there is a mirco switch above the walkers fastened to the top of the housing under a flapper. This is a warning that the chopper_spreader has plugged. There is a pickup for the shaft speed on the front walker shaft.
Oh the end of the walker shaft there should be a round plate with some rectangular holes in it with a monitoring switch on it. it operates magnetically to monitor the shaft speed. These switches can need adjustment or replacementto monitor correctly. There is another switch above the walkers that may also be disfunctional. Good luck!