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Cause of BMW blower Motor RESISTOR Failure?

Discussion in 'Electronic Design' started by amdx, Mar 25, 2013.

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  1. amdx

    amdx Guest

    Hi Guys,
    This is a long thread from, but I think it hit
    the wrong audience. I thought you guys might have more incite into
    the root cause of this common part failure.

    From the OP,

    "Does anyone have insight into what is the root cause (and repair) of
    the FSU failure that plagues almost every 1997 to 2003 BMW?

    Also, does anyone have an idea HOW TO TEST a "repaired" FSU?

    The "blower motor resistor", which also goes by FSR (Final Stage Resistor)
    or by FSU (Final Stage Unit), is known to fry itself in almost every single
    E46 (3-series), E39 (5-series), and E38 (7-series) BMW.

    The problem with replacing this ~$100 part is that the new replacement FSU
    fries itself just as often as the old one did, so you end up repeatedly
    replacing your fried FSU every few years or so.

    That's fine for most people (although the DIY is a PITA) - but I ask
    this newsgroup whether anyone has any insight into WHAT is actually
    breaking - and - why?

    Here is the best (admittedly sketchy) wiring diagram we have so far:"
  2. amdx

    amdx Guest

    Yes Jim,
    Just so ya know, I picked up the post from another group because there
    was a long thread without a real answer. I thought this group would have
    more knowledge, I mean, I'm sure there is more knowledge here :)
    From what I see it is, it is not PWM and not even sure about the linear
    regulator. I posted the group in my original post, there is a bit more
    about the regulator in the thread. Some thought maybe the motor might be
    bad, causing resistor failure, but it is often 3 years after the
    resistor replacement.
  3. Jamie

    Jamie Guest

    THat is a linear controlled speed unit. It has the heat sink sitting
    in the path of the air box to help remove the heat. Most likely that unit
    fails more when you are using the heater over the air conditioner.

    I find it hard to believe that BMW would make such junk like that.

  4. miso

    miso Guest

    Generally if the module is a resistor, there is one position where the
    motor is fed directly. So when the resistor goes open, your only speed
    is full blast or off.

    If you have more speed values than could be explained by taps in a
    resistor, then you have an electronic control of some flavor. Usually
    when the electronic control fails, you have no fan at all.

    I've replaced these in VWs, Mercedes, and Infiniti. The reality is the
    expensive cars have just as shitty fan controls as the cheap cars.

    Exar has/had a group that did nothing but reverse engineer the
    electronic modules for replacement parts suppliers. You can't believe
    the crappy designs used by the manufacturers. It was always a problem if
    the replacement parts some use good engineering, or just copy the
    moronic designs of the OEM.

    I used to think they potted all these modules to protect the circuitry
    from the elements, but in reality they were just kind of embarrassed at
    the poor engineering in the design.

    Most of the time the bipolar designers would spot circuitry that had
    potential to damage on start up. As you probably know, breakdown
    voltages in bipolar can be a function of pins being floated or shorted
    to other pins.

    Resistors or electronic controls need heat sinks, so I don't think you
    can tell based on the outside of the package.

    The replacement part for my Infiniti blower control was a Delphi make in
    Mexico! The original part, which looked a bit different (bigger heat
    sink) was made in the USA. The Infiniti part isn't potted. I saved it
    just to open it up and see if anything smoked. It looks clean. It is a
    PCB with 4 semiconductors, one power device, and what looks like a surge
    suppressor. The PCB has Picotech on it. It has a 0.004ohm current sense
    resistor on. I suppose I can salvage it for yucks. The board is
    conformal coated, which makes it hard to read chip numbers.

    If I had to fault them on anything, I would say too much heat sink
    grease. It is oozed all around the power device. [3 pins, so MOS or BJT].
  5. amdx

    amdx Guest

    Blame it on Jim, his rates are high. :)
  6. Nico Coesel

    Nico Coesel Guest

    That doesn't need to be the case. The cause can be something simple.
    In my car the resistor module has a fuseable link which corrodes and
    makes the module fail. Ebay to the rescue...
  7. amdx

    amdx Guest

    The original thread had six links from 6 people that had attempted to
    remove the potting. All ended in failure.
  8. Some of them nevertheless succeeded in removing enough of the potting to
    make a repair. It would be interesting to reverse engineer the whole thing.
    But someone seems to have done so already as there are second source units
    available. Meanwhile, time is running out. These cars from the nineties will
    not last much longer I suppose.

    petrus bitbyter
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