[V8] Headlight question

Dave Saad dsaad at icehouse.net
Wed Aug 6 06:46:36 PDT 2008

Well - yes and no -

First off, I have not looked inside this particular module (one of  
the only things on my car I have not opened up btw :-)
but to measure current (or amps) the easiest and cheapest way is to  
measure a voltage drop across a very small resistor. I am pretty  
confidant this is how the Audi system works.
Inside there has to be a current sense resistor somewhere around 0.1  
ohms. When you know the wattage of the bulbs in the circuit, you can  
easily calculate what the voltage drop across the sense resistor  
should be. If you have too little or too much current, the voltage  
drop across the resistor will be out of range and the system turns on  
the warning light.  An open circuit means no voltage drop, and too  
much current means too much voltage drop.  I doubt the system checks  
for over current, but who knows.  This all means that the warning  
system acts as a current limiting device that will dim your total  
possible light output somewhat.  When you relay the system, you are  
eliminating as much of those tiny current limiting resistances as  

To fake out the system, you either need to fool it into thinking the  
current draw is in range (not easy), or disable the warning (probably  
easy).  I don't know what the wiring mod does, but I suspect it  
simply removes the signal wire to the autocheck display - right?

Can anyone tell that school is out, and I have very little to do?


On Aug 5, 2008, at 5:18 PM, Tony and Lillie wrote:

> The headlight warning system works on resistance, I believe. So, if  
> the
> resistance of the bulbs is significantly different than the  
> resistance of
> the relays (about 200ohms each) then it will trigger the warning. I  
> simply
> wired past it on my car. I had installed relays for the stock DOT's  
> on the
> V8, and installed 100W high beams also. Didn't want to burn up that
> (expensive) headlight switch.
> Tony Hoffman

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