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That would apply to a gasoline engine, or a diesel engine with an electric shutoff. The #1 terminal on the alternator has power FED to it at startup, to power the field in the alternator, so it will begine to charge. (Excite) Once the engine is running, and the alternator is charging, the stator in the alternator generates AC current, which is rectified by the diode trio, and fed to the field windings. This current is also available on the #1 terminal, where it can "backfeed" to the "load" side of the ignition switch, powering the ignition system, or the fuel shutoff valve, even when the operator shuts off the key switch. Having a diode, and "idiot lamp", or a suitable resistor in that circuit eliminates this backfeeding of current, or at least limits it to a level that will not operate the ignition system or fuel valve, and the engine shuts down. In the other thread, I posted a couple of diagrams
showing what can happen if there is a failure in the charging sircuit, and the diode, reistor, or lamp is not in the circuit. That is why it is a good idea to use the diode, the lamp, or the resistor, even IF the system is on a diesel tractor with a MECHANICAL fuel shutdown.
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