You will need a resistor that equals the resistance of the original bulb in parallel with the LED.

A 5 watt 12 volt bulb should take a 30 ohm resistor to use an LED.

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Satan creates nothing: he only ruins everything. He does not invent: he tampers. And his followers are no different ~ Archbishop Carlo Maria Viganò

Thank you ! .

I don't see how the resistor will make the bulb light .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

On this particular bulb I would just stick with a 5W bulb. The current through the bulb as you’ve astutely stated starts the field. The resistance also isn’t constant - that is you have some dynamics going on right as you turn the key due to the inductance of the field coil and the changing resistance of the bulb filament as it starts to glow. Both components are current limiting over the split second after you turn the key unlike a resistor. I wouldn’t mess with this just because I had to have an LED there. Just my 2c…that circuit is deceptively simple and you need it to work as designed to guarantee reliable charging.

If you put in a 30 ohm resistor in parallel that burns 5W make sure it can dissipate 5W. That is going to be one big warm resistor.

I think in the owner’s manual it says something like if the light doesn’t turn on at all take your car in immediately. So by bypassing the LED with a resistor you’re kind of bypassing the warning light from actually detecting the absence of the start current as you turn the key. In other words the absence of the glowing bulb is just as important as the glowing bulb and with a parallel resistor you’ve bypassed this indication.

I know, I’m being a nit picky nerd about it. My feeling is this LED mod on this particular circuit is actually a downgrade not an upgrade.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

IMO ;

Being nit picky is a good thing .

I am always learning new things .

I just like brighter lamps is all, I'll leave the charge lamp alone, I'm sure (I certainly HOPE !) all here know that when the Bosch alternator stops charging it doesn't turn the red dash lamp on.....

Guess how I and many others discovered this in a car with fully mechanical fuel injection ? .

One more reason why I insist upon ONLY BOSCH or HELLA voltage regulators ~ they "leak" a tiny bit so the alternator is self energizing once it gets past 3,000 + RPM .

The shared knowledge and training here is priceless and far better than most of the factory / dealer / college training I've had over the decades .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better