There's something to be said for that. The alternator light goes inline in the exciter wire so when the ignition is on but the alternator is not working it sees 12v on the battery side and 0v on the alternator side and illuminates. When the alternator is running properly there is 12v on both sides and the bulb will turn off. If the alternator is on its last legs and or loaded down the light will be dim and may go out if you rev it. The diode is to prevent back feeding. It acts as a check valve that only permits current flow in the battery to alternator direction. Without it the ignition may not turn off the car (depending on how it's wired) and even if it does there can be some nasty current spikes and whatnot at shutdown which may pop the fuse for that circuit. Edit: Illustration from our own forum: https://www.jalopyjournal.com/forum/threads/alternator-idiot-light-necessary.1195329/#post-13605725
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