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  #1  
Old 07-05-2003, 05:31 AM
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How to remove the 3rd brake light and not get the "bulb out" light go on?

On my W124, I have a wing, and a 3rd brake light. I don't need the one behind the rear window anymore. I Took the bulb out, and even the assembly out (minus the part that is bolted (?) on the rear board) and it just keeps coming on.

How can I make something that would not gimme the light out warning on the dash?

Here's a pic of my setup



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Old 07-05-2003, 10:41 AM
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I have heard that guys who eliminate their side maker lights replace the bulb with a resistor. I don't know what value resistor to use, but I can't imagine it's too critical. Hope this works for you.

Rex
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Old 07-05-2003, 02:11 PM
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Thanks for the tip, I was actually thinking about the very same thing!

But no clue on how to do it
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Old 07-07-2003, 04:41 PM
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ttt
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Old 07-07-2003, 05:24 PM
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I'm wondering where you got power for the lights in the wing. Have you tried powering it from the leads for the original bulb? Then you would have bonus of the lights in the wing being monitored by the original circuit.

BTW, it looks fantastic.
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Old 07-07-2003, 05:28 PM
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Hmm, measure the resistance on the bulb with a multimeter and pick up the same value resistor . . .

Replace the bulb but mount the bulb where it won't be see, like in the trunk . . .

Black out the lens on the brake light housing . . .

Black out the bulb . . .

So many ways, so little time
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Old 07-07-2003, 05:40 PM
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Do what MTI said.

Measure the resistance and get the proper resistor. Wiring your spoiler brake light directly to your 3rd brake light will not resolve your problems, because there is a difference of resistance/draw.

Any bootsy car audio shop can hook you up if you can't do this yourself.
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Old 07-07-2003, 09:31 PM
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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.
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Old 07-27-2005, 01:54 AM
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Solved!

May not be relevant anymore, but use Ohm's law to calculate the resistance. The bulb's wattage tells you. Let's say for argument that it is a 25 watt bulb. Watts = Power = Volts x amps. So since we can assume the volts to be constant at 12.5 volts, then 25 watts / 12.5 volts = 2 amps.

Since Volts = Amps * Resistance (ohms), then Ohms = 12.5 Volts / 2 amps. This equals 6.5 ohms.

OK, that is quite a bit of resistance and heat to put into one small resistor. What I would do is put a bunch of resistors in series to add up to 6.5 ohms, thus lowering the heat flux through each resistor. Total dissapated heat is still going to be 25 watts, but the surface temperature of the resistors in series will be lower. I'd make a cable of resistors in a series loop, say about two feet long and well wrapped, and then stuff it up in the trunk overhead where the wires from the 3rd brake light come down and the vacuum lines for the rear headrests are.

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