The torque mode method of command does not require or use a tach signal, the loop is closed only through the encoder back to the cnc controller.
Your drives use SCR control and while being very rugged and not requiring a DC power supply, have been replaced by modern PWM methods.
You could look at A-M-C drives for replacements if you want to go the Torque mode route, you may get enough back on selling the old drives to almost re-coupe the cost of new, as there are still alot of Fanuc 3 out there.
You can keep the motors, just do not connect the tach.
Also confirm you have encoders and not resolvers, as these are probabally incompatable and would have to be replaced.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.