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211 Brake Sensors - do they work?

Discussion in 'Suspension, Steering & Brakes' started by AlastairD, Oct 25, 2012.

  1. AlastairD

    AlastairD Senior Member

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    Hi,

    A recent service informed me my brake pads were 40% worn (4k miles ago), however while on holiday this weekend I get the telltale scraping of metal to metal from the brakes kindly informing me that the pads are down to the rivets. Oh well I'm due for new discs anyway. But how could this be?

    I used the forum discount and picked up discs and pads from ECP. Both sets of pads I removed had both wear sensors completely obliterated and not a hint of this from the car! The one down to the rivets was on the piston side, the opposite side still has 7mm on it! Do I have a dodgy caliper or is this normal.

    Is it that the wear sensors simply were not plugged in properly or should I be looking for something else?

    Any advice or even opinions appreciated.
     
  2. Corned

    Corned Senior Member

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    Wear sensors are only on one side of the caliper, and not on one of the rears at all. I.e. there are three sensors - one each for the fronts, and one for one of the rears. They fit into one of the two pads on each caliper.

    It is therefore quite possible for the un-sensored pad to wear faster than the other, and of course not give you any warning that it is worn out.

    As to why one pad is wearing faster is a different question. They should wear evenly in theory, but this does not always happen in practice. If the caliper slider is not free to slide (in a single-piston assembly) then uneven wear is likely. If one disc face is corroded, pitted or scored, this may cause the respective pad to wear faster.

    Sods Law usually dictates that the inward face is the one which causes trouble - because it is the most inaccessible and difficult to inspect!
     
    Last edited: Oct 25, 2012
  3. OP
    AlastairD

    AlastairD Senior Member

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    The pegs on the wear sensors had totally gone, one was set to be worn away earlier than the other. Surely the car should have highlighted this?
     

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  4. Blobcat

    Blobcat Moderator

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    I've been "advised" to replace pads before by the dealer, my response has always been I'll wait for the sensors to let me know. I've never gone down to the metal and the sensors have never failed to alert me in plenty of time. It looks like your sensor circuit isn't working as it should be.
     
  5. Corned

    Corned Senior Member

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    Ooooh, nasty. The sensor should have alerted you to that one.

    They are plugged in under the wheel arch somewhere, and then connected into the front SAM iirc. Best to check all connections back to the SAM for security.

    But to answer your original question - yes, the car should have given a warning for that.
     
  6. Corned

    Corned Senior Member

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    One other question.

    Have you removed that sensor from the pad yet? and if so, was it worn down? It could possibly not have been pushed fully home, and therefore not have actually come into contact with the disc.

    Or is it ground down to nothing, just like the pads?
     
  7. wireman

    wireman Senior Member

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    Re-using old sensors results in them not working next time they should, perhaps its been previously attended to by a cheapskate.
     
    Last edited: Oct 25, 2012
  8. OP
    AlastairD

    AlastairD Senior Member

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    Thanks all,

    There's nothing left of the pegs on the sensors at all.

    So when the peg has been worn away I should get open circuit and a warning? Or is it more complicated as is the German way?

    I'd like to test and make sure it will work next time round but if I do I'll probably just get errors to get cleared. I could use the old sensors and check all the connections until I get a message.

    I thought I could rely on the information from the service but that didn't work. If it happens next time I'll be trashing a new set of discs!
     
  9. wireman

    wireman Senior Member

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    There are two types of sensor in use on most cars, the older kind are a single wire which contacts the disk when the pads are worn and makes a circuit to switch on the warning lamp, this works only when the pedal is pressed and can be a wee shade iffy by not always lighting or flickering the lamp. These sensors have a single wire and require nothing more than a lamp to complete the warning system.
    Leaving them disconnected disables the warning system.

    The other kind have two wires, they have a shorting strip inside them which is cut once the sensor contacts the disk, this causes permanent warning, as does leaving them disconnected.
    Some form of interface is required to swap the sense of the signal, its usually electronic and often integrated into one of the many ECU's and the like on the car.
    The cheat so that the warning lamp does not come on all the time is to short circuit the wires of the sensor(s), a strand of fuse wire bridging the sensors connector pins is a common cheapskate trick.

    New sensors must be fitted with the new pads with either type of system.
     
    Last edited: Oct 25, 2012
  10. Ricardo_e220

    Ricardo_e220 Senior Member

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    Oh by the way having just done the fronts on mine recently (2009 model) there is only 1 sensor on the offside front wheel, and the daft thing is that its on the outer pad which possibly due to the location of the caliper piston pushing wears less than the inner one.....

    Be interesting to see your thoughts on the ECP ones, the pagids I got have been squealing like a banshee, until I took the car out on some quiet roads and gave them the 'bedding in procedure' of half a dozen 50-20 slows followed by 4 70-10 slows...they have been quiet for the rest of the week
     
  11. brandwooddixon

    brandwooddixon Senior Member

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    I must admit, I use Pagid discs but tend to stick with MB pads. Admittedly they're pretty dirty in use, but never squeel.
     
  12. OP
    AlastairD

    AlastairD Senior Member

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    I have fitted the Pagid one's, squeal free right from the start. It's the first time I've fitted pads with built in anti squeal pads and I must say the new setup feels pretty good.
     
  13. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    I think SBC 211's have 2 on the front and one on the back, non SBC are like the other mercs in that they have 1 on the front and one on the back.
     
  14. OP
    AlastairD

    AlastairD Senior Member

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    Mine is SBC, defo 2 on the front.
     
  15. Cole@MBS

    Cole@MBS Forum Supporter Authorised Forum Supporter

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    N/S one is a 50% wear sensor, never fit this one, it screws up the radio reception
     
  16. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    Ahh, I did wonder why they were in different places on the pads when I did my mums 211.

    No issues with radio reception on hers though, wonder why it interferes?
     
  17. Bigstat

    Bigstat Guest

    W211 brake sensors SBC

    Hi! my light warning for service brake!,visit workshop! has just come on.Has anyone in N.I. had this problem,and did Mercedes Benz offer to repair it for free???.I notice that they have been repaired in the USA for free.
     
  18. E320mark

    E320mark Senior Member

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    Anything free from agnews!! Not a chance! Save yourself some time and probably money and go to Alan mcgookin beside George Logan cars! Porsche specialist but does anything German!!
     
  19. Bigstat

    Bigstat Guest

    SBC

    Cheers! Mark,i know Alan well,have you had this problem yourself?
     

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