W209 Third Brake Light Reliability
 

Car is a 2007 CLK 350.

In 2011 the third brake light was changed by the dealer under warranty.

The problem is that now (Jan 2016) the vehicle dash warning system is indicating the third brake light is out. A visual of the brake system verifies all three lights are working properly. The likely problem is current out of spec going through the third brake light LEDs

Before I drop $150 on a new light I was wondering if there are any reported problems with the Factory (or aftermarket) third brake lights.

It is a little disturbing that a LED third brake light has gone bad twice in 8 years.

Thanks for the insight

I replaced both the third brake light and side mirror light, both worked but had intermittent errors. I could see corrosion on the connector on the mirror light that was likely the cause of the problem. Upon inspection the brake light appeared fine, no cracks or corrosion. The new lights fixed the warning messages. I believe the brake light was $104 on Amazon, MB OEM part. From what I've heard slamming the trunk will break the brake light, mine showed no evidence of any damage. I was happy to drop $150 to get rid of those annoying messages.

Check the battery voltage (12.6 at rest, 11v during cranking, 14v+ at idle). Mine was throwing 3rd brake light errors right before my battery died. I put in a new battery, and no more errors.

Thanks for the replies fellas

I replaced the battery in August 2011 with a 60 month one, it SHOULD still have life in it but certainly worth checking

I also forgot to mention that the dash warning of the inoperative third brake light comes and goes. Sometimes popping up at start up, at other times while driving. Currently I have been without a warning for three days, but quite possibly may get a warning today on the way to work.

Thanks again for the insight

Don't go by the age of the battery - check the voltage. The last thee MBZ batteries I've had have lasted less than three years each.

Well with Rodney's direction I put a LED based voltmeter in the cigarette lighter to check the charging system (it has three LEDs , yellow + green = low. Green=OK. Green + red=overcharging)

I was running fine for nearly a week with no "third brake light" error, but noticed today the yellow led was flickering, indicating low charge status, and coincidently the "third brake light" warning message appeared.

In addition I checked my records and noted the battery was installed 8-11-2011, nearly the end of the life of a 60 month battery.

I have a digital cigarette lighter voltmeter on order and will verify an actual number of the charging system.

Thanks again for the help

UPDATE
 

I do agree with Rodney that bad batteries/charging can wreak havoc on electrical systems. When the battery went bad on my Cayenne (after 8 years) I thought the car was possessed.

Today I had the nearly five year old OEM battery checked at my local AutoZone. The person testing the battery said it checked out OK and was testing good.

Occasionally the "third brake light" warning message pops up, but when I shut off the car and restart it I will not get the warning message for sometimes over a week. The Third brake light does in fact work when the warning light comes on.

My cigarette lighter charge indicator diagnostic tool will occasionally flicker "low charging," but mostly indicates normal charge status.

Does the brain trust still lean towards a bad battery? I ask because I may trade in the car for coupe with 5 liter or (gasp) 6.3 liter motor and don't want to drop $150 unnecessarily.

Thanks

Vehicle Voltage 13.8-13.9v at Freeway Speeds
 

UPDATE: I have been monitoring the system for a month now, Autozone checked my battery and reported it was still good. I recently purchased a numerical volt meter for the cigarette lighter and have been monitoring the system.

The car will run without issue a few days with no THIRD BRAKE LIGHT warning on the center display, but then suddenly while cruising on the freeway at 70 MPH and 13.8-13.9v charging the THIRD BRAKE LIGHT warning will pop up.

The third brake light was changed at the dealer under warranty 4 years ago. At this point I am beginning to think the issue is a fault within the electronics of the third brake light.

Curious to what the Brain Trust Thinks.

Thanks for your input

You know, after no problems for months now with the new battery, TODAY I get the warning! I guess it's time to take it out and check for breaks.

UPDATE
 

Man I hope I'm not jinxing myself here . . .

So this is what transpired,

After receiving the first third brake light warning on the dash I replaced the battery to see if that would solve the problem. The third brake light had been replaced about four years ago at the dealer under warranty so I was hoping that was not the issue. I still got the third brake light warning on the dash even after a new battery was placed, I went to AutoZone and had the battery tested, it tested out good.

