When the ignition is first turned on at start up, the ignition switch supplies "exciting signal" voltage to the electronics of the internal regulator through Terminal1 (or L on a Hitachi). The regulator sees this voltage as a reason to start charging. When the alt spins up, the charging voltage (internal) is switched to that terminal by the electronics of the regulator, and stays on that terminal as long as it is charging. When the ignition is shut off that internally supplied voltage is still being supplied from the regulator, and with no diode/light/resistor, it supplies the ignition coil with voltage to run the tractor. When first turned on the voltage (using a light bulb example) the voltage goes through the light bulb filament and to what is now an apparently grounded terminal. The bulb is lit. When running and charging, the regulator puts 14.2 volts on the formerly grounded side of the filament, thus with both sides at 14.2v the filament is cold light off. When shut off, and coasting down, the ignition side of the filament goes to 0volts but the coil appears to be a usable path for the charging 14.2. the filament has a high enough resistance that even though it finds a "ground" through the coil, it limits the current (by glowing) as the alternator winds down, passing voltage and becoming lit, but insufficient current to run the ignition system. thus no spark. When below the speed at which it will supply charging voltage (maybe 500RPM at the alt shaft) the regulator cuts the supply voltage to the #1 terminal and resets it as an apparent ground. Turn on the key, light on Start engine---charging, light off Shut off engine light on as it coasts down then light off. Alternator failure (some of them) light on because it goes back to no charge state and the light is grounded as though the alt was not turning. I hope this helps. Jim
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