Are you sending the PWM drive voltage and sending back the tacho signal within the same cable bundle. If so then "cross-talk" is your problem. Simply run the PWM power and the tacho signals in completely separate cables.
Are you sending the PWM drive voltage and sending back the tacho signal within the same cable bundle. If so then "cross-talk" is your problem. Simply run the PWM power and the tacho signals in completely separate cables.