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Old 11-12-2008, 08:19 PM   #1
Kensmith
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Default 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
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Old 11-12-2008, 08:56 PM   #2
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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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Old 11-12-2008, 08:56 PM   #3
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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
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Old 11-12-2008, 09:04 PM   #4
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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
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.
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Old 11-12-2008, 09:05 PM   #5
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Originally Posted by GCD1962 View Post
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.
I'm interested in this, any way it can be reached from under the dash?
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Old 11-12-2008, 09:22 PM   #6
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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.
Ken, if the tach is spot-on at idle, it's entirely possible that it's also spot-on at highway speeds aaaannnnd the gear calculators are inaccurate. I've used a few of the online gear calculators and they all pretty much look alike. If the original developer made a small error in programming, I suspect that many clones/copies will have the same error. Before doing anything, I'd suggest doing a quick hookup of an electronic tach, to compare to your tach. If you don't happen to have an old tach laying around, I suspect your mechanic buddy might.

My 2 cents.

- Pat
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Old 11-12-2008, 09:50 PM   #7
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I'm interested in this, any way it can be reached from under the dash?
Don't really know on a C-2, I have a '62. If it is similar you need to physically get to the tach head within the unit. I doubt you could get to the adjustment with it still in the car
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Old 11-13-2008, 12:00 AM   #8
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I'm interested in this, any way it can be reached from under the dash?
Nope, you'll have to yank the whole instrument cluster and remove the tach.
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Old 11-13-2008, 07:47 AM   #9
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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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Old 11-13-2008, 09:05 AM   #10
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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.
That's a very good point. The spec'd or measured height of a standing tire is not going to be the same when the tire is spinning (rotational speed and temp will cause variances).

larry
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Old 11-13-2008, 11:44 AM   #11
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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.
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Old 11-13-2008, 02:07 PM   #12
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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.
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?
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Old 11-13-2008, 03:36 PM   #13
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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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Old 11-13-2008, 04:34 PM   #14
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Quote:
Originally Posted by JohnZ View Post
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.
John is 100% correct as usual. My NCRS Chapter had a tech session a number of years ago at Corvette Specialties of Maryland.

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.

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Old 11-13-2008, 07:11 PM   #15
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Quote:
Originally Posted by JohnZ View Post
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.
So it must be removed to calibrate then. Thanks John. I will just calibrate my brain to +200 RPM while cruising down the road!
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Old 11-13-2008, 09:41 PM   #16
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Quote:
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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.
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.)
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Old 11-13-2008, 09:48 PM   #17
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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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Old 11-13-2008, 10:09 PM   #18
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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.
That's what I must have been thinking of "the small arm". I needed to adjust that when I installed a nos unit in the tach. Thanks for the clarification. In the "fine tune" how much of an adjustment is there?
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Old 11-14-2008, 07:19 PM   #19
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Quote:
Originally Posted by Brian@CSofMD.com View Post
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.
Yep, speedo is correct, tach is off at higher speeds. 200 lower at 65 mph, 300 lower at 75 mph. So it is progressive.
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Old 11-14-2008, 08:12 PM   #20
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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.

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Last edited by Willcox Corvette; 11-15-2008 at 11:15 AM.
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Old 11-14-2008, 08:12 PM
 
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