PeachParts Mercedes-Benz Forum - View Single Post - How to remove the 3rd brake light and not get the "bulb out" light go on?

Greg in Oz · #8 07-07-2003, 09:31 PM

The most elegant solution would be to power the brake light in the wing from the source for the original 3rd brake light. This may not give the desired result though if the light in the wing has LEDs or multiple bulbs (as it appears to have) with different current draw.

Replacing the original bulb with a resistor may not work due to the very different resistance characteristics of an incandescant bulb versus that of a resistor. The resistance of a resistor is essentially constant regardless of temperature. The resistance of the filament of a bulb changes dramatically with temperature. I have just measured the cold resistance of a 12V 21W bulb as 0.7 ohms. At 12 volts this would result in a current of 17 amps equating to 204 watts! Obviously this occurs only at the instant the power is applied to the bulb. When hot the filament of a 12V 21W bulb should draw 1.75 amps or have a resistance of about 6.9 ohms. The change in resistance between cold and hot is about 10 times. For this reason DO NOT measure the resistance of a cold bulb and replace it with the same value resistor. The best thing that would happen would be a blown fuse. Other scenarios might be damaged wiring or bulb failure unit. Replacing the bulb with say a 6.8 ohm resistor (nearest prefered value) may work although it will need to be at least a 20 watt resistor which is large and gets very hot. It also does not give the inrush of current that a bulb does when cold, something the bulb failure unit may expect.

Have you considered leaving the 3rd brake light in place? My 190E Sportline still has its 3rd brake light operational despite having a brake light in its factory fitted wing. I have heard that this may not comply with some regulations (even here in Australia) but obviously has not been a problem in the 13 years mine has been on the road.