I'm not familiar with the innards of an 863C, not gonna be much help. But are you certain you were working on the lift/tilt solenoid valve? Not sure which coil it is myself. One thing your going to have to be thorough about though, you mentioned you parked the machine for a month and now have the problem, you need to check every little part of the wiring harness, field mice can do a good bit of damage to a wiring harness in a months time. Just a thought.