I have an m4900 and had the same problem. Checked everything and the ignition switch was the problem. Replaced it and all is well. Even called to get a starter rebuild before I checked all voltages and the rebuild center told me to check the switch as they had just had another person with the same problem and the swithch was defective. However it could be any of the components mentioned by others.
hey kubmech. got the starter relay as you recommended, but there are only #4 wires (#2 purples & black & red) on the new relay. the existing relay on my Kubota B21 is behind the instrument panel & has #7 wires (black, red w/ gray stripe, red w/ whit stripe, green w/ white stripe, white w/ blue stripe, white w/ green stripe, & brown). how do I wire the new relay in? thanks. jeff