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Old 05-14-2019, 10:21 PM
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Third Brake Light issue

Hey everyone, new X5 owner here. I've owned the car for a few months, and the third brake light hasn't worked since I bought it. I used a multimeter to check the connection in the spoiler where the third brake light is, and no power is reaching it. I've checked the fuses, all are good. I checked where the wiring goes through the tailgate into the spoiler. A few of the wires showed wear from the opening/closing over the last 15 years, but only in the outer rubber covering. No visible breaks to the wires inside. I thought it could possibly be the LCM, but every other light in the vehicle works. Even all the lights in the door handles, etc. The third brake light is the only light in the entire vehicle that doesn't work. Even purchased a new light to no avail. Anything else I should be checking? It's really starting to irritate me. Any help would be greatly appreciated. Thanks!
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Old 05-14-2019, 10:53 PM
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Codes?

That'll tell you usually if there's a fault before the light or because of it.

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Old 05-14-2019, 11:26 PM
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I had some problems with a couple of segments in my LED 3rd brake light bar in my 2001 3.0i. I fixed that by reflowing the solder on the LEDs. The individual lights are in 6 segments of 4 LEDs each.

The light bar assembly is powered with 12V at the Blue + Brown wires. So with the light bar removed / disconnected, stepping on the brake, you should be getting 12V at those wires. If not, the problem is upstream of that point.

The bar is wired internally so each of the 6 4-LED segments gets its own 12V, which then drops across all 4 LEDs in series. So if you have any single LED or solder joint failure, you are likely to lose the whole 4-LED segment, or 1/6th of the light bar. Minor failures like that did not cause codes for me.
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Old 05-16-2019, 10:49 AM
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Yeah, thatís my problem. No power at the plug in the spoiler. So trying to find out where the disconnect is at. Do the blue + brown wires come off the taillight assembly in the rear? If so, I might just run new wires from that point up to the third brake light.
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Old 05-16-2019, 12:04 PM
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I don't know where that 12V on those two wires comes from, since I never needed to trace that back. So my story is just to confirm that those wires are simply 12V when the brakes are on, and 0V otherwise. Nothing weird due to the LED, vs. the other lights which are all non-LED (on my car at least).

If you can't find someone else with the answer, or an accurate wiring diagram ...

If you find a blue+brown pair of wires further upstream, you might assume they should be 12V / 0V for testing purposes, and go from there.

Too bad you bought an unneeded light bar. Those things were close to $200 when I had my issues, which is why I went for the repair vs. replace option. The repair is holding up solidly after solder reflowing.
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Old 05-16-2019, 03:16 PM
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Are you familiar with the online TIS website? You can look up the wiring schematics: https://www.newtis.info/tisv2/a/en/


Enter the last 7 digits of your VIN...then type some keywords or phrase into the search box to see if you get any results that might lead you to the wiring diagram for the high mount brake light. Or you can try to find it on your own perusing through the wiring diagrams and functional description link.


EDIT: click link and look for H34 (raised brake light): https://www.newtis.info/tisv2/a/en/e...centre/hKBHqmp



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Old 05-16-2019, 05:00 PM
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Great links, Qsilver7...

It looks like the positive power to the third brake light connector (X138) is a single continuous wire, right back to the LCM. My next test would be to check if there is power between the black/yellow wire (X138, pin 2) and a known good earth point, as the issue may be with the earth connection rather than the power connection (as the earth connection has more joints).

Clearly, you need to test this with the foot brake applied!
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Old 05-16-2019, 09:32 PM
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Thanks for the info guys. Since I don’t have a garage to use at the moment, I have to wait until the weather clears up in order to tear into this again
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Old 07-01-2019, 12:31 PM
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update

I still haven't been able to track down the problem with my third brake light (high mount, center in the spoiler.) I have a Foxwell NT510 scanner, and it shows no issues with the LCM or brake lights. No fault codes. Brake switch controller checks out ok, too. When I ran the live tests through the scanner, it tells me the third brake light is activated, even though it's not lighting up. Just to make sure, I did a live test on all the other lights, and they all work correctly. And to double check, I disconnected the third brake light from it's connector in the spoiler and hooked it up directly to a different power source, and the LED's came on. So I know it's not an issue with the LED's or the unit itself. If there is a break in the wires somewhere coming from the LCM, wouldn't I be getting a fault code on the scanner? Just wondering what my next step should be.


2003 X5 4.4i, 6/03 build date

Last edited by Bradleylee; 07-01-2019 at 12:35 PM. Reason: add signature line
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Old 07-01-2019, 02:32 PM
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Check the wiring in the rubber boot that goes from vehicle body to hatch. Very common for fraying and breaks in the wiring at that locale.
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