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Thread: Brake lining light on now... but pad sensors long unplugged

  1. #1
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    Brake lining light on now... but pad sensors long unplugged

    I’ve had my Pad sensors unplugged for maybe a year but just the other day the brake lining lamp on the dashboard illuminated and has stayed on since. Is it the case that something may have closed the circuit causing it to illuminate or should the light have been illuminated ever since I unplugged the sensors and for some reason it just decided to come on now?

  2. #2
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    Assuming the sensors are still looped it could be a bad connection, typically at the plugs. My light has been on since time began and I keep them wrapped around the strut. Go figure
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  3. #3
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    With the sensors unplugged the light SHOULD be on. The lamp is triggered when resistance across the circuit drops below a preset value. In normal operation the sensor loops are completing the circuit and the bulb is off. There is a transistor on the instrument cluster PCB that controls the switch. The lining wear circuit is supplied with 12v through a 220 ohm resistor on the instrument PCB, which goes through the right rear sensor first, up to the left front sensor, then finally back to the instrument panel.This resistor often overheats because it is a 1/4 watt component and something like a prolonged short to ground (a cut sensor) can cause it to have to dissipate over 1w of heat, which in turn causes it to desolder itself from the PCB.

    If you have double and triple checked all physical connections on the undercarriage, wheel wells and found no fault then pull the cluster and inspect the PCB for a broken solder joint and reflow as needed.
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  4. #4
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    Great explanation for brake pad warning light! To eliminate the whole problem, when you pull the cluster, you can pull the warning light bulb.

    Do you know how the low beam headlight warning light works. Mine is on but the headlight is OK.

  5. #5
    richardodn's Avatar
    richardodn is offline Old Guy BMW CCA Member
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    There are 2 trigger modes that cause the light to light. As Stuck described, it's basically a big loop from and back to the circuit board in the cluster. Trigger mode 1 is an open circuit such as when you simply unplug the sensors. Trigger mode 2 is when the circuit goes to ground such as when the rotor wears into the sensor. This second one is the case that Stuck describes as causing an overheat in the resistor.

    When you removed the sensors, that caused trigger mode 1 and it should have lit up. The fact that it didn't implies there was already something wrong with the board or the bulb. Now that it's lit, it's actually working correctly.

    I suppose the next question is how to make the bulb turn off. Here are the choices.

    1. Install the two sensors and hook them up.
    2. Install jumper wires where the sensors would be plugged in to complete the circuit.
    3. Remove the cluster and remove the bulb.

    I recommend option 1.
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  6. #6
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    Unless this system is vastly different from the E38, you can simply take the 2 brake pad sensor wires coming from the "car" side of the wheel well, and splice them together. (solder and heatshrink) If your dash/control board is working properly, the lights will go out, because this completes the circuit. You can remove the sensor at the wheel completely. Checking the brake pads for wear is a simple process, so the sensor isn't really needed. Hope this helps!
    Last edited by goinstrong; 12-29-2019 at 01:57 AM.

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    Thanks all. And please remind me which wheels have the sensors?

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    richardodn is offline Old Guy BMW CCA Member
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  9. #9
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    That bulb/socket in the cluster is a bit of a "loose fit" for me, new brakes all rounds (rebuilt calipers, lines, discs, pads etc) and sometimes over bumps the light goes away, sometimes it come on and stays on. A gentle tap on the dash sometimes makes it go away as well, bad socket connection for me.

  10. #10
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    Quote Originally Posted by H.J. 1981 View Post
    That bulb/socket in the cluster is a bit of a "loose fit" for me, new brakes all rounds (rebuilt calipers, lines, discs, pads etc) and sometimes over bumps the light goes away, sometimes it come on and stays on. A gentle tap on the dash sometimes makes it go away as well, bad socket connection for me.
    In my case, it was the resistor itself was bad. I ended up sourcing a working one from ebay. I would like to try to fix the old board, but would need help sourcing the correct resistor.
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  11. #11
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    Order the correct resistor on Mouser or Digikey. They are literally pennies...
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