see if the neutral switch on the destroking pedal (left hand foot pedal) under the cab has power on both terminals when the pedal is depressed. just an idea where to start.
Check for wire continuity. If there is a lot of vibration the heavier copper wires on the starter_solenoid seem to crack off at connectors. I chased the same headache on my 860 for days. Good luck.
With the key in the start position you should have power to the small wire on the selenoid.if you have no power on the small wire check all of your neutral start switchs.if you have power to the small wire,check the grounding of the selenoid to the engine block.