Thank you for the ideas Mike,
Last night I verified the tachometer output is getting to the driver inputs okay. When I was turning the motor by hand, I would see ~+- 0 to .3 volts.
Question, should the tachometer shield be connected at the motor, or the driver, or both?
I next verified the drivers are using differentiated input.
Will I have to change the drivers input mode to use your idea of a variable voltage input with a battery and potentiometer?
Thanks,
Bryan