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Root cause insight into the common BMW blower motor resistorfailures

Discussion in 'Electronic Repair' started by Bimmer Owner, Mar 21, 2013.

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  1. Bimmer Owner

    Bimmer Owner Guest

    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. jim beam

    jim beam Guest

    that looks like a linear semiconductor controller - an incredibly
    antiquated concept for a modern car.

    old resistor packs for fans were open wire that sat in the fan's air
    stream for cooling. they were generally very reliable if their alloy
    wasn't too susceptible to salt.

    that unit looks like it still sits in the air stream with that honking
    great heat sink and i estimate it's trying to dissipate >100W. that can
    only mean it's a linear controller because a modern pwm device can
    control high motor currents with very little heat dissipation <10W.

    bottom line, a linear controller is always going to get hot and end up
    frying itself over time. the only thing you can do is either replace it
    with another unit that will ultimately meet the same fate, or undertake
    a significant modification.

    for the latter, you can try putting an even bigger heat sink on it - but
    i doubt there's a lot extra room available. you can also "pwm" it. i
    built a similar unit to deal with a linear controller over-heat issue on
    my 89 civic.

    <[email protected]/5068043855>

    depending on how much time you want to spend on a project like that, pwm
    can control superbly and offers benefits like motor speed not being so
    susceptible to supply voltage [engine idle voltage drop] etc.

    the down side of pwm is that it can generate electrical noise. [poor
    stereo installations can be particularly susceptible.] the ideal
    solution is to implement pwm with "soft switching", but that's getting
    quite advanced.
  3. jim beam

    jim beam Guest

    if it were an adequately designed unit, it should tolerate that and worse.

    analyze the actual problem - don't just waste money replacing stuff. a
    $30 dvm will diagnose this for you, and you should already own one if
    you have any ambition to repair any modern vehicle.
  4. the will

    the will Guest

    Blower motor drawing too much amperage taking it out. Change the
    blower motor anytime?
  5. Nate Nagel

    Nate Nagel Guest

    My thought as well. Have you measured current draw on a new blower
    motor and compared it to one that is installed in a car where the FSU
    has failed? that would tell you whether there's any merit to this idea
    or not.

  6. Scott Dorsey

    Scott Dorsey Guest

    My inclination is to do exactly the same thing I do with the cooling system
    issues: blame German engineers who seem to believe that their climate is
    typical of the entire world.

    I don't see why it is so hard to unpot one of these things and repair them
    directly, especially if it's a semiconductor failure. Put a bigger transistor
    in there.
  7. jim beam

    jim beam Guest

    for an "engineer", you're simply not of this planet.
  8. Nate Nagel

    Nate Nagel Guest

    Did you have any suggestions for the OP, or did you just show up to
    snipe without contributing anything as per usual?

    You do know that most electrical/electronic components have a maximum
    current rating, yes? And that electric motors tend to draw more current
    when the bearings are going or they are otherwise subjected to loads
    higher than that for which they were designed? Does any of this sound
    remotely familiar to you?

    Really, what the will suggested seems to be a logical first step.

  9. jim beam

    jim beam Guest

    that's not going to fix it though. and the germans sell a LOT of these
    vehciles in the middle east - it's a good deal hotter there than here.
    they know exactly what they're doing.

    unpotting is a nightmare - it will take much less time to build your own
    pwm controller. who knows, maybe you can switch the existing unit???!!!
  10. jim beam

    jim beam Guest

    don't lecture me on electronics nate.

    if you don't know what the **** you're doing and don't know how to use a
  11. Bimmer Owner

    Bimmer Owner Guest

    Here are pictures from the last half dozen who tried that approach:

    Most who try to unpot fail, mainly due to damage caused to the
    surface-mount circuit board during the initial mechanical degooping

    Those deft few who avoid knocking off the surface-mount components
    with the debriding chisel, are left with a badly bruised board,
    where some have said they've resoldered solder cracks (see pics).

