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You should have a brake pad sensor on at least one of the front brakes. It is inserted in the brake pad itself. This needs to be replaced anytime the warning message comes on. Once the sensor comes in contact with your brake rotor it turns the warning message on and is no longer usable. They are cheap at Autohaus
 

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CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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The sensor is basically a ground loop. When circuit is completed to ground, the warning is activated. It's possible your sensor wire insulation is worn off and grounding against the caliper or other suspension parts. In this case, you just need to replace the sensor (it comes with a new wire). Mine was doing this when it was raining and it ended being a loose connection where the sensor plugs into the wiring harness.
 

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2003 CLK 320 Coupe
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Not sure I understand.

In BMW products (at least I know, MINI Cooper, specifically) the brake sensors...also one per axle, alternate side...consist of a wire loop to complete a circuit. Ie: the circuit is "normally CLOSED". When the circuit is broken, or the sensor is unplugged (removed...whatever), the warning is triggered. The business-end of the sensor consists of a small exposed wire loop inserted within the pad, that when exposed to the rotor surface via pad-wear, breaks, opening the circuit and must then be replaced (or "cheated" at the connector).

What I think I'm understanding is that the Benz wear sensors work on the opposite principal...triggering the warning only when an exposed wire touches the rotor, thereby completing the circuit to ground ("normally OPEN"). If this is the case, my question becomes what is to prevent someone from simply unplugging the sensor at the connector and calling it done (never to be grounded/triggered again). Also--where the BMW system forces you to replace the sensors once broken, by design--can the Benz sensors be reused?
 

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Yep, it's normally open, so if the sensor fails, you can unplug it and you'll never get an error. Of course you can then wear your pads down to the metal with no worries about those annoying messages! ;)
 

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Discussion Starter · #10 ·
The old pads did not wear to the sensor. I tried to reuse it but broke it and it triggered the warning. I pulled it and the warning stayed on. Btw, I have never replaced the pads and not have to replace the rotors..
 

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2004 CLK 240 Coupe
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Also--where the BMW system forces you to replace the sensors once broken, by design--can the Benz sensors be reused?
As I see it, the MB brake wear sensors do not get 'broken'. They are embedded inside the brake pad, so that when the pad material is almost gone, the sensor touches the rotor and is grounded. So if the pads are replaced quickly, in theory you can re-use the sensor. But they are so cheap that folks normally renew them.

The warning provided is in the group which can be termed 'pre-alarms'. These are generated by software (from Front SAM N10/1) and are displayed in the cluster window as text - same as an exterior lamp failure, which is picked up by the routine diagnostic check at Start-up.

More serious faults wil illuminate an icon instantanously on the cluster.
 

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The old pads did not wear to the sensor. I tried to reuse it but broke it and it triggered the warning. I pulled it and the warning stayed on. Btw, I have never replaced the pads and not have to replace the rotors..
So you unplugged the sensor from the connector there at the hub? If not, and you jest left some bare wire hanging out there, it may be grounding and causing the fault. Also, there should be at least one more sensor on a another wheel so maybe that pad is worn and its sensor is grounding?
 

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Discussion Starter · #13 ·
Back brakes are good. No other sensors triggering. I completely unplugged the front sensor from the connector and the warning stays on. Is there no reset method such as pressing a series of buttons? I don't want to unplug the battery and erase all my settings.
 

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Something is grounding the sensor. Try unplugging the rear one - maybe its shorted.
 

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I remember unplugging the sensor, cutting the two wires, removing some wiring shielding, wrapping the two wires together, wrapping the exposed wires with electrical tape, plugging back in the sensor and the error message went away so that my wife would not be annoyed (and thus letting me know so) until the new sensors I ordered came in.
 

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Discussion Starter · #17 ·
I have solved the problem with help from the forum. I bought a new sensor ($8), plugged it in and the warning went off. Unlike others and I thought, as long as the senor was not grounding out, the warning should not come on. Obviously the sensor turns the warning on as well as off. Thank you all for your help. Geno
 

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You have made a good point! :)

I have now realized that if the sensor worked as I suggested in post #11 it wouldn't need 2 wires, just one. So there must be a continuous circuit via the sensor, which get interrupted if the brake pad wears down to the limit.

So an open circuit (or no sensor) will produce an alarm. I have a WIS drawing that shows S10/2 (front) and S10/4 (rear) brake wear sensors feeding into N10/1 Front SAM. But no pin numbers are shown.
 

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There is something odd about these sensors! Mine triggered yesterday. The same thing happened last year, but I think it was just water in one of the connectors because when I checked things, the pads were fine and so were the sensors, but after "messing with" them, the messages went away. Well, I checked tonight and guess what? I need new brakes! Oddly enough, though, the pads are not quite worn down to the sensors - just "almost". It makes me wonder if these are somehow inductive and sensing the closeness of the rotor? This is definitely not how they used to work. On other MBZ's I've worked on, the sensors were simply ground circuits and singe wires. These have two wires, so there is more going one here.
 
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