W203 High Level (3rd) Brake Light

[Dichtung]

Hello Guys,
Anybody had this fault - A few weeks ago, up popped a message `3rd brake light. Check lamp` . I knew that only about a third of it worked, but SWMBO stood at the back and reported total failure.
I replaced it with a new one off ebay, probably Chinese. but still no joy.
I removed the new one and tested it with a 12v power supply and it works.
I can`t really see the feed wire breaking and am wondering if this lamp has a separate fuse somewhere or is it `electronically` supplied and monitored.
Since then, the car passed its MoT with one advisory - Non obllgatory high level brake light needs repair.

Any ideas ?




















I removed it

 

[BTB 500]

I can`t really see the feed wire breaking

Wiring which is frequently flexed back and forward could fail eventually (after 15 plus years?) ... certainly heard of this on folding mirrors and front doors.

 

[mioba]

Have you tested the voltage of the feed wire....

 

[Dichtung]

BTB 500. Thanks. Yes , at the boot lid hinge area. That thought did cross my mind. That would be a nightmare to fix.

Mioba. Thanks. The tiny plug on the feed wire end defies ordinary tester probes. But yes. logic suggests this needs checking somehow.

I`ll keep you posted.

 

[BTB 500]

The tiny plug on the feed wire end defies ordinary tester probes. But yes. logic suggests this needs checking somehow.

I've got a set of test leads terminated with small insulated croc. clips - these hold sharpened pins made from copper wire (stripped from mains cable) ... I have a couple of sizes:

 

[Dichtung]

Thanks BTB, Now in the process of cobbling similar probes.

 

[Dichtung]

Bad news I`m afraid. I tested the supply plug using fine probes and the meter just flickered maybe up to 0.2V . To eliminate possible supply wires to plug problem, I snipped the plug off and connected the meter directly onto the supply wires.
Same result, so it is looking now that the old car will have to live without that 3rd brake light.
And I will have to live with the constant reminder to check that lamp. But HeyHo.

 

[Justintyme]

And the two other brake lights are working OK? Can you not piggy-back a feed from one of the cables from these two ? That car does not have the Hi-Tec canbus system that came with newer models, so should not be problematic Plus....its accessible, unlike the later E class brake light in the rear window..and the rear brake lights are incorporated into the park lights....,single cable powering both brake and park lights.

 

[Dichtung]

Thanks Justintyme, That may be worth a try. The car is an 03 reg. so there`s not a lot to lose if it totals the system.

 

[Alps]

will be a wire in the hinge of the bootlid, same happened in my E55, found a few wires which had snapped, re-joined and all were good again (for a few more years anyway )

 

[Dichtung]

Yeah, thanks Alps. I`m pondering JustinTyme`s suggestion, which will probably work, but will still most likely beep and message me everytime I brake forever; or I can bite the bullet and try to find a broken wire in the hinge area.
But not today, it`s sleeting out there.

 

[Alps]

will be a wire in the hinge of the bootlid, same happened in my E55, found a few wires which had snapped, re-joined and all were good again (for a few more years anyway )

Yeah, thanks Alps. I`m pondering JustinTyme`s suggestion, which will probably work, but will still most likely beep and message me everytime I brake forever; or I can bite the bullet and try to find a broken wire in the hinge area.
But not today, it`s sleeting out there.

it honestly won't take that long to find, , probably take longer to do the workaround! better do it properly first time

 

[Dichtung]

An update to the problem. I cannot find a broken wire in the lid hinge area, so I am assuming that the fault lies at ` the other end` so to speak.
I tried an independent wire straight from the battery negative to the lamp supply in case of a bad earth connection, but that didn`t work either.
Before I replaced the boot lid liner, I extended the lamp wires in case I resort to connecting to the offside stop lamp as Justintyme suggests.
Thanks all.

 

[Justintyme]

An update to the problem. I cannot find a broken wire in the lid hinge area, so I am assuming that the fault lies at ` the other end` so to speak.
I tried an independent wire straight from the battery negative to the lamp supply in case of a bad earth connection, but that didn`t work either.
Before I replaced the boot lid liner, I extended the lamp wires in case I resort to connecting to the offside stop lamp as Justintyme suggests.
Thanks all.

I have a similar ( if slightly different 3rd brake light problem) I need to take a feed from the live cable supplying the 3rd brake light in the rear window. I need it to power the towbar relay brake lights. I can get the park light feed fom the Nr plate lights. I need to do it this way because my 2014 E class w212 brake and park lights are energized through a single cable, when the brake is pressed it sends an increased pulse to the LED light which causes it to "glow" brighter, and you have your brake light. I can't get this to work, even using a "Smart" relay, so I have to find individual brake and park feeds to power the relay. Hence using the 3rd brake light, and Nr plate light to get the feeds I need. The Nr plate light is not a problem.. its easy enough to access, The 3rd one is a different kettle of fish...I removed the rear seats, no problem, but when it came to the platform behind the headrests and the vertical 1/4 panels containing the airbags, after some serious tugging and pulling, I chickened out. So after much ( and more ) searching, I managed to get the wiring diagram for the rear section of my car, and it shows exactly where I need to go to get the feed for my brake light, without disturbing the rear window at all.
Now for your problem, if you can get the rear wiring diagram for your model, it will save a lot of hassle, but my suggestion still stands. The wiring in your car is the "old Fashioned" type...one function, one wire, so no need to be afraid of doing damage. The fact that you have already put cables in place, just extend one of them back to the rear brake light, and test it before making it a permanent arrangement. You will need to disconnect the original wire from the light of course. Once it works, you will be able to route it into the boot lid at the rubber grommet. Good luck.

 

[Dichtung]

Well, Justintyme, my problem is miniscule compared to yours. But you sound like a capable fella, so good luck with that one.