Brake lights won't turn off?

George.Smith

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C Class AMG Coupe 2012 1.8 Turbo
I have a 2012 Mercedes C Class Coupe, and the brake lights won't turn off.

I had this problem about 2 months ago, but it was fixed by replacing the brake switch. Now, the brake lights have been temporarily staying on again. By temporarily I mean they'll be fine, then I'll turn on my car and they'll be on, after a while they might turn off by themselves, or the next time I turn the car on the lights will be off. It's happened again but now they really won't turn off.

I'm thinking it's a fuse problem, but there are 3 different fuse boxes and so many different fuses relating to ABS, brake lights, etc.

Any ideas?
 
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George.Smith

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Does having the sidelights off/on/auto make a difference?
Nope, I've just tried with the light switch in all positions and the brakes stay on. Also just removed the fuse for 'brake light' and the brake lights are still on. There's supposed to be a fuse box under the dash, but I've had a look and that's not there so I don't know if that's because its a different model. I'm honestly so confused
 

Ken_R

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W203 2006 C230 Estate
There's supposed to be a fuse box under the dash, but I've had a look and that's not there so I don't know if that's because its a different model. I'm honestly so confused

Different car, but mine is at the end of the dash. Open door [driver's] pull off end cap, and, if it's like mine, there will be a folded piece of paper there with the 'fuse legend'.

W204fuseboxlocations1799x1012-65419.jpg


I believe the image show a LHD variant so, reverse locations.

That's assuming, I've got the correct model variant. I'm not fully au fait on such matters.:(
 
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George.Smith

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Do you have a towbar with wiring?
I have the button for a towbar in the centre light console, but whenever I've pressed it I've not noticed anything so I'm not 100% sure
 
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George.Smith

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Different car, but mine is at the end of the dash. Open door [driver's] pull off end cap, and, if it's like mine, there will be a folded piece of paper there with the 'fuse legend'.

W204fuseboxlocations1799x1012-65419.jpg


I believe the image show a LHD variant so, reverse locations.

That's assuming, I've got the correct model variant. I'm not fully au fait on such matters.:(
I've taken the bit off at the end of the dash and these is no fuse box there. I do have the bit of paper with all the fuse information though
 

Ken_R

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W203 2006 C230 Estate
Getting back to basics. The brake light switch was replaced. By whom? Yourself/friend/mechanic/garage? Was the old unit tested and found to be faulty?

Do you have a multimeter? They are 'cheap as chips' compared with decades ago!
 
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George.Smith

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Getting back to basics. The brake light switch was replaced. By whom? Yourself/friend/mechanic/garage? Was the old unit tested and found to be faulty?

Do you have a multimeter? They are 'cheap as chips' compared with decades ago!
The clip broke on the old switch, so it was replaced by a mechanic. I do have a multimeter
 

Ken_R

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Not my territory, as I have never had to delve below the dashboard into the pedal area, and I'm not even sure that I still have the dexterity;), but I'd be looking at removing the connections to the brake light switch and checking for 'open circuit' across the unit, and testing for functionality in relation to pedal movement.
 

EmilysDad

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Brake switches normally have some kind of adjustment ..... my Smart Roadster's does as did my Omega ..... the plunger that hits the pedal usually clicks in or out.
 

PINBALL

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I think its only a 'cherry' type micro-switch, that could have been dislodged (by carpet removal n refitting or even by your toe) if its not the correct type and may need to be swivelled around again particularly if its been fitted loosely.

Problem is getting to it as its up under dash.

Personally I'd order the proper one from Merc & ask another garage to fit it this time.

Can't see it being anything to do with a fuse unless they are not working at all.
 

PINBALL

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Also check to see if cruise control still works regardless, as this can only work when brake lights switch is OFF.
 

Uncle Benz

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Might sound stupid, but have you tried lifting the pedal up, to make sure it is fully returning to top of travel?
 
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George.Smith

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I think its only a 'cherry' type micro-switch, that could have been dislodged (by carpet removal n refitting or even by your toe) if its not the correct type and may need to be swivelled around again particularly if its been fitted loosely.

Problem is getting to it as its up under dash.

Personally I'd order the proper one from Merc & ask another garage to fit it this time.

Can't see it being anything to do with a fuse unless they are not working at all.
That's what I thought. If it was the fuse it would mean the brake lights wouldn't work at all? The cruise control test is a very good idea, I didn't even think of that. I'll try that when I leave work and let you know
 
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George.Smith

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Might sound stupid, but have you tried lifting the pedal up, to make sure it is fully returning to top of travel?
Doesn't sound stupid! I have tried pushing the peddle down then getting behind it and lifting it, but still no luck unfortunately
 

kid-jensen

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Definitely a job for the multimeter.

You should be able to see the switchj change state at the point where you apply pressure to the pedal. Measure the volts across it, and they should appear or dissappear when the brake pedal is moved.

If there's no change, then most likely the switch is faulty, or out of adjustment, such that it is thinking the pedal has already been pressed.
 

mattkh

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Doesn't sound stupid! I have tried pushing the peddle down then getting behind it and lifting it, but still no luck unfortunately
Hi
Have you had a look at the brake light bulb itself.
If possible, please put up a picture of the brake light assembly.
 

Qwertyco

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Thread from the dead, I know, but I just had this problem.
'04 facelift C220 CDI Sportscoupe, UK, 646 engine.

Seems like my problem was damp in the front fuse box.

Pulled all the fuses one at a time and they were all a bit oxidised, and there was a lot of condensation in the fuse box lid.
As soon as I pulled fuse 53 the brake lights went out,(*** BEWARE - the engine fan also started up at full speed - BEWARE***)
Cleaned up the fuse, and now all seems well.
I pulled every plug, relay and fuse and cleaned the contacts, used contact cleaner spray and set my workshop vaccuum on BLOW and pushed the nozzle deep in the fusebox to try to dry it out.

Fuse 53 - 25a - Starter relay
Rear SAM control unit with fuse and relay module
Valid for engine 611/612/642/646: CDI control unit

thanks to this: https://fuse-box.info/mercedes-benz/mercedes-benz-c-class-w203-2000-2007-fuses-and-relay
 

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