If you turn your key to the mow in reverse and push the button so red light is lit, does it work ok?
Most all 1000 and 1500 series have the ground switch on the rear end. This is on the right side right above the brake. There is one wire that breaks off the reverse linkage, I think it's yellow with a white tracer. This wire gets moving around as you drive the tractor and if it goes to ground, your blades will shut off. But this is not the issue if you turn the key back to the reverse mow, and it still shuts down.
If it still is shuting down it after all that, it maybe the relay.