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Thread: Question on Brake Lining light...

  1. #1
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    Question on Brake Lining light...

    After replacing my brake pads, rotors, and sensors, I took my car in to have them reset the light, which they apparently did. But now when I have the key in position 2, all of my service lights come on EXCEPT for that one. Did they actually reset it, or did they just kill the light all together? I kind of like having all the service lights functional and in working order...

  2. #2
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    The brake lining warning light doesn't come on until the sensors contact the pads. There's no reset of that other than replacing the sensors.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

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    Quote Originally Posted by thejlevie View Post
    The brake lining warning light doesn't come on until the sensors contact the pads. There's no reset of that other than replacing the sensors.
    So are you saying they just killed the light to make me happy? If they did, I'm gonna have a talk with them tomorrow because they basically just robbed my car of that function...

  4. #4
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    I could double check the wiring diagrams, but I don't remember any "smarts" in that warning function. If the light was still on after installing new sensors, there was a problem in the sensor circuits. If the shop was intimately familiar with E30's, they may have just fixed whatever was wrong. It should be easy enough to find out if that function is still operable by unplugging one of the wear sensors, which should turn on the light.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  5. #5
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    Quote Originally Posted by thejlevie View Post
    I could double check the wiring diagrams, but I don't remember any "smarts" in that warning function. If the light was still on after installing new sensors, there was a problem in the sensor circuits. If the shop was intimately familiar with E30's, they may have just fixed whatever was wrong. It should be easy enough to find out if that function is still operable by unplugging one of the wear sensors, which should turn on the light.
    A picture of my dash with key in position 2 is in my sig. It still looks like that now with all of the service lights on EXCEPT for the Brake Lining one (like it no longer works). Is that how it's supposed to be?

  6. #6
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    I'm curious about this as well. My brakes are in dire shape and I apparently don't know how the brake lining light is supposed to work.

  7. #7
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    Here is my write-up.

    Quick summary: there is no memory or "reset", the light just shows the current condition, and the lamp test is done during start.


    Brake lining indicator light

    Summary: The light is triggered either by the wire grounding (usually
    against the brake rotor as the sensor is partially worn away) or the
    wire breaking (usually as the sensor is further worn away). If the
    sensors are fine, re-flow the solder on the instrument cluster resistor.

    Operation

    Most e30s with four wheel disk brakes have a brake wear monitoring
    system. The system uses two replaceable sensors to monitor the front
    and rear brake pad wear. The sensors are mounted in a notch on the
    outer edge of the inboard brake pad on the left front and right rear.
    As the pad wears, the sensor faces are ground away with the brake pad
    friction material.

    The sensors are simply a wire embedded in an insulating material. The
    brake wear circuit is similar to alarm system wiring: the indicator
    light illuminates when the sensor wire is either grounded or is broken.
    When a sensor face first starts to wear against the rotor, the wire is
    initially exposed and grounds against the rotor only when braking.
    Later the wire is worn through, breaking the circuit. Thus the lamp is
    initially illuminated only when braking, and later stays illuminated.

    The sensor circuit is fed +12V through a 220 ohm resistor on the
    instrument cluster circuit board, through the right rear sensor, then
    through the left front sensor, then back to the circuit on the cluster.
    The schematic is on page 3435-0 of the e30 electrical manual, although
    it's admittedly not clear.

    If the wear indicator circuit doesn't work and the sensors are fine, the
    most common source of problems is the 220 ohm resistor. This can be
    easily tested without taking apart the instrument cluster. Turn on the
    ignition to power the circuit and illuminate the wear indicator.
    Measure the voltage and available current on the right rear sensor
    connector. One of the contacts should read +12V, and provide 55-65 mA
    to ground. The contact is the blue/white wire, not the green/yellow,
    but it's trivial to probe both rather than look for the wire color. Use
    the brake rotor as a clean ground.

    If you aren't getting voltage or the correct current, the problem is
    very likely on the cluster circuit board. Remove the board and re-flow
    the solder around the 220 ohm resistor. This resistor is mounted to the
    outside of the C1 blue connector on the PC board, just above the
    temperature gauge connection. It may stick off of the board on long
    leads, or may be a larger resistor mounted close to the board.

