Tachometer is acting funny

[godsmack635]

my factory tic toc tac in my '68 Charger was working fine until I fixed an overcharging problem. After picking up two new alternators and finding both were shorted out, i finally got a third alternator that was good and installed it along with my new voltage regulator. I had all three alternators tested before I left the auto parts store and was told all were working correctly. My charging system is working as it should now but i noticed my tach only goes up to 2000 rpm on the freeway when it used to read 2600. Any idea what could cause this? There were a few people messing around with wires at the alternator as we were trying to figure out the overcharging of the electrical system. Not sure if someone touched a field wire where they weren't supposed to or what..............Any help is appreciated. Thanks!

 

[Durandal25]

.........need to have a calibration done on it I'm afraid. Tac units get get hoakey when grounded, but 600 RPMs is a lot to be out....

 

[Meep-Meep]

I don't think I ever seen a factory MoPar tach from the 60's work worth a damn.

Seems odd that it worked well when the charging system wasn't. The increased average voltage might be the reason for the RPM discrepancy. You can test a tach with a signal generator and a bench power supply.