drjay: The switch could be your problem and it's the easiest to fix. So work that first.
If I'm looking at the right schematic your start circuit is really simple: The key switch, the relay. and the cruise unit, which provides the ground for the relay. The cruise unit also lights the neutral light on the instrument panel and prevents starting (by removing the ground from the relay) if it thinks you are in gear. Do you see the neutral light on the instrument panel when you try to start the tractor?
The cruise unit determines if you are in neutral via a potentiometer (variable resistor) on the gear select lever and some owners have had problems with it. But if you have a good neutral light, the potentiometer isn't your problem.
The schematic also shows an inline diode between the relay and the cruise unit, which might fail (rare) or you might have a poor electrical connection in that circuit. I have nor read of a failed cruise unit, particularly where the neutral light worked but the starter didn't. But I guess it's possible.
Are you sure you checked the right relay? An identical relay, also on the firewall, controls the glow circuit and one owner reported the connectors were swapped to the other relay (which works fine but is confusing). If you have a helper, you should be able to feel the relay click in (with your hand) as your helper turns the key. The glow relay should click when the switch is turned to "on" and the start relay should not click until the switch id turned to "crank."
If the start relay doesn't click, you could try providing a "bypass" ground to the ground side of the coil to diagnose a possible problem in the cruise unit or associated wiring. Good luck. By the way, the relays will work fine even if unscrewed from the firewall, which may be useful for troubleshooting. They are just mounted there so they don't flop around. Good luck.