    There are on-line pictures to locate the resistor, and you can confirm
    the correct part by looking for the red-red-brown color bands.

    http://e30.bmwdiy.info/brake-lining-...g-light-03.jpg

    This might sound daunting if you haven't soldered before, but this is the
    easiest soldering task ever. Let the soldering iron warm up for a few
    minutes, then touch the soldering iron tip to each resistor lead for two
    to five seconds. You should see the solder surface change texture as it
    melts, filling in the cracks that have grown over time.

    Add a dab of silicone to mechanically support the resistor so that the
    solder doesn't crack again. Don't cover the resistor, as it has to
    dissipate almost 1 watt of heat if the sensor wire is touching ground.

    That heat is likely why the resistor is mounted so far off the board to
    start with -- the designers didn't want to use a expensive 1.25W
    resistor, when a common 1/4W resistor was adequate most of the time.
    But they didn't consider that worn-through sensors and holding the
    brakes on for an extended period (e.g. stop-and-go traffic) are
    correlated. The resistor is at 4X its power rating, and the only place
    for the heat to dissipate is through the resistor leads. Now you have
    the mechanical leverage from mounting the resistor far from the board
    "working" the solder combined with thermal expansion cycles from the
    overheating. No wonder there is cracking!

    Worst-case power analysis for the 220ohm sensor supply resistor.
    220ohm, 10% tolerance = 198-ohms, 5% tolerance = 209-ohms
    Nominal 12V power is typical 13.8V operating, 15V hot load.
    Power = V*I = V*V/R
    Worst-case 15V*15V/198ohms = 1.14W
    Worst-case, 5% 14.4V*14.4V/209ohms = 1.00W
    Typical/nominal 13.8*13.8/220 = 0.87W
    Design safety worst case 18V*18V/198ohms = 1.64W
    So the designer should have specified at least a 1.25W part.

    Note: One instrument cluster I checked out was a late 1989 (12/89 or
    01/90) MotoMeter. It had a physically larger 1W or 1.5W resistor
    mounted close to the board, but still had developed bad solder joints on
    both ends. Based on this sample, mechanical vibration does not appear
    to be the dominant problem. My guess remains thermal expansion cycles,
    however there was no evidence of board overheating or degradation.

    Note: I previously assumed that the lamp check was done by providing the
    sensor power through the unloader relay. That would test the lamp drive
    circuitry as well as the lamp. Instead the lamp check is done with a
    diode isolated feed from the start circuit.

    While you have the cluster out, with soldering iron at hand, re-flow the
    solder around the temperature meter mounting hole. One of the contacts
    is through the sleeve around the brass mounting stud. The stud pulls
    the sleeve into the soft solder on circuit board to make contact. Over
    time the solder deforms, resulting in bad contact. Tightening the
    mounting nut will help for a bit, but building up the solder is a longer
    term fix. Better, solder on a thin brass washer.

  8. #8
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    question, what did your brake supplies cost you? My uncle, an american car guy, still tries to maintain that my car will be ridiculously expensive to maintain despite the fact I've owned it for 3 years now and it hardly requires much money. He was saying a brake job would be like 2K for a shop to do. I wanted to laugh continuously in his face but it wouldn't work. He asserted rotors would cost 150-200, I then pulled out my iphone and showed him rotors on pelican parts for 47.99. Next he tried to tell me calipers were hella expensive, my response="since when do calipers need to be constantly replaced?"


    Anyway I'm ranting. Since he unjustly got my grandpa's 2800 cs when my mom had been granted it and eventually wrecked and sold it I'm kinda bitter. Not to mention he simply doesn't realize how not rare and inexpensive/reliable these cars are.

  9. #9
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    E30s are easy and inexpensive to get parts for.

    Front brake rotors start at $20 -- in California you should be able to find them at local stores for that price.
    Caliper rebuild kits (just seals and boots) are under $5.

    Brake pads are the same as most other cars. Cheap ones are $10-$15. Ones that you would find acceptable are $30-$40. Track pads are readily available, and tend to be less expensive than the larger pads for other cars.

  10. #10
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    Quote Originally Posted by djb2 View Post
    E30s are easy and inexpensive to get parts for.

    Front brake rotors start at $20 -- in California you should be able to find them at local stores for that price.
    Caliper rebuild kits (just seals and boots) are under $5.

    Brake pads are the same as most other cars. Cheap ones are $10-$15. Ones that you would find acceptable are $30-$40. Track pads are readily available, and tend to be less expensive than the larger pads for other cars.
    my point exactly, but he takes the know it all road when his only experience was on that 2800 in the early 90's. And again, how the hell often do you even need to fuck with the calipers? idiot. I usually get work done at a shop and remember getting a quote which is why I almost vomited with laughter when he said it'd be 2 grand.

