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    Thread: Brake pad warning lights

    1. Semi-n00b
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      pheaton 3.0d V6
      04-15-2011 12:50 AM #1
      My warning lights keep coming on for my brake pads. Took it to my dealer who checked it over confirming that all the pads are fine. Still happening tho. Any solution, any one?

    2. Member
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      May 04 V8
      04-15-2011 04:50 AM #2
      Easy diagnosis: disconnect individually each of the connectors to determine which wheel is faulty. This will give you an idea of where the short is taking place.

    3. Moderator PanEuropean's Avatar
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      04-15-2011 10:26 AM #3
      Hi Pierre:

      I don't think your idea will work. The brake service message works by detecting a discontinuity (an interruption in the circuit) of the single wire leading to and then from the brake pad.

      If you disconnect any one of the connectors, you will see a continuous brake service message.

      A much easier way would be to run a diagnostic scan and see if the vehicle reports which of the four wheels is sending the message (I'm not sure if it gets that granular, but it can't hurt to try).

      An alternative method would be to create a jumper plug that MAKES the circuit across the connection where it plugs into the brake pad, and then try using this jumper plug on each of the four wheels - checking individually to see when the warning message disappears.

      Michael
      Please don't send me technical questions via IM - instead, post your questions onto the end of the most appropriate thread in the FAQ, so that everyone can benefit from the answer, and everyone can assist in providing the answer. Thanks, Michael

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      04-15-2011 10:32 AM #4
      About 10kmiles ago I had the same experience: brake pad light stubbornly on, but visual inspection indicating there was still enough material for maybe another 7-10kmiles. I changed the pads anyway, since at almost 50kmiles I felt I had gotten excellent service from them, and the light wnet off immediately and has been off ever since. I think the moral of the story is that the sensors are designed to give ample warning long before the pads are completely worn out.
      Stefano

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      04-17-2011 11:54 AM #5
      Quote Originally Posted by PanEuropean View Post
      Hi Pierre:

      I don't think your idea will work. The brake service message works by detecting a discontinuity (an interruption in the circuit) of the single wire leading to and then from the brake pad.

      If you disconnect any one of the connectors, you will see a continuous brake service message.
      THis does not sound correct to me.

      The way brake service messages work is by embedding a small wire in the brake pad, so that when the brake pad is too thin, a contact is made between the wire (that now protrudes from the pad) and ground (the brake disk).

      So the car is expecting a contact to take place, and not an absence of contact (or, as you say, a discontinuity in the circuit).

      Having an interruption in the circuit means the pad is healthy, as it prevents the contact from taking place.

      So, disconnecting the wire would make the car think that the pad is good.

      Disconnecting all wires should remove all warning, unless, there is a short between the wire leading to the brake sensor and some part of the car.

      Either the short takes place between the brake sensor connector and the car computer (in which case it can happen anywhere in the wire mesh) or (more probably) the problem lies in the sensor.

      Reconnecting each wire one by one should tell you which wire is faulty (connecting the faulty wire will light the brake warning).

      P.

    6. Moderator PanEuropean's Avatar
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      04-17-2011 12:21 PM #6
      Hi Pierre:

      Unless I have grossly misunderstood the principle, there is continuity in a new brake pad, and when the brake pad eventually wears down, it wears through the wire and breaks the continuity - rather than establishing continuity.

      I suspect that we are probably both in agreement about the general concept, but are using different terms to describe it.

      Michael

      Brake Pads - New and Worn (indicator wiring at top)

      Please don't send me technical questions via IM - instead, post your questions onto the end of the most appropriate thread in the FAQ, so that everyone can benefit from the answer, and everyone can assist in providing the answer. Thanks, Michael

    7. Moderator Prince Ludwig's Avatar
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      04-17-2011 12:56 PM #7
      After having a spurious brake pad warning after changing all my pads and discs last year, I can confirm that the system works the way Michael says. Any break in the circuit activates the warning.

      Harry

    8. Member
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      04-17-2011 03:18 PM #8
      Err.... Then, there must be 2 wires and not only one.

      On my old merc, there was one wire only, that lit up a lamp when the wire was touching the surface of the disk.

    9. Member n968412L's Avatar
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      04-17-2011 03:47 PM #9
      I think if you look carefully you could believe that there are actually two wires in a common sheath. In the self study guide (posted in a thread regarding long life servicing recently) there is a circuit diagram that shows the two wires.

      self study guide here: http://www.volkspage.net/technik/ssp/ssp/SSP_224.pdf
      Regards
      M
      Last edited by n968412L; 04-17-2011 at 03:51 PM.

    10. Moderator Prince Ludwig's Avatar
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      04-17-2011 04:52 PM #10
      Quote Originally Posted by Zaphh
      Err.... Then, there m]ust be 2 wires and not only one.

      On my old merc, there was one wire only, that lit up a lamp when the wire was touching the surface of the disk.
      Yup, the old Mercs do work like that (or at least the W140 I have some experience with does...) but if you look at a worn sensor pad from the Phaeton, you'll see two little copper points where the wire has worn away. I think the logic is that once the sensor is worn, the warning is displayed constantly rather than just when you press the brake pedal.

      Harry

    11. Member n968412L's Avatar
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      04-18-2011 12:08 AM #11
      Quote Originally Posted by Prince Ludwig View Post
      I think the logic is that once the sensor is worn, the warning is displayed constantly rather than just when you press the brake pedal.

      Harry
      And also if there is a fault in the circuit it fails safe with the warning light coming on, forcing you to investigate.

      M

    12. Member
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      04-18-2011 10:31 AM #12
      So you should then be able to check the circuits by unplugging all four sensors, jumping them with a small wire (this should remove the pads light) and then, as you reconnect each sensor, find out which is the faulty one.

      P.

    13. Moderator PanEuropean's Avatar
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      04-18-2011 11:03 AM #13
      Quote Originally Posted by Zaphh View Post
      So you should then be able to check the circuits by unplugging all four sensors, jumping them with a small wire (this should remove the pads light) and then, as you reconnect each sensor, find out which is the faulty one.
      Exactly! (But this presumes the fault is in the pad unit, not the vehicle side of the wiring...)
      Please don't send me technical questions via IM - instead, post your questions onto the end of the most appropriate thread in the FAQ, so that everyone can benefit from the answer, and everyone can assist in providing the answer. Thanks, Michael

    14. Member
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      04-18-2011 12:05 PM #14
      This will also tell if the fault is in the vehicle, if all sensors are jumped and the light is still on.

    15. Moderator PanEuropean's Avatar
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      04-18-2011 03:01 PM #15
      Correct, it will indicate that the fault is in the vehicle, but it won't indicate WHERE the fault is in the vehicle.

      So - let's hope the fault is in one of the brake pads, or; that a diagnostic scan will reveal which corner of the car is reporting a fault.
      Please don't send me technical questions via IM - instead, post your questions onto the end of the most appropriate thread in the FAQ, so that everyone can benefit from the answer, and everyone can assist in providing the answer. Thanks, Michael

    16. Moderator ruddyone's Avatar
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      04-18-2011 03:35 PM #16
      Perhaps check that the wires are in place. I actually had a sensor wire brake which caused the sensor to go off. Unfortunately, some of those wires are built into the pad which caused me to replace the entire pad. Another possibility (although remote) is that the wire came unplugged.

      Best regards,

      Nate
      No. Try not. Do... or do not. There is no try.

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