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      09-01-2014, 09:45 PM   #1
SamuelL421
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My third brake light is out, replaced today and still broken?

About a week ago, I found that my third brake light (LEDs in the middle) had gone out and that the dashboard "light out" indicator was on. Knowing this to be a common problem, I ordered a new light thinking it would be a simple fix.

After the light arrived earlier today, I removed the old light and attached the new one. Unfortunately, it hasn't solved the problem – I still have no third brake light and the dash indicator is still on . I next tried a multimeter to check the voltage going to the brake light when the pedal is depressed. With the brake pedal pressed down and the other brake lights working, I still get no noticeable voltage to the third brake light (my multimeter is set to 20v DC).

I was thinking that I could possibly test the third brake light outside of the car to see if it is even working correctly. I'm not sure how to go about doing this, does anyone have some advice on the topic? How could I test my third brake light and what sort of power source could I use?

Alternatively, maybe there is some way I could check if the power line / ground for the third brake light is broken somewhere? I know these cables run through the rubber connection at the edge of the trunk, could that be a possible failure point?

Has anyone else had this or similar problems with the third brake light? Any ideas what to try next? I welcome any and all advice (I'm definitely no electrical guru like some of you)
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      09-02-2014, 12:00 AM   #2
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Assuming you don't have a bench power supply, you can test it with a 9 V battery.
It won't light very brightly, but it will light enough to see if it's good.
It's probably polarized, so if the 9 V battery doesn't light it, turn the battery around.
The common problem I'm aware of is the lenses shattering, not the LEDs failing.
Did you check the fuse? Not sure exactly which one it is, or if the third even has its own fuse.

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      09-02-2014, 02:05 PM   #3
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Do what Stickmon says but I'm hoping its simply a fuse, and not messed up wiring. Have a look at any exposed wiring for damage or fraying.

Check the connection at the bulb to be sure its tight just in case its making a poor connection.
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      09-02-2014, 08:45 PM   #4
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My 3rd brake light on my coupe is also burnt out but with no light on the dash. I was going to buy a new 3rd brake light but now I want to see what comes of this. BTW is there a fuse for the 3rd brake light?
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      09-03-2014, 09:38 PM   #5
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I know this sounds obvious but there are a two identical connectors in the boot lid (on my 06 anyway) and only one is for the third brake light. Make sure you are using the correct connector.
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      09-10-2014, 10:28 AM   #6
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I had the same issue, then opened up the wiring loom and saw that it had frayed from the years of opening and closing the trunk. Check there.
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      09-10-2014, 02:48 PM   #7
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Quote:
Originally Posted by ferrarif1 View Post
I know this sounds obvious but there are a two identical connectors in the boot lid (on my 06 anyway) and only one is for the third brake light. Make sure you are using the correct connector.
What is the second identical connector for? A spoiler brake light?
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      09-30-2014, 09:40 PM   #8
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OK, I finally had some quality time with the z4 and a multimeter and I got some odd results...

After testing the old 3rd brake light (badly cracked) and the brand new one, both seem to work. I then turned to the wiring itself, with the brake depressed, I had no voltage at the connector. I then moved back along the wire itself and tested again - same result, no voltage. Next I moved back to where the wiring goes through the small rubber boot between the trunk and the body. I pulled back the boot and checked the wires on leading into the body and had the same result again, nothing happening on the multimeter.

Any ideas for what I might try next? Could it be a fuse issue everything else is still working correctly?

Electrical gremlins... *sigh*
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      09-30-2014, 11:19 PM   #9
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Here's an older post showing the full E85/E86 fuse chart, if you want to check a couple other lighting circuits, or ones in that area.
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      10-01-2014, 12:04 AM   #10
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Here's the entire schematic from the WDS, FWIW.
Fuse 14 is all the brake lights, so not much help there.
You're sure you aren't plugged into the "mystery connector"?
When I replaced my cracked light, I noticed a similar connector hanging up there, but didn't investigate.

Assuming that's not the case, I would next check for voltage at pin 16 on connector X12 of the Light Switch Cluster.
As for where that's at; I'll leave it as an exercise for the student.

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Last edited by StickMon; 10-01-2014 at 12:53 AM..
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      10-01-2014, 12:31 PM   #11
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Am I the only one that does not understand StickMon's last post?
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      10-01-2014, 12:35 PM   #12
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Sorry. Did I lose you there?
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      10-01-2014, 01:14 PM   #13
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Quote:
Originally Posted by SamuelL421 View Post
How could I test my third brake light and what sort of power source could I use?
If I read the wds diagram correctly you might not be able hook up the 3rd brake light directly to a 12v power source (battery or so). There is a transistor in between (in the light switch cluster) that might be there just to turn on the light, but it might just as well be there to limit the current (the diagram is not conclusive in that). You can try hooking it up in series with a 5W resistor, 120 ohm or so (that's a guess, the leds are paired in clusters, so the right amount of current depends but I think this is a safe setting: 120 ohm limits the current to 100mA). If you dont see them dimly lit, theyre broke or get not enough current (use them without the resistor).
If you have a current limiting power supply you can use that (but you probably haven't or you wouldn't have asked this question)

But I expect that you have a broken wiring loom somewhere. At the trunk hinge is the most likely area provided your car hasn't been tempered with by an incompetent mechanic in the past.

