SWITCH NOTE:
I had a thread months back on my L3800 with a "chattering" starter. Replace the battery. Only got worse. Last week, on occasion, you'd turn the key and nothing would happen.
This tractor was left sitting outside for four months by the previous owner. The battery was down quite away and start attempts probably didn't engage the starter solenoid quite hard enough and attempts thereafter only fried the contacts more. Partly a sticky solenoid is my first guess.
Some time you get a totally normal start. Sometimes, silence. When the hamsters don't make a noise, I cycle the clutch pedal. No difference. Then I bounce up and down on the seat. Click...Vrooom!
I put my seat switch on the volt meter and it shows a good clean make-or-break. What
was crusty was the switch block plug. That had a fair amount of dirt/dust in it and I am sure that held moisture. I shot the block out with contact cleaner and all the starts have been strong with little or no chatter. I am sure the solenoid contacts are "crusty" so, when I get time, I am going to order a set from the Alternator Man in Dallas.
The seat switch is a sealed switch block. To spray it, you'd have to drill a hole and flood it with contact cleaner. Then you'd have to seal up the drill hole with JB Weld. Easier to just replace the switch block.
I'll let you know how things progress. At least, with a Denso starter, parts are plentiful and inexpensive.