pat |
Originally Posted by Kensmith
(Post 1567826678)
I'm interested in this, any way it can be reached from under the dash?
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Originally Posted by Kensmith
(Post 1567826678)
I'm interested in this, any way it can be reached from under the dash?
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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.
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Originally Posted by ejboyd5
(Post 1567830066)
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.
larry |
Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment. :thumbs:
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Originally Posted by JohnZ
(Post 1567832421)
Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment. :thumbs:
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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.
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Originally Posted by JohnZ
(Post 1567832421)
Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment. :thumbs:
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 |
Originally Posted by JohnZ
(Post 1567832421)
Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment. :thumbs:
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Originally Posted by JohnZ
(Post 1567832421)
Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment. :thumbs:
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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.
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Originally Posted by Brian@CSofMD.com
(Post 1567839846)
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.
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Originally Posted by Brian@CSofMD.com
(Post 1567839846)
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.
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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. |
The fine tuning , by moving the regulator, affects the tension of the hair spring and this of course will position the pointer higher or lower. If for example, you are reading 200 rpm high at say 3500 rpm, you can move the regulator to make it read where it should be. Problem is, now with more spring tension, it will read low at the bottom end. The regulator is only meant to fine tune after the proper procedure of magnetizing and demagnetizing are completed. Again, I would leave it alone.
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Originally Posted by Brian@CSofMD.com
(Post 1567850872)
The fine tuning , by moving the regulator, affects the tension of the hair spring and this of course will position the pointer higher or lower. If for example, you are reading 200 rpm high at say 3500 rpm, you can move the regulator to make it read where it should be. Problem is, now with more spring tension, it will read low at the bottom end. The regulator is only meant to fine tune after the proper procedure of magnetizing and demagnetizing are completed. Again, I would leave it alone.
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I agree too, it's way to close to worry about taking it out!
Willcox |
I have the instrument cluster out of my '64. I've calibrated the spedo and figured I would check the tach calibration. Does anyone know the correct tach cable RPM to tach needle reading?
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Why would you test a tachometer with math instead of another tachometer? Too many variables to accurately determine with math. Tire size +/-, gear ratio +/-, speedometer gear +/-, correct reading on tachometer dial +/-, clutch slippage, etc. Your tachometer could be dead on right now, but the accuracy of your other inputs and decimal point rounding could give you a false answer.
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Count the teeth on the dist shaft gear and the driven gear. Provide this information to an experienced speedometer shop and they should be able to provide a calibration table. From there you can decide whether to make corrections to the head's magnetic flux.
It can be tough to get the tach dead on at all speeds due to some combinations of offset and gain error, but my recommendation is to get it as accurate as possible at the top of the rev range and don't worry about idle speed accuracy. When doing maintenance work that requires accurate idle speed use a good test tach. Duke |
I found the answer. Since the distributor spins at half the engine RPM, you multiply the cable RPM times 2. Or, tach reading = cable RPM x 2.
Luckily mine was dead on at 4000 RPM and a little off higher and lower than that. I can live with that. |
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