two tacs. which one is right?

[cazsper]

Along with the factory tach, my '73 Mach-1 also has a tach mounted to the steering column (aftermarket of course). The factory tach is about 200rpm off from my column tach. Is the factory tach that "off" or can the wiring of an additional tach affect the signal to the factory tach?

I'm just thinking about getting rid of the column mounted tach, but I would like whatever I am using to be accurate..

 

[rutkak]

Along with the factory tach, my '73 Mach-1 also has a tach mounted to the steering column (aftermarket of course). The factory tach is about 200rpm off from my column tach. Is the factory tach that "off" or can the wiring of an additional tach affect the signal to the factory tach?

I'm just thinking about getting rid of the column mounted tach, but I would like whatever I am using to be accurate..

I'd say it is anyone's guess which one is right unless you have a 3rd way of checking. The only way I REALLY knew my aftermarket tach was off by 150 rpm was to verify with my electronic timing light that has a built in tach.

Despite this, I'd probably assume the 40 yr old instrument cluster tach is wrong. If you haven't done so, check out Rocketman Bob's RCCI webpage. He offers a number of great services and can refurb your tach and probably calibrate it as well. I had my idiot lights converted to tach and it looks amazing.

http://www.rccinnovations.com/index.php?show=menu-tach-all

 

[midlife]

Yes, the addition of items attached to the coil can affect the factory tach, as it relies upon the amount of current flowing to the tach from the ignition. The adding of another tach adds another load to the system and thus affects current.

A simple test is to simply disconnect the column tach and see if your factory tach improves.

 

[Don C]

All analog tachs are subject to being inaccurate. They detect the pulses caused by the coil charging and discharging and convert it to a steady voltage that changes as the RPMs change to move the tach needle. Any changes in voltage levels or current will affect the readings. They do have internal adjustments, but having an accurate reading at one RPM doesn't guarantee the tach will be accurate for the entire sweep of the needle.