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Red brake warning Light

Discussion in 'A4/A4 cabriolet/S4 forum(B6 chassis)' started by R32 DBP, Jun 18, 2012.

  1. R32 DBP
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    R32 DBP New Member

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    [Jun 18, 2012]
    Hi guys, on my 2004 audi a4 a.8t 190 quattro i get a red brake warning light flash up on start up and then clear after a few seconds.

    the vag techicians are at a loss as it does not come up on vag com, all pads and discs are fine, and they put a new brake fluid sensor on it.

    does anyone know what could be causing this? The light doesn't always come on either btw

    thanks
    simon
    #1
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  3. Mike B
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    Mike B Member

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    [Jun 18, 2012]
    Could be a sign of a weak battery .May need charging or replacing
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  4. slappy
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    slappy Member

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    [Jun 18, 2012]
    I had the same thing, put in a new battery and it sorted the issue.
    #3
  5. Turkster
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    Turkster Bro.Paul 2.5TDI Quattro Sport

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    [Jun 19, 2012]
    battery is on its way mate, the volt drop from the cranking the starter and all the other bits is draining the power away from the break sensor thing,
    #4
  6. murran
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    murran Active Member

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    [Jun 19, 2012]
    they put a new fluid sensor on it? by this you mean they fitted full reservoir with the sensor built in then filled it up?
    #5
  7. R32 DBP
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    R32 DBP New Member

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    [Jun 19, 2012]
    no i think they just changed the sensor- believe me they no what theya re doing- have been VAG specialists since 1983 and i would highly recommend them.

    Cheers for the heads up about the battery guys, il try it with another
    #6
  8. murran
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    murran Active Member

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    [Jun 19, 2012]
    the whole reservoir with the float and sensor comes as one unit from tps.
    i too have worked at a vag independant for 11/12 years. the 4 or 5 a4's that weve had in with this fault have all been ok since replacing the whole reservoir.

    whether its owt to do with the battery i dunno. these guys seem to think so, so try it and report back.
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  9. pj123
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    pj123 Member

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    [Jun 19, 2012]
    It'll be the battery.

    Before you start the car after its been sitting overnight check the voltage. 12.6v or more is normal. Around 12.2v or less you will most likely get the warning and even more likely if it has been a cold night as there will be greater resistance in the circuit and less energy from the battery cells. I'm guessing you dont get the warning after a journey when the battery is charged and the car is warm?
    #8
  10. N11KDJ
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    N11KDJ Member

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    [Jun 19, 2012]
    I had the same warning light but not on start up, it happened at the same point on my journey to work when I turned right at a roundabout, replacing the reservoir fixed mine but as yours comes on at start up maybe it's the battery in your case.

    If you have already had the sensor changed then it sounds like they would have changed the reservoir so looks like the battery could be the culprit here..
    #9
  11. BahnStormer77
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    BahnStormer77 BahnStormer

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    [Jun 19, 2012]
    if it's as you start up, then the Audi technicians said that there's a design fault where the surge from the battery to the glow plugs as you turn the ignition on where the current in the cables causes a sensor to trigger: this quite understandably happens at very low temps and quite inexplicably when the battery is very low or on it's way out... if it's while you're driving, then it could be what I jsut had: the sensor wires degraded a couple of months after getting new brakes and hoses fitted... but I think that would / shoud show up on vagcom.
    #10
  12. Mike B
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    Mike B Member

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    [Jun 19, 2012]
    I found the wire from the battery was pressed against the sensor so loosened this and putting some foam insulation between as well as charging the battery cured it .My son did it by buying a new battery cost£120 mine £0
    #11
  13. R32 DBP
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    R32 DBP New Member

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    [Jun 19, 2012]
    Only the sensor was swapped, not the whole reservoir. apparently this is easy to do. As it does it on start up after not being started for a few hours it makes sense that it is the battery.
    #12
  14. boost-addict
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    boost-addict Active Member

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    [Jun 19, 2012]
    your not listeneing the sensor is part of the lid/cap to the reservoir lol

    of course its easy, even my 6 yr old boy could swap it over, unplug, undo, replace, do up. plug in,
    #13
  15. quattrojames
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    quattrojames Moderator Staff Member Moderator

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    [Jun 19, 2012]
    This typically affects diesel engines which require a higher cranking force to turn over.

