Rear Brake Light Stays on - Mercedes-Benz Forum
 
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#1 (permalink) Old 10-01-2012, 10:41 AM
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Red face Rear Brake Light Stays on

Hello everyone,
can anybody help me sort this problem out,
my "99" W220, has developed a brake light problem,
the brake lights work, but when you park up and remove the ignition key, the nearside brake light stays on, not fully on (as in fully lit up) but the lighting around it (you know what i mean) and it will not go off, i have had to disconnect the battery to stop it being drained.
i have checked the bulbs and all seem ok, there is a little rust, on the side light holder, which i have cleaned, but that was working anyway.

Any Ideas......

Thank's in anticipation.
Les.
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#2 (permalink) Old 10-01-2012, 11:41 AM
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Its maybe a bit obvious, but have you left one of the parking lights on?
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#3 (permalink) Old 10-01-2012, 12:39 PM
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That's weird, for sure.

If you disconnect the rear lights assembly you can use a multimeter to check voltages on the connector pins. You can compare the left and right sides.

Without the brake on, you should have no voltage on the pins below. With the brake on you should have 12 vdc. Also check for good continuity of the earth pins to chassis.

E3e4 pin 3 is the supply for the left stop lamp. Earth is pin 7.
E4e4 pin 5 is the supply for the right stop lamp. Earth is pin 1.

If you have a supply voltage there with no brake switch operation, it sounds like a fault in the rear SAM.
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#4 (permalink) Old 10-03-2012, 10:43 AM
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Thanks for your reply, i have posted a video on You Tube
Just in case it helps...
Thanks
Les.
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#5 (permalink) Old 10-03-2012, 05:51 PM
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Just noticed from my WIS that your rear stop lights (and also the center brake light) are LED and not the same as the others in the rear light assembly.

I wonder if the LED is picking up a stray voltage from somewhere. As you said earlier it does not seem to be the normal brightness.

Does it work as a stop light when operated? Or just stay on low permanently? As it is LED, it probably would not flatten your battery overnight. Try swapping with the other side to see if the fault remains.
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#6 (permalink) Old 10-04-2012, 12:49 PM
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Thanks for your reply keyhole,
the brake lights work perfect, and although the problem lighting is very low, in the dark it looks a lot brighter, i have to admit i would'nt know how to remove the lcd's to swap them over...?

Regards
Les.
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#7 (permalink) Old 10-04-2012, 06:23 PM
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Just a thought - the fog light setting is for one tail light to be on, and very bright. To activate it, one turn the headlights on and pulls the switch outward to the second stop.

I do not know what the fog lights do if the ignition is off and the key is removed. But you might check that you don't have the switch positioned so that the rear fog light on.

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#8 (permalink) Old 10-05-2012, 06:14 AM
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Hi Les

Without using a multimeter, and checking voltages/resistances from a wiring diagram, I don't see how you can trace this electrical fault.

Maybe it is 'auto-electrician' time here...
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#9 (permalink) Old 11-18-2014, 03:37 PM
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Hi Les, I have the same problem have you managed to fix the problem?
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