Can you put a LED bulb in the ALT spot in the dash, and still have a correctly functional charging system? I don’t think so, and I’ll explain my thinking.

Since the alt bulb has power on both sides of the bulb with the ignition on (the regulator supplied power of everything is working right, and a ground when it isn’t) the bulb won’t light. Power goes through the bulb to the regulator to “wake” it up when the ignition is on. Since a LED is a diode, depending on how it’s installed you will either have a regulator that never knows the ignition is on, or a warning light that won’t work when the charging system fails.

Am I missing something?

I’m at work, babysittering machines that flat won’t run today. Just letting my mind wander while waiting on 11:42.

 

You are correct. The regulator relies on the resistance of the filament in the original bulb to turn on. An LED wouldn't provide that. More importantly, as you correctly point out, the "D" in LED stands for diode. For that circuit to operate properly, current has to flow in one direction for normal operation and in the other direction for charging failure.

 

Pardon my lowly mechanical engineering self, but how about two in parallel with one reversed in the circuit? And a resistor.

 

Yeah, but how do you put two into a holder for a single 194 lamp?

I was wondering if one of the polarity-agnostic LEDs would work. Apparently they have circuitry that works in either direction.

 

I question what benefits that provides over an incandescent bulb. The main benefit of LEDs is reduced current and heat, but the resistor negates both of those benefits.