First question is has there been any recent electrical work done on the machine.
So a quick explanation of how the system works. First off there is a battery disconnect that has two large terminals and two small terminals. You will have one of two types. Basically they only differ in that one type hooks to the positive cable from the battery and the other connects to the negative terminal of the battery and the other large terminal goes to ground. One of the small terminals will be hot when checked with a test light or volt meter and the other will be grounded. The small hot wire goes to the key switch and then back to the small terminal. Turning the key switch on sends current through a coil in the master switch which closes it and connects the batteries to the rest of the machine system. The alternator is the other main supply component to the system. There are three terminals to most of the Komatsu alternators, B terminal is the battery connection, the ground terminal connects to the case of the alternator and the R terminal goes to the computer. People usually think that R means regulator but in this case it doesn't. The R terminal will have no voltage when the engine is not running. It will have some voltage when the engine is running. Usually it will be charging voltage. Think of the R terminal as a signal that tells the computer the engine is running. The R terminal is hooked to a diode inside the alternator. That signal to the alternator has some logic. First off it is a safety latch that will disconnect a safety relay and prevent the engine starter from engaging when the engine is running. Second item is that the computer has an algorithm or logic piece that knows if the R terminal has voltage then the engine is running but there is no signal from the oil pressure switch so it starts making noise on the monitor panel. There are some twenty or more circuits in the machine that have something to do with coils. Coils can be a problem with computers because when you put current through the coil and then stop, a voltage can be induced that goes the opposite direction. To stop this from happening there are diodes in all the circuits related to a coil. As I recall in the larger Dash 6 machines there were somewhere around 17 diodes. They are all usually taped to the wiring harness. When a diode goes bad it provides another source of ground which means current flows and a coil won't drop out.
Knowing all this should give you some clues to the problem. First symptom is the engine won't start. Does the engine oil pressure alarm come on when you turn on the key? Yes, check the R terminal on the alternator for current when the key is turned on. No, check it anyway to be sure. Now try to jump the starter by touching the battery connection of the starter to the solenoid terminal and the engine should start. If not, you need a starter safety relay or the starter needs a rebuild or replace. You can check the terminals of the starter safety relay by disconnecting the harness connector on it and checking for voltage when you turn the key to start. I don't remember which lead it is but as I recall, there are only three. Next item concerns the master switch itself. Turn the key on and then turn it off. If the master switch is still closed, go back and smack it with a big screw driver handle. If it drops out, you can take it apart and clean it and reinstall and try it again. If that doesn't work I would check all the fuses first and if all good then I would start looking for diodes. Just a warning, they are a pain. They are easy to check, finding them and getting them out of the harness connectors is the pain.
You might want to start another thread and keep everyone informed as to your progress.
Good Luck
