Not sure where you would get a wiring diagram but your problem sounds like one of the safety switches is stuck on (or off). When you go to reverse without disengaging blades on some rideons, a safety switch will cut the engine. The same thing will happen if you go to get off the mower without disengaging the blades or putting the park brake on. Check your safety interlock on the seat, it may be faulty and not making contact even if you are sitting on it.
Gazza, May 2009