Electrically deleting 3rd brake light

[DaveSmed]

Dave (you fucking genius you), would that same deal work to drive LEDs in the 3rd brake light? Can you use the ground and the green/black and not have a false reading?

Well, looking at the diagram, I would think simply grounding the green/black wires would eliminate it completely.

Should that cause a problem, (check control will think the brake lights are on all of the time w/o seeing an input from the brake switch) a relay would fix that easily.

One interesting thing to note, it looks like BMW stuck a current limiting resistor in there between the ground and check control. If you have one handy, might as well toss it in.



The way the check control monitors lights is through special relays that work the opposite way you would think. The contacts of the relays are wired in series for all of the monitored lights on that circuit. (ex. the left, right, and center brake light bulb relays are all wired together) and they are wired as such that all the relays must be energized to complete the circuit to ground. (all wired normally open)

That way, with the last relay in the series circuit wired to ground, if you see a ground at the first relay, check control knows all three relays are energized for that circuit to be complete to ground.


What makes these relays special is the construction of the coil and contacts. The contacts are not the mechanical ones you would expect in a normal automotive relay, rather they are a reed contact enclosed in a sealed glass tube under a vacuum. The coil is different in that it has considerably thicker windings than a normal relay would, resulting in a much lower resistance. With the relay wired up to +12v and a ground, the low resistance would allow too much current to flow it would turn into a short duration heater rather than a relay.

Instead, the relay coil is wired in series with the bulb it is monitoring. The bulb is the highest resistance item in the circuit, so it works as normal but the current flow through the circuit also causes the magnetic field generated in the windings of the relay coil to close the sensitive reed contacts, completing the circuit mentioned earlier.

When the bulb burns out, the relay coil no longer has a ground, and as in any series circuit with an open anywhere, current flow stops and the relay de-energizes.

The Check Control system judges lightbulb function based on current flow, not resistance (though in a round-about way.....) Which is why if an improper bulb is installed (say a taillamp bulb where a brake light bulbb belongs) you may still end up with a "Brake Light" fault despite the complete circuit. The Taillight bulb is simply not a good enough path to ground for the brake light relay. (The relays are rated for different current draws based on the bulbs used, headlights have the biggest windings=need the most current flow, while the license plate lights have the smallest.)


Now that thats clear, can anyone provide ANY reasonable explanation as to why the color of the lightbulb base (brass vs. silver) would have any effect on anything? A dirty connection could sure cause it as your bulbb will be dimmer from the added resistance in the circuit, and thusly flowing less current which will cause the relay to remain open, but not bulb base color, which has nothing to do with anything.

[Van Westervelt]

Hmmm

[StereoInstaller1]

Dave (you fucking genius you), would that same deal work to drive LEDs in the 3rd brake light? Can you use the ground and the green/black and not have a false reading?

[DaveSmed]

<sigh> No, much easier than all that stuff mentioned.

IIRC, there are two green/black wires going to the third brake light, in addition to a brown, and a something else. Take the brown and the something else and tape them up and out of the way. Connect the two green/black wires together.

No more light.

[rodrigo]

Why? Just bundle up a 3rd brake light, bulb and all and put it under the rear deck.

...yeah...i guess i should do that then...i was hoping there would be somehting like a light bulb simulator from radio shack or something.LOL.

I'll route it down into the trunk and make a little box for it. :)

[StereoInstaller1]

Why? Just bundle up a 3rd brake light, bulb and all and put it under the rear deck.

[rodrigo]

The issue is that if you do, you will need to dissipate the heat. Unfortunately, your best bet for heat dissipation is a lightbulb....

...Goddammit!!
Next mod will be a check panel delete!!!

[StereoInstaller1]

So how do I do that...replicate the resistance of that bulb?

The issue is that if you do, you will need to dissipate the heat. Unfortunately, your best bet for heat dissipation is a lightbulb....

[Sam Cogley]

So how do I do that...replicate the resistance of that bulb?

With a resistor?


:p

[rodrigo]

You need to replicate the resistance of the bulb, then all is good.

Luke

So how do I do that...replicate the resistance of that bulb?

[StereoInstaller1]

everything is on bus circuit, so when you depress the break the it completes the circuit and the bulb goes out. All you have to do is to complete the circuit from the two wires going to the third break light. .

WRONG!

All that will do is pop the fuse, then no "break" lights at all (learn to spell, fuckers!)

You need to replicate the resistance of the bulb, then all is good.

Luke

[thereisnoyun]

In still trying to make it so that the third brake light in my car gets deleted...I should be done with rear shelf modification soon. pics will come.

my next problem is the check panel...and the 'BRAKE LIGHT' indicator always on...how the HECK do i get rid of that??? I dont want that red light always on...pulling out the bulb is out of the question!


I'm not too good electrically to resolve this.

everything is on bus circuit, so when you depress the break the it completes the circuit and the bulb goes out. All you have to do is to complete the circuit from the two wires going to the third break light. .

[rodrigo]

In still trying to make it so that the third brake light in my car gets deleted...I should be done with rear shelf modification soon. pics will come.

my next problem is the check panel...and the 'BRAKE LIGHT' indicator always on...how the HECK do i get rid of that??? I dont want that red light always on...pulling out the bulb is out of the question!


I'm not too good electrically to resolve this.