Current for bulb recognition

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BobFisher

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I replaced some time ago my daytime running bulbs and number plate bulb with LEDs. Of course, the controller now thinks there is are open circuit bulbs. Since the warning is now dysfunctional for these locations, I'd rather never see it than always see it. Anyone know what current the controller needs via a shunt resistor to assume the presence of a bulb? Thanks.
 
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WtA3WtQ5

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An easier solution is to find LED bulbs that are the same or similar wattage. The only down side is that you have much brighter DRL's.
 
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Uwe

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What kind of car?
What kind of bulb?

If you know the wattage of the bulb that has been replaced, you can figure it out based on Ohm's Law.
V (volts) = I (amps) * R (ohms)
and
Power (Watts) = Volts * Amps

Example: So a 24W bulb being supplied with 12V would pull 2.0 Amps of current (once it has warmed up, more on that below)

Since V=IR, we can move the I to the other side to get R. V/I = R.

So 12V/2A = 6 ohms of resistance.

It should be noted that the filaments in incandescent (including "halogen") bulbs have substantial positive temperature coefficient of resistance. This means if you measure their resistance when they are cold/off, you will see a much lower value than what you will get from the above calculation. This may be a factor if the control module does "cold diagnostics" on the bulb(s) in question.

In addition, the resistor in question must be able to handle the power that is being dissipated. If you apply 12V to a typical 6 ohm 1/4 watt resistor, it will fail instantly.

-Uwe-
 
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kczach

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When I had my 2011 S4 I switched out my bulbs to LED including the license plate bulbs. I went to ECS tuning to get the bulbs . All of them worked without and error messages. There was a resistor added to the license plate bulb which was not a problem.
 
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BobFisher

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Thanks for the responses but there is some misunderstanding evident in them. The issue is that an LED bulb of similar or even significantly higher wattage than the equivalent incandescent one takes only about 15% of the current of an incandescent and this is below the threshold of the controller to recognize a bulb as being present. I was asking if anyone knew what that threshold was - it appears evident no-one does. Regarding sourcing from ECS tuning, the particular bulbs i need fixed are my running lights: they are on all the time and the brighter LEDs make the car significantly more visible. Unfortunately they are also on the radar of the controller in a way that the interior trim lights (I believe) are not. I say this becuase I have replaced a number of the interior lights with LED and get no error codes. It is also why lots of people sell LED kits to replace the interior lights but no LED lights are advertised for B6/7 Audi running lights. I can solve this myself by experimenting using a variable resistor but i was asking first to save myself unnecessary work. I can do the math: I have a physics degree.

Part of me was hoping that somewhere in the coding was a generic threshold setting for the current to recognize a bulb as being present, maybe based on the smallest bulb being monitored in that model.

Don/DV52: thanks for jumping in with my car details, although i doubt this threshold had changed much between models (up until LEDs became standard). It may even be an industry standard. I've been and looked for it but found nothing.
 
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Uwe

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Part of me was hoping that somewhere in the coding was a generic threshold setting for the current to recognize a bulb as being present, maybe based on the smallest bulb being monitored in that model.
In a B6/B7? Nope; module that controls the lights doesn't have an adjustable threshold. Some much newer cars have settings for bulb type, but the B6/B7 modules are late 1990's tech, before LED lighting was really a thing.

-Uwe-
 
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stefdds

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It is also why lots of people sell LED kits to replace the interior lights but no LED lights are advertised for B6/7 Audi running lights.
It is typically handled with these:

 
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WtA3WtQ5

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Never changed out the halogen drl bulbs in our B7 with adaptive bi-xenons. Weren’t too many good choices back then.

I did find led bulbs that used within a few watts of the factory halogens in a B8 Q5 and no codes.
Took a few tries to find ones that were not junk. Much of the China stuff is dodgy. Like to to think things are getting better but not so sure.
 
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