Delco 10SI, and other common alternators used used as upgrades in vehicles, rely on magnetism in the spinning rotor. The magnetism is created by electricity that is turned on by the internal voltage regulator. when the alternator is stopped and the ignition is off, the regulator shuts off all current to the rotor, and there is no drain. The regulator's #1 terminal in this condition appears as a ground. When the ignition is turned on, but not started, electricity from the key ignition goes to a small filament type light bulb. The ground for this bulb is the #1 terminal, so the bulb lights up (the same as when the car key is on and the dash light (ALT) lights up). When the regulator senses this new found voltage applied to its #1 terminal, it is ready to start giving the rotor electricity to make magnetism. When the alternator begins to turn, as the engine starts, the regulator turns on the voltage to the rotor, and changes the #1 terminal from a ground to 12v positive. Now there is 12v from the ignition on one side of the bulb, and 12v from the regulator on the other, and the bulb goes out. There is positive 12v on both terminals of the bulb, so nothing flows through it. Inside the alternator is a complex array of diodes (one way check valves for electricity) this array changes the alternating current from the windings in the alternator housing, to direct current to charge the battery, and feed the regulator some charging information. The #2 terminal on the alternator is connected to the output so the regulator "understands" how much voltage is being created, and regulates it correctly. When the ignition is shut off, the light bulb has no voltage on the ignition side, and that now appears to the light bulb as a ground because it is connected to the coil through the ignition wiring. The light cannot flow enough electricity to make the coil function, so the engine stops running. If the light was not there (or a diode or resistor) there would be enough electricity coming from the #1 terminal to keep the ignition on, and the engine would not shut off.
I cannot make this simpler, and actually explain it, so I will move on to what happened to your's. First the fact that your tractor did shut off tells me that there is either a resistor, light bulb, or diode placed in the circuit (called Exciter circuit). The fact that the battery went dead is probably related to failed diodes in the Array I spoke of earlier. When a diode fails it has two options it can be like a switch that is off, or it can be turned on in both directions (shorted). if turned off, the alternator can still work some with reduced total output. If shorted, it lets battery voltage flow the wrong way through it to ground and discharges the battery when the tractor is shut off. So it is possible that the alternator failed much earlier but had enough output to keep going, then failed radically and died. It is also possible that there is a diode, resistor, or hidden light bulb in the exciter circuit. If you put a good alternator in it (three wire same as it was) it will probably be fine. If it still discharges, there may be no bulb, diode, or resistor, and it might not shut off. Replacing the amp meter is easiest, and at this point necessary to make sure it is charging. Jim
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