Ford 7710, Ford 550, Ford 1910, Ford 4430, KD Forklift, Ford LGT17H, New Holland L255 Skidsteer
Does it ever stop? Sounds like a hydraulic “wet clutch” PTO and either the solenoid valve that the switch controls is stuck or not hooked up.
Also, the PTO brake is worn, the brake actuator is bad or the hydraulic system for the PTO is leaking by internally, but this usually causes the PTO clutch to slip under heavy load.