RBoots is right on with the voltage from a bad alternator diode, to the R terminal, causing the machine to think it's running, and not start. Ever time I have seen that happen it wouldn't do anything, even clicking.
Often there is also a separate solenoid on, or near the starter besides just the one that causes it to pull in. Basically it's a solenoid with a low amp draw that is made by the start switch and, in turn, sends a higher amp signal to the solenoid on the starter. It's possible your hearing that one click, but the contacts inside are bad and not allowing power through to the actual starter solenoid.
Komatsu is also known for putting a powered master switch on one side of the batteries or the other. Usually it is on a circuit separate from everything else. When you turn the key switch to the on position, it powers the master switch and causes it to make the main leg to the battery, be it the hot or the ground. Once again if this goes bad, the machine will do nothing.
Some of the older Komatsu's also had diodes in the wiring that allowed power to drain off when the master switch broke. Not saying that is causing your problem, but I've seen them blow and cause some strange problems also.
One thing for sure, and certain, if a machine has an electrical problem it's usually something pretty simple......but only after you pull out nearly all your hair trying to find the problem.
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