The small solenoid (safety solenoid) is there so the higher amperage required by the actual starter solenoid doesn't have to pass through any switches, or electronic components. SO, first thing to do is check that solenoid. First insure you have 24 volts going to one of the big posts on it. Next, insure you have voltage going to it, from the key switch/start circuit. If no voltage here, suspect the safety switch in the left arm rest. Now insure you have 24 volts on the second large post when the solenoid is energized. This should be providing power to the small post on the solenoid attached to the starter. You can easily bypass this solenoid with a piece of heavy (say 10 or 12 gage) wire. Check it by running your wire from the big, hot post to the small post, and see what happens. Too, you can jumper the two big posts and see what happens. Either way, with it jumped, the starter solenoid should be powered and engage.
One thing, when doing any of these tests, the key switch has to be ON. If it's off, the master switch won't be making the system ground, and you won't read voltage anywhere.
On that note, the master switch could possibly be going bad. When it pulls in and makes the ground, it's doing so good enough for the small solenoids to pull in. When the starter begins to pull a higher amp draw, the connection breaks down. The connection breaks, the solenoids lose power, and the starter stops...........and the cycle repeats as long as they key switch is held in the start position. This would account for the chattering your seeing. You can also bypass the master switch by jumping the batteries ground straight to the frame.
Too, you can unhook the battery wire to the starter solenoid and check everything with it unhooked. If everything works like this, but not with the starter actually working, I would suspect the master switch problem I just described.
I got a bit off track there, but with the small solenoid checked, go to the solenoid on the starter itself. Once again, insure you have 24 volts, from the battery, to the big post on the solenoid. If the small solenoid is working, the small post on the starter solenoid should read 24 volts when the key switch is actuated. This should cause the starter to pull in.
This pretty much covers everything I can think of on any Komatsu starter circuit I can remember working on. Beyond the idiot switches, like the one on the safety switch, and the whole deal with the alternator telling the computer the machine the engine is running so it can't start, the circuit is actually pretty simple. Check out the things I mentioned above, and get back with us. I'll pick my brain some more if necessary, but given the info so far, this is all I can think of.
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