tach calibration

[bly]

I am not sure my tachometer is reading correctly. How can I check this, and how, if I can, calibrate it to read correct? It is a vdo tach. There are no switches on the back. Any help would be great.

Thanks Bly.

[Ron61]

Bly,

I use the VDO tac also and mine is the electronic one. I was fortunate enough to find a patrol office who had a radar set up and he let me run through at 60,70, and 80 miles per hour and I wrote my tac readings down and then compared them to the speedoimeter speed. Using that and the math both my speedometer and tach were right on at all speeds. Your tire circumfrence and gear ratio also figure into this.

gear ratio= tire circumference X rpm X 60
-------------------------------------
speed X 63,360

You can use this to find any of the unknowns. For example it is hard to read my tac at a 25 rpm mark but at 60 I was showing just over 2500 rpm. The actual rpm was 2525. My gear ratio is 3:27 by the way.

Ron

[YerDugliness]

EDIT---GEEZ, I should have looked at how old this thread is....12 years old

...but, I guess you can tell that I use the search function, right?????

Originally Posted by bly

I am not sure my tachometer is reading correctly. How can I check this, and how, if I can, calibrate it to read correct? It is a vdo tach. There are no switches on the back. Any help would be great.

Thanks Bly.

I have a VDO tach and it seems to read at twice the RPMs of the engine...for example, it says my box-stock 1989 5.0 HO motor will turn 10,000 RPM....I sincerely doubt that.

I thought that there were three switches on the back that, depending on their settings, calibrated the tach, too...in fact, I have posted a thread about just that issue but got no replies. I figured out why once I managed to get my head under the dash...I can see no switches of any kind on the back of my VDO tach (but they may be there...see the link I posted below).

By doing further research I found out that on some VDOs there is some sort of electrical component inside the tachometer that needs to be replaced for the tach to display the correct RPMs...and the most usual price I found for that modification is about $100 plus shipping both ways. I suppose if you knew which piece it is and could disassemble the tach to replace the part yourself you'd save considerable $$, but then there's that old "...what if" thing going on and without thorough knowledge regarding the VDO's internal workings I'm certain I'd muff it up...

A new VDO tach costs about the same as paying for the parts to be replaced and shipping. I think I'm going to go with the new tach rather than rebuilding mine.

BTW...this bit of info did come up on one of my VDO calibration threads...three little switches that adjust the tach for 4, 6, and 8 cylinder motors. Maybe this will help:

http://www.springfieldstreetrod.com/..._DIAGRAM_C.jpg

Hope you get yours fixed....I'm not giving up on mine, but the "journey" continues

Cheers!

Dugly