    I've got less experience on a petrol engine, quite likely it's the same, but on a petrol engine in June I'd not just rush out for a new battery before testting it first.
    #14
  16. murran
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    murran Active Member

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    [Jun 19, 2012]
    you know what to do then if the battery doesnt fix it.

    from stripping (breaking up/cutting open) an old reservoir, the sensor looks like a chunk of plastic but has no switch or moving parts as such and doesnt actually contact the float mech at all. i believe it meerly has a variable resistance to it that is effected by the magnet within the float mech. it picks up on the magnet getting close to it, this changes the resistance thru it and effects the return voltage to the instrument cluster.
    i believe the float mech within the reservoir somehow goes faulty allowing the magnet too close or maybe the magnets magnetism some how increases/changes/lessens.
    ive no idea why it seems to mainly occour on start up particulally when the ambient air is below a certain temp.
    its not to do with glow plugs tho, as the last one i changed was on a 04 plate 1.8tq end of last year.
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  17. R32 DBP
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    R32 DBP New Member

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    [Jun 22, 2012]
    Hi, i swapped the battery and that didn't fix it. I have had the sensor changed so not sure what the difference is??

    Can you explain what you think needs doing and what the cost is likely to be?

    thanks
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  18. R32 DBP
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    R32 DBP New Member

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    [Jun 22, 2012]
    is it possible to just disconnect the reservoir sensor so that doesn't happen?
    #17
  19. boost-addict
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    boost-addict Active Member

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    [Jun 22, 2012]
    There's no point guessing . Get it scanned on bag on. Could be a number of things. Why waste money trying to fix things that turn out to be good anyway
    #18
  20. murran
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    murran Active Member

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    [Jun 22, 2012]
    this fault doesnt show up on scans. the abs has nothing to do with it, the fluid level sensor its connected to the instrument cluster which doesnt show fault codes for it. its a two wire sensor for gods sake.... how ecu controlled do you think it is?????
    replace the whole reservoir (sensor/float/reservoir it comes as one new unit).
    i dont know how many times i have to post this before people take notice! im not a total shmuck who doesnt know what im banging on about!
    Last edited: Jun 22, 2012
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  21. R32 DBP
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    R32 DBP New Member

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    [Jun 23, 2012]
    cheers. is is possible to just disconnect the sensor? it seems to do more harm than good!
    #20
  22. murran
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    murran Active Member

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    [Jun 23, 2012]
    i dont think so, the resistance of the sensor is critical so the instrument custer see's the correct return voltage.
    #21
  23. Mike B
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    Mike B Member

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    [Jun 23, 2012]
    I had the res changed first before finding out that it was the battery.Cost about £100 plus labour
    #22
  24. murran
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    murran Active Member

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    [Jun 23, 2012]
    r32 dbp has already changed the battery and its still doing it.

    im sure the reservoir is roughly £30 from tps. so i dunno where your garage got £100 plus labour from?
    #23
  25. Mike B
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    Mike B Member

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    [Jun 23, 2012]
    I thought it was dear too although it is a2.5tdi and the sensor is underneath .I moved the wire away cos I thought it it should not be pressing hard against the sensor and assume that helped
    #24
  26. R32 DBP
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    R32 DBP New Member

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    [Jun 29, 2012]
    replaced the sensor, just the sensor! a 10 second job! not the whole reservoir and the light has gone! wrong sensor used on 1st replacement.

    Do not bother replacing the reservoir, the sensor can be done at home and costs peanuts
    #25
  27. murran
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    murran Active Member

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    [Jun 30, 2012]
    how do you mean wrong sensor? what other sensor fits?
    #26

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