PTO SWITCH PROBLEM

I REPLACED THE DRIVE BELT ON A TORO #74412 ZERO TURN MOWER. NOW WHEN YOU START THE MOTOR THE PTO IS IN THE OFF POSITION AND THE BLADES ARE SPINNING. IF THE PTO SWITCH IS ENGAGED THE MOWER SHUTS DOWN. THIS OCCURRED WITH PARKING BRAKE ON BUT NO OCCUPANT IN THE SEAT. ANY HELP OR SUGGESTIONS WOULD BE APPRECIATED. THANK YOU.