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:
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
replace "roosters" with "cox" to reply.
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
"C'est un Nagra. C'est suisse, et tres, tres precis."
On Thu, 21 Mar 2013 09:47:31 -0400, Scott Dorsey wrote:
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?
On Thu, 21 Mar 2013 08:14:12 -0700, jim beam wrote:
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.
That isn't necessarily the case. For example,t hey could be correctly
designed, rated for the application, etc but have a manufacturing
defect in just one of the components.
A better questions is why BMW apparently doesn't give a damn
to do the failure analysis to find out what's wrong. I have a friend
who has an X5 and had this problem with the blower resistors.
Even worse, the only symptom was it was draining the battery
and it took a huge number of hours to track it down.
While you're all wondering about that problem, might as well
add the fancy aux radiator fan to the list. This car had that go
and now the replacement one has failed again. And the
symptom there is, again, it drains the battery even when the
car is off. That fan is a real POS. Instead of just a simple
fan motor, it's a fan that's variable speed, driven by a PWM
signal. So, instead of just a motor, that fan sitting in front
of the hot radiator has electronics in it. A real genius of a
design. And for what? Like the fan can't just be on or off?
Only reason I can think of is that they want to save a few
watts of power to try to get better fuel economy. And for
that their customers get to shell out $500 for a new fan
every few years.
If by PWM you mean pulse width modulation, then it would allow for
variable speed, but a DC motor is an inductive load and is not sensibly
controlled by such a system unless there is something in the circuit to
allow the peak voltage generated by the motor at pulse cut of to be
shunted to earth.
That "something" could be as simple as a diode. PWM is
commonly used to vary the power to a motor. BMW, for
example, uses it on the aux fan motor of the X5. And I
would suspect that it's also used for the blower motor
because you wind up wasting a lot less power that way.
And every little bit of power saved adds up and effects MPG.
PWM is the most common method of controlling the speed of DC motors -
a flywheel diode is part of the "system" to handle the inductive
kick-back. Virtually all battery operated variable speed power tools
use PWM. So do virtually all electric bicycles with brush motors and
the vast majority of electric forklifts.
In fact, just about any application of a brush type DC motor that
requires reasonable speed control has switched to PWM control of some
sort over the last 20 years, including power wheel chairs (except
those using 3 phase brushless motors)
On Tue, 26 Mar 2013 19:24:08 +0000, Bimmer Owner wrote:
I only found 1 transistor.
This is what is suggested in this quote below:
[QUOTET0iman]There is only ONE mosfet in the FSU and I would forget
about trying to replace it even if you had one. Also, we have no reason
to believe the mosfet has caused any FSUs to fail. I have yet to see the
resistors in any of the picture shown anywhere in this thread.
Can someone circle the resistors for me? I would think they would be fairly
decent wattage so they would be very easy to see, but I don't see any
If they are resistors, I've never seen any that look like that.
Also, given that you want to thermally bond any components
that generate major heat, why are they not heat sinked?
With any power design I've seen, the key components, eg
the transistors are directly bonded to the heat sink.
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