Quote:
Originally Posted by V6'er
^ And, it's ALMOST correct. 
To truly calibrate a frequency meter, which is what the tach is, you MUST use more than one calibration frequency. Simply using a 120Hz signal is not enough (because you are modifying the frequency response of the meter).
Sengk is very close to hitting it right, but, you have to remove the needle on almost every tach to get the "offset" correct. The pot allows you to adjust "gain" (needle swing) only.
So, to get it really accurate, you need high and a low frequencies like sengk is using with an aftermarket tach, and, you need to remove the needle and put it at the correct spot once the full needle swing is achieved.
I also found that using 15K, not 22k, results in greater reliability. (This resistor sets the input voltage range to the tach chip, which is actually a frequency to voltage converter.) |
I actually did remove my tach needle also. I had it off a few times already so When i did the final calibration I held the aftermarket tach at a constant rpm and replaced the needle on the stock tach at the same position. I didn't put this part in the write up because i was unsure whether or not this made a difference. I've also had a problem with bending the metal pin before on a different car while installing gauge faces so I didn't want other people to take more risks than necessary.
Thanks V6'er for clarifying this. It sounds like you have a better understanding on this than I do so please add more info if you can to improve on this.
thanks,
sengk