W203 Brake lights on continuously with ignition

Looops

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Joined
Sep 11, 2011
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45
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CLK230
Evening all.
Today I was about to send my W203 C Class 240 in for MOT and noticed the brake lights are on all the time that the ignition is on.

I've got a good scanner and getting B1048/Stored & Current. "Circuit 54 has Short circuit to positive".

Can't see any damp around the rear SAM but one of the connections (large black cable) had seen some overheating. I cleaned it up but no difference.

Any thoughts anyone?
 
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Looops

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Joined
Sep 11, 2011
Messages
45
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CLK230
I've changed that already troubleshooting a problem with BAS. Also I can see real-time on my Foxwell code reader that the switch is ON/OFF etc.
 
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Looops

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Joined
Sep 11, 2011
Messages
45
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CLK230
Update to this. Changed the rear SAM for a used one from eBay but the symptoms persist.
Brake lights stay on with ignition.
However on the one I got the fuel pump relay the car wouldn't run, the fuel pump relay contacts were badly corroded. I'm going to send it back for another or a refund.
 

Syncropete

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Aug 8, 2012
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w463
Hi, Did you ever fix this. I have the exact same fault on our W463 G400 (same electrical system as w203)
Regards
 
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Looops

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Joined
Sep 11, 2011
Messages
45
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CLK230
Sorry, I scrapped the car. The CAN BUS went from bad to worse and I ran out of patience after hours and hours of troubleshooting. One thing to look at, was ok on mine but apparently it's common, look at the CAN BUS connector by the right and left footwells, they get wet and corrode and drive the network mad.

Good luck.
 

Qwertyco

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Aug 29, 2017
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The Cold North East
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C220 SE CDI Coupe
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: Fuse Box Diagram Mercedes-Benz C-Class (W203; 2000-2007)
 

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