I put a digital voltmeter in the cigarette lighter to monitor the charging system, perhaps an electrical issue may be tripping a fault. I was consistently getting 13.8 and 13.9 V while driving the car.

Typically the fault would occur after driving the car between 25 to 35 minutes and it would come up unexpectedly. It did not matter whether I was on the freeway or city streets when the fault appeared.

After receiving that fault if I shut the engine off and restarted often I would get another 10 to 15 minutes of driving without receiving the fault, but invariably it would always arise again.

So I bit the bullet, and purchased an OEM third brake light for $115 off Amazon. Today I replaced it and drove the car for an hour and did not receive any faults or indication of trouble. Hopefully this is the end of my problem because I certainly do not want to trace any breaks in the wires because it does not look like an easy job.

Thanks to everybody for your insight and help I hope this thread help somebody in the future.

Regards

All my lamp messages have stayed gone after replacing the lamp assemblies (knock on wood).

Congrats Brother RN, fancy seeing another Pelican here :-D

side note, the cig VMs tend to be .5 volts lower than the battery terminals. I would double check with the DVM on the terminals vs source at the Cig Lighter.

Crazy how MBZ has so many freakin electronics even for a brake lamp, geez. I bet if you open it up like Rudeney you will find broker solder joints. Heat expands the joints ergo short in the assembly.

Thanks pal, "Birds of a Feather . . ."

I bet it would be difficult to find the broken solder joint in the old third brake light. I wonder if the old unit could be repurposed as an under counter light. If I succeed I'll post a photo,again thanks everyone for your help

Ok if anyone is interested in this repurposing of the defective third brake light here are the results:

It is very simple to connect power. An old 12v 1.5A transformer I had lying around lit up the brake light with no problem, but (wisely) the Mercedes engineers used Red LEDs in the unit, so even with the red plastic lens removed the light produced is Red, unless you are using it for a dark room it won't be of much use as an under the counter light.

Mood lighting for the 911 :D

Well, thank you all for jinxing me! :) I got the error yesterday while driving. I checked battery voltages and all is in order. I got it again today every time I started the car and pressed the brake pedal. I guess a replacement or "surgery" is in my near future.

No voltage to 3rd brake light, Left and right lamps are OK
 

I removed the LED assembly and bench tested the unit. LEDs all light with bench power supply. Could not get it to flicker even when flexing and tapping the assembly. I checked the power at the wires on trunk lid just before the LED connector and there is no voltage (with brake depressed). The left and right brake lamps operate normally. I did confirm continuity between the brown wire and chassis ground, zero ohms. Where do I trace the hot wire back to the N10/2 distribution panel? I cannot find where the 3rd light gets it's power from.
I also tested the trunk latch operates normally.

It's fed directly from the rear SAM. If the SAM detects that it is not working, it will electronically shut it off. You can't volt-check the circuit without the light assembly connected.

Thanks Rodney, I made rookie mistake. All this time I thought the brake lights were on, they were the park lights. Once I turned the key to the #2 position the brakes lit and so did the 3rd light. I did the messy surgery to open up the 3rd light and re-solder the connector pins that were wobbly. I believe that was the source of my bad LED in the first place. I used a dremel to cut open the LED housing. I then re-soldered the connector pins and the diode. I used JB weld to reassemble and reinstalled. All working well now. For others who are thinking of trying this... I don't recommend it. A lot of time and mess and you stand a chance of breaking the housing or the lens. Just spend the $120 for a new MB part.

@RN, I am curious to know if replacing the light has eliminated the intermittent dash alert.
We have been living in a parallel universe with the issue you describe. If so, is replacing it very difficult on a scale of 1-10?
Many thx.

Not difficult but a lot of plastic fasteners to remove before you can access the nuts
 

Not difficult, just lots of fasteners to remove to access the lamp assembly.

What I found out is that all those third brake lights from w209 (at least the ones I came across) are suffering from the same thing - 2 metal pins in the connector has a fractured solder joints on where they are soldered on the brake light circuit board. Crappy lead-free solder from the factory.
Even the light is partially working the vibration gets the contact disconnected from the board and then you have the warning message on the cluster.