If you want to measure continuity on the wiring loom, the light switch cluster (A3a) is located here:

There is a black&yellow wire going from that light switch cluster all the way to the 3rd brake light. And from the 3rd brake light there is also a brown wire going to earth. Thats the easy one to measure
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Last edited by GuidoK; 10-01-2014 at 01:21 PM..
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      10-01-2014, 04:23 PM   #14
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The current limiting for the LEDs is built into the light, so you can run it directly off 12V.
The transistor acts like a switch. Where it says "30" is the voltage source.
The transistor will be open when the light's off, closed when it's on.
The little rectangle above it is a small value resistor that's used to measure the current going to the bulb for the sake of the bulb out warning.

I think Samuel established in post #8 that the light is good, just not getting power.
Next thing would be to get at pin 16 on the back of the Light Switch Cluster, where the black/yellow wire comes from, and see if there's voltage there (with the brakes on, of course.)

If not, it doesn't look good for the Light Switch Cluster.
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Last edited by StickMon; 10-01-2014 at 06:47 PM..
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      10-05-2014, 02:50 PM   #15
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Thanks for all the help and feedback guys, I'm going to have downtime later this week (thurs/fri/sat) to do more testing - I'll report back what I find out.


Quote:
Originally Posted by StickMon View Post
The current limiting for the LEDs is built into the light, so you can run it directly off 12V.
The transistor acts like a switch. Where it says "30" is the voltage source.
The transistor will be open when the light's off, closed when it's on.
The little rectangle above it is a small value resistor that's used to measure the current going to the bulb for the sake of the bulb out warning.

I think Samuel established in post #8 that the light is good, just not getting power.
Next thing would be to get at pin 16 on the back of the Light Switch Cluster, where the black/yellow wire comes from, and see if there's voltage there (with the brakes on, of course.)

If not, it doesn't look good for the Light Switch Cluster.
^
I'm going to walk back through everything I have tested so far to rule out everything that I can. After that, I guess I'll have to start looking at the X12 connector (*cue ominous music*).

Quote:
Originally Posted by GuidoK View Post
If I read the wds diagram correctly you might not be able hook up the 3rd brake light directly to a 12v power source (battery or so). There is a transistor in between (in the light switch cluster) that might be there just to turn on the light, but it might just as well be there to limit the current (the diagram is not conclusive in that). You can try hooking it up in series with a 5W resistor, 120 ohm or so (that's a guess, the leds are paired in clusters, so the right amount of current depends but I think this is a safe setting: 120 ohm limits the current to 100mA). If you dont see them dimly lit, theyre broke or get not enough current (use them without the resistor).
If you have a current limiting power supply you can use that (but you probably haven't or you wouldn't have asked this question)

But I expect that you have a broken wiring loom somewhere. At the trunk hinge is the most likely area provided your car hasn't been tempered with by an incompetent mechanic in the past.

If you want to measure continuity on the wiring loom, the light switch cluster (A3a) is located here:

There is a black&yellow wire going from that light switch cluster all the way to the 3rd brake light. And from the 3rd brake light there is also a brown wire going to earth. Thats the easy one to measure
^I managed to get dim light from the LED strip, but my testing setup was extremely crude. I'll look at these again with some more precise measurement when I check back over everything.

- *off topic* GuidoK, your VANOS guide saved my @$$ last year - so I'll take suggestion you've got
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      10-06-2014, 09:57 AM   #16
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Quote:
Originally Posted by ferrarif1 View Post
I know this sounds obvious but there are a two identical connectors in the boot lid (on my 06 anyway) and only one is for the third brake light. Make sure you are using the correct connector.
I noticed exactly the same thing in my Z4M! In fact, I hooked my new 3rd brake light up to the wrong one first. When it didn't come on, I thought the light was faulty and just by luck I noticed the other connector when getting ready to put the old (cracked) light back on. Weird!
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      01-14-2021, 10:03 PM   #17
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where is the light switch cluster (A3a) located?

The location http://www.bmw-planet.com/diagrams/release/zi_images/GREO_E85_61_0032.png[/img]
does not seem to exist anymore.

de
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      01-16-2021, 06:04 AM   #18
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It is the module that has your light switch on the left of the steering wheel (lhd car).
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