C1 Tach Calibration

[Al C.]

I had the speedo and tach redone by CSM, and finally got the car back together. The re-bushinged tach is reading about 30% low, according to two separate electronic testers. E.g., tach says 1000 while electronics say 1300.

In my prior experience with the car, I was getting reasonable tach correlation with the electronic instruments, and with calculation from road speed (clocked by our following car). But my generator pulley is 4.1 inches vs. should-be 3.74 inches. This larger gen pulley should make the tach read only about 10% low. (Actually the ratios would depend on the effective diameter at which the belt rides, so these numbers are only approximate.)

I'm wondering if my tach's original calibration may have been matched to my incorrect setup, and then the tach was corrected to component spec? And if so, something else is now off (e.g., are there different gear choices in the generator tail?).

It seems that, even if I had the correct 10% smaller pulley, the tach would now still read about 20% low? I'm attempting to understand whether the current calibration is correct, and if so what else could be off. Is there a way that I can double check the tach (and generator head speed) without removing it from the car? Suggestions for inexpensive stroboscopes?

Thanks for any info, ... Al '58 #378 245 hp

[Dennis C.]

Re: C1 Tach Calibration

Al - Your wonderful description is making my eyes cross. I feel like there are way too many if's, an's or but's. Maybe buy a cheap JCW tach and use that to help tell you where you are in calibration land. (Inexpensive stroboscope) !!!

[Dennis C.]

Re: C1 Tach Calibration

Al - Your wonderful description is making my eyes cross. I feel like there are way too many if's, an's or but's. Maybe buy a cheap JCW tach and use that to help tell you where you are in calibration land. (Inexpensive stroboscope) !!!

[Al C.]

Re: C1 Tach Calibration

My frustration led perhaps to overdescription. A simpler version of the question:
Is there any credible way for a properly calibrated tach to read 20-30 percent low?
Can the generator'stach drive gear set have an incorrect ratio, or are they only available one way?
It's probably not belt slippage, it's too steady.
I do trust the two electronic tachs and my ears that the tach is low.
I have an extra tach cable, so I could rig up to measure the cable speed by observing their junction if I could find a way to do so.
In days of yore we used the Stroboscope to measure the rotation speed of rotating machinery - it's basically an electronically variable pulse rate strobe light, you set strobe rate to freeze the motion then read the speed dial. Haven't seen one in 40+ years.
... Al '58 245hp

[Al C.]

Re: C1 Tach Calibration

My frustration led perhaps to overdescription. A simpler version of the question:
Is there any credible way for a properly calibrated tach to read 20-30 percent low?
Can the generator'stach drive gear set have an incorrect ratio, or are they only available one way?
It's probably not belt slippage, it's too steady.
I do trust the two electronic tachs and my ears that the tach is low.
I have an extra tach cable, so I could rig up to measure the cable speed by observing their junction if I could find a way to do so.
In days of yore we used the Stroboscope to measure the rotation speed of rotating machinery - it's basically an electronically variable pulse rate strobe light, you set strobe rate to freeze the motion then read the speed dial. Haven't seen one in 40+ years.
... Al '58 245hp

[Doug Flaten]

Re: C1 Tach Calibration

I am no expert on the parts availability, but I have never seen a choice of gears in any of the catalogs. I guess it is possible that some cars used different tooth counts, but I have never seen any info stating such. Are you positive that the speedo shop recalibrated the tach after working on it?
The other thing to verify is whether you have the correct diameter crankshaft pulley.
Doug

[Doug Flaten]

Re: C1 Tach Calibration

I am no expert on the parts availability, but I have never seen a choice of gears in any of the catalogs. I guess it is possible that some cars used different tooth counts, but I have never seen any info stating such. Are you positive that the speedo shop recalibrated the tach after working on it?
The other thing to verify is whether you have the correct diameter crankshaft pulley.
Doug

[Al C.]

Re: C1 Tach Calibration

Doug, I too have never seen a selection of gears. The other pulleys are correct.
I'm grasping for straws because (1) it it hard to believe that CSM would have incorrectly calibrated the tach instrument, and (2) it's such a chore to pull the darned tach back out (I mumbled incoherently throughout the removal and reinstall, on my backside, with the seat adjuster putting a permanent dent on my back), (3) hate to leave the car undrivable again it's had a bad spring-summer of not-able-to-drive. Aarghh.
Thanks, ... Al '58 245hp

[Al C.]

Re: C1 Tach Calibration

Doug, I too have never seen a selection of gears. The other pulleys are correct.
I'm grasping for straws because (1) it it hard to believe that CSM would have incorrectly calibrated the tach instrument, and (2) it's such a chore to pull the darned tach back out (I mumbled incoherently throughout the removal and reinstall, on my backside, with the seat adjuster putting a permanent dent on my back), (3) hate to leave the car undrivable again it's had a bad spring-summer of not-able-to-drive. Aarghh.
Thanks, ... Al '58 245hp

[Mike E.]

Re: C1 Tach Calibration

The only gears in the equation are in the tach drive unit on the generator--and are one metal, one fiber, and the only ones ever used by GM. Calibration of the unit, whether tach or speedo, is by controlling the degree of magnetization relationship between the rotating magnet (driven by the cable on the back of the tach)and the speed cup, which causes the needle to move.

[Mike E.]

Re: C1 Tach Calibration

The only gears in the equation are in the tach drive unit on the generator--and are one metal, one fiber, and the only ones ever used by GM. Calibration of the unit, whether tach or speedo, is by controlling the degree of magnetization relationship between the rotating magnet (driven by the cable on the back of the tach)and the speed cup, which causes the needle to move.

[Al C.]

Re: C1 Tach Calibration

Mike, thanks, that sounds pretty definitive - I guess (sigh) I'll have to pull the tach back out.
... Al '58 245hp

[Al C.]

Re: C1 Tach Calibration

Mike, thanks, that sounds pretty definitive - I guess (sigh) I'll have to pull the tach back out.
... Al '58 245hp