  11. #11
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    My brake lining light was on for about 2 years until a couple of weeks ago when I replaced the pads, sensors, and rotors. All of my lights turn on except for the brake lining light. I think that's just how it's suppose to be.

    1989 325is 5MT 200k miles

  12. #12
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    Quote Originally Posted by crazycast2 View Post
    My brake lining light was on for about 2 years until a couple of weeks ago when I replaced the pads, sensors, and rotors. All of my lights turn on except for the brake lining light. I think that's just how it's suppose to be.
    mmm...that's very good to know. Thanks!

  13. #13
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    The light bulb to the pad warning light on your cluster could have burned out too. Mine works.

  14. #14
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    The brake warning lamp should light when the ignition switch is turned to start and the motor cranks. This is the "prove out" feature that tells you the bulb is working. If your bulb is not lit when cranking then the bulb is burnt out or removed. As djb2 described, it should be off at all other times in a properly functioning system.
    "Howdy, Folks!"

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  15. #15
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    Alright, so I went out to check. The light doesn't come on with all the other ones when the key is in position 2. However as mentioned above, when I crank it, it comes on briefly, then turns off again when the engine is running. Is it functioning correctly? Thanks.

  16. #16
    richardodn's Avatar
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    Yes. The light coming on when cranking shows that the lamp is functional. It should then be off at all other times.
    "Howdy, Folks!"

    1986 Delphin 528e - Roof rack equipped lumber hauler.
    1989 Zinnoberrot 325iX Sedan - I miss this car. (Deceased)
    1998 Avus Blau 328iC - Someone else's project now
    2008 Platinum-Beige X3 3.0si - Current project
    2012 Alpine White X3 xDrive35i - My new snowmobile.
    2020 Estoril Blue 440i xDrive cabrio - This car is a blast to drive.

  17. #17
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  18. #18
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    Exactly the info I was looking for, thanks Richard.

  19. #19
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    In my '84 323i there is a third wire in the car rear harness, a brown one. It is ground, but where should it be connected at the rear wheel? It is separated from the sensor plug. Wire length is about one inch, so it cannot be connected very far from the sensor plug mount. But there is no place for any wires in my brake protection plate...?!? I guess it's for grounding the hub and disc etc. But where it should be connected? The brown wire connector is broken, that does not give any clue.

    Edit: picture added

    sensorplug.jpg
    Last edited by MikkoRan; 05-09-2022 at 03:43 PM. Reason: Picture added

  20. #20
    richardodn's Avatar
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    I assume that's the right rear. That cable is actually 2 wires. The brake wear sensor is inserted into the brake pad and then connected to that cable. I don't remember exactly how it's routed as I no longer have my E30. Maybe someone else has some pictures.

    The brake wear circuit is pretty simple. It starts at the cluster. The wire goes to the front left, then to the rear right, and returns back to the cluster. If the circuit is broken or shorts to ground, the light comes on.
    "Howdy, Folks!"

    1986 Delphin 528e - Roof rack equipped lumber hauler.
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  21. #21
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    Yes, I know the schematics. And yes, that's right rear. But, my cable has 3 wires, as shown in the picture... Sensor itself has 2 wires, as always. The 3rd wire is brown, ground. Maybe I just attach it to the brake protection plate, it will ensure ground to the whole trailing arm and rotor etc.

  22. #22
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    Quote Originally Posted by MikkoRan View Post
    In my '84 323i there is a third wire in the car rear harness, a brown one. It is ground, but where should it be connected at the rear wheel? It is separated from the sensor plug. Wire length is about one inch, so it cannot be connected very far from the sensor plug mount. But there is no place for any wires in my brake protection plate...?!? I guess it's for grounding the hub and disc etc. But where it should be connected? The brown wire connector is broken, that does not give any clue.

    Edit: picture added





    sensorplug.jpg

    it may have been added to defeat the sensor. a lot of aftermarket brake pads do not have a sensor pad. it may have been added if the op didn't want to cut the harness to ground the wires together.

  23. #23
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    I'm pretty sure the wiring harness is original and wiring diagram also has this third wire connected to somewhere in rear wheel. Maybe I just bolt the wire into the protection plate.

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