Welcome to NTT.
On some older Craftsman machines the switch opens when the operator is present. If so, just unplug the switch connector under the seat. If that doesn't do it, then try shorting the terminals as Jerry said. If you hear a 'click' when trying to start, I would suspect the solenoid.
Follow Mith's advice to eliminate starting circuit interlocks. Make sure the blades are off and it's out of gear. Have someone sit in the seat, push in the clutch and turn the key to run. Connect one jumper cable from the positive battery terminal to the big red connector on the starter. Don't clamp it to the starter connector, just touch it.
Whatever you try, always expect it to start. Don't let it surprise you when it does. Never stand in front or between the wheels, etc.
All in all, interlock switches are pretty reliable. I've never come across a failed switch, just the mechanisms that open or close them, or the wiring.
Good luck and let us know how it goes.
Doc: Might want to take another look at that "
welcome
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forum" smilie.
