There are two identical switches on the GT18. One for the clutch and one for Deck/PTO engagement. If either switch doesn't work the tractor won't start. I tried to locate new ones thru Sears and other tractor suppliers but ended up buying a used one for twenty bucks. I was looking at the old switch and saw the word Indak. I Googled it and found, Indak, the company who makes the switch, it's their part #J634. They don't sell directly but Kochsales.com in Texas does. Koch uses part #7755R which I believe is the original Sears part number. I got two switches for $15.00 and they are brand new. No more erratic starting problems from worn out switches.

Thanks for the tip!

I suspect that most people just bypass the safety switches when they go bad.