Bench test factory Mopar tachometer

Get an old points distributer, and put it in your vice. Use a drill motor to spin it.

Hook up the tach lead to the wire that would connect to the coil.

Ground the dust to the negitave side of a battery, hook the tach plus and minus to battery plus and minus. Be sure the rotation is correct and crank away.

The speed of the drill should be enough to get 2 readings fast and slow.

You can not calibrate the tach but you can see if it functions. And if you know the drill rated rpm you can see if you're even in the ball park