Repairs toro 580d large area mower, high speed not working

any one out there know much about these commercial toro mowers? Talked to Toro, and of course they want me to bring it in, I like to work on my own stuff. It has a high speed switch, which works on occasion if I reset all faults, works until I do anything to take it out of high speed then won't go back in. is there a way to read codes on the 580, or what would cause this problem to test inputs. Has a black box under deck controls but I don't see any of the lights the manual talks about. must be an older version? I believe it is a 1999-2001 model 580d two wheel drive 85 hp diesel. thanks for any help