Engaging the mower deck the engine will die.

Pete, I have a John Deere rider, so your machine may not work exactly the same, but there's a "kill" switch on the seat that won't allow the engine to run if the mower deck is "on" but the operator isn't on board, sitting in the seat. I suspect the proper operation would be for the switch to be in the "normally open" position when the rider is aboard, and "closed" or grounded when the seat in unoccupied. You can check it with an ohmmeter. It should be in one position when the seat is up, and the other position when the seat is occupied. Your machine may have a "kill" switch also attached to the deck activating lever. Your schematic will tell you. If you can access the seat switch, just try disconnecting it and see what happens.