    One problem with "put a bigger xtor" is that nobody on this planet
    has produced a decent circuit diagram of the FSU.

    Does anyone here have access to an FSU circuit diagram?
  12. jim beam

    jim beam Guest

    if people priced their time and ignored the damage in which attempts to
    unpot invariably result, it's cheaper to just buy a new one.

    you don't need it any more than you need the circuit diagram of a chip's
    internals - all you need is its function parameters - which you pretty
    much already have.

    you might be able to pwm the unit itself thus pretty much removing the
    heat component thereby prolonging its life [literally] exponentially.
  13. Bimmer Owner

    Bimmer Owner Guest

    This is an interesting approach, given that the vast majority of
    bimmer owners do NOT replace the blower motor - they replace the FSU.

    While the blower motor replacement procedure is a major PITA, one
    'can' test the leads from the FSU harness connector pins #5 and #1
    which are power and ground respectively to the blower motor.

    Again, we don't have a circuit diagram, but it has been said that
    the blower motor takes about 6 amps (variously, depending on the speed)
    but it would take a test jig to test that in operation.

    To my knowledge, nobody has created that test jig (although I know
    of only one attempt, which failed):

    It's easy enough to test the resistance of the blower motor though,
    and those results have come out at about 0.4 to 0.6 ohms.

    It would be expensive to change a blower motor on a whim, so, how
    would YOU suggest the blower motor be tested in situ?
  14. Nate Nagel

    Nate Nagel Guest

    link times out?
    ooh, or what? ITG gonna kick my ass? Sorry, I'm more interested in
    helping the OP than your delicate little feelers.
    WTF is that supposed to mean?

    OP can dissect the thing all he wants but it doesn't do him a damn bit
    of good to know *what* has failed unless he knows *why* it failed.

  15. Scott Dorsey

    Scott Dorsey Guest

    Static resistance doesn't tell you anything, but operating current measured
    with a DMM would tell you a lot.
    I've never tested one, but I put a drop of turbine oil on the motor bearings
    every five years or so. I do the same on the window and seat motors too.

    I'd imagine if you listen carefully and have good hearing you can tell if the
    motor is binding at all, but many people do not.
  16. Bimmer Owner

    Bimmer Owner Guest

    That's exactly what we've done - yet - we need help since nobody to
    date has figured out HOW to test an FSU that is fried.

    Note: It appears to be an active component, but it probably does
    dissipate 100W.
  17. Bimmer Owner

    Bimmer Owner Guest

    To be clear, that's what 99.99999999% of the BMW owners do.
    But that's not the point of this thread.

    The point of this thread is to get a handle on WHY they are all

    Specifically, how to figure that out is the question.
  18. Bimmer Owner

    Bimmer Owner Guest

    It 'can' be done, but would require a test jig inserted inline
    as the FSU is deeply ensconced under the dash while the blower motor
    is even more deeply so.
    While that preventive work might be prudent, the sheer effort
    to remove the entire dash simply to access the blower motor
    would be problematic.

    Still, if the problem is that the blower motor is merely using more
    current as it gets older, why wouldn't a NEW FSU burn up within a
    few weeks of insertion?
  19. Ismo Salonen

    Ismo Salonen Guest

    If the unit is near its limits it might just get very hot and parts
    start aging very fast -> semiconductors will just fail after a little
    while. The new unit should fail sooner than the old one but who knows
    who soon, maybe after a few years. Peugeot's ( and Citroen) used just
    one huge pnp darlington which failed quite often , it was working too
    near its operating limits.

    (just my 2 cents)
  20. Scott Dorsey

    Scott Dorsey Guest

    Unpotting is fun, it's a nice change in the day to just sit down for a couple
    hours with a dremel tool and a dental pick.

    But I agree, building an aftermarket controller replacement would not be
    a tremendously difficult thing to do, and it might be a highly profitable
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