LED`s in Generator / Red bulb

[Lars Hvidsteen]

I have fitted the nice LED`s from Jules Dielen in my 356 B T6 instruments, also the coloured ones i oil, generator, flasher and long lights.

I have read here on this forum that the one for generator light far left (red) NOT should be replaced with LEDS? but need the original bulb for chargeing properly??

Is this correct?

thanks

Lars

[Doug McDonnell]

From this link several using LEDs without problem: viewtopic.php?f=1&t=33320

[Norm Miller]

Lars,

It shouldn't matter ig you are running a generator.
If an alternator the light is the exciter to start charging.

Norm

[David Jones]

LED is Light Emitting Diode. A diode can only pass current in one direction. The generator light comes on as a function of the differential in voltage between the battery and the generator. When you turn on the ignition the generator light turns on because current flows from the battery through the light to ground via the regulator. As the generator voltage rises the light will go out because generator voltage equals the battery voltage. If the generator voltage rises enough above the battery voltage the generator light will begin to glow. This cannot happen with an LED because being a diode it can only pass current in one direction. if you install it backwards it will not work at all. Not a good application for an LED.

[Fred Winterburn]

I haven't used LEDs in my old cars, but it seems to me that despite the high 1.8V drop of a red LED that it still might work just fine. The real question is how does that compare to the equivalent incandescent bulb that probably only faintly glows with 3V differential ? The LED might perform just as well or even better despite the 1.8V threshold voltage before current can flow. I would need to experiment on a red LED and check voltage drop that gives full illumination. Probably less than 3V.
In a couple of cases with the CDI, (a 6V VW and an old 12V two stroke SAAB), the CDI was being back fed from the charging system through the generator bulb even after the main ignition switch was turned off. This provided just enough current to the CDI to keep a hot engine idling. This can happen if a higher wattage bulb(less resistance) is used for the generator bulb. Although it hasn't yet happened with a 356, I send a 1N4007 in with the CDI just in case. The voltage drop is only 0.5V and if oriented properly will stop the back feed to the CDI, but still allow the gen light to illuminate if the generator voltage drops far enough below battery voltage. The extra 0.5V drop isn't a problem. The real cure if it happens is to install the correct wattage gen bulb. Or try the red LED. I'll bet that would work fine too. Fred

LED is Light Emitting Diode. A diode can only pass current in one direction. The generator light comes on as a function of the differential in voltage between the battery and the generator. When you turn on the ignition the generator light turns on because current flows from the battery through the light to ground via the regulator. As the generator voltage rises the light will go out because generator voltage equals the battery voltage. If the generator voltage rises enough above the battery voltage the generator light will begin to glow. This cannot happen with an LED because being a diode it can only pass current in one direction. if you install it backwards it will not work at all. Not a good application for an LED.

[Jules Dielen]

Hey lars, no problem with the led when your car has a generator, just switch the 2 wires on the back of the bulb holder and you are all set. It has no impact on your charging system.

Dont use the led if your car has an alternator.

[Martin Benade]

So the regular bulb (but not an LED) will indicate an overly high charging voltage, a feature that exists but is seldom noticed/needed?

[Fred Winterburn]

Yes, you are correct. A voltmeter in combination with the idiot light is probably the best combination in my opinion, Fred

So the regular bulb (but not an LED) will indicate an overly high charging voltage, a feature that exists but is seldom noticed/needed?

[Jules Dielen]

I don't believe i have ever seen a 6v gen OVERcharge, but frequently see them underperform.

Owners used to run 6v cars with 8v batteries without hurting anything for many years. Not saying that is the "smart" thing to do, but a 6v gen puts out about 7.7v before the regulator.