When the ignition is switched on, terminal 15 is energized. But the alternator is not running yet, so the de-energized D+ contact acts as «minus». The control lamp lights up, because there is a difference between the voltage of the on-board network, which is powered by the battery, and the voltage of the generator still not running.
When the engine starts and the alternator reaches the charging speed, then the voltage regulator connects the alternator to the onboard electrical equipment. The positive current now flows from terminal 15 and additionally from terminal D+. As a result, there is no longer a difference in voltage, and the charging indicator lamp goes out.
When the ignition is switched on, the lit control lamp must «pre-excite» alternator. Only in this way can it supply current even at low speeds. Although pre-excitation is only necessary when starting the generator.
Charging doesn't happen all the time
Turning off the control light does not yet prove that the battery is being charged from the generator. It only says that there is no longer a difference in voltage between the battery and the alternator. If, for example, all electrical consumers are switched on when the engine is idling, the charging indicator does not light up, although more electricity is taken from the battery than a weaker generator can provide: nevertheless, there is no difference in voltage with the battery.
Tip: You may have noticed that sometimes the charging indicator light stays on if you start a warm engine without gas and it continues to run at low idle. In this case, the pre-excitation of the alternator is too weak, it still does not give current. As soon as you tap on the gas pedal, the red light goes out. This is normal and does not indicate a malfunction.
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