Followups set to sci.electronics.repair .
In sci.electronics.repair Bimmer Owner said:
Does anyone have insight into what is the root cause (and repair) of
the FSU failure that plagues almost every 1997 to 2003 BMW?
At a guess, one or both of the big power transistors that are inside the
FSU are failing. If the failure is that the blower motor doesn't run at
all, they are probably failing open. If the failure is either that the
blower motor runs at maximum speed, or a fuse blows, then they are
probably failing shorted. The blower motor probably draws more current
as it ages, and it may eventually be exceeding the power-handling
capability of the transistor(s). When the motor is switched off, it may
also generate a bit of a voltage spike, which may be above the voltage
rating of the transistor(s).
A possible solution is to replace the transistor(s) with ones with a
higher power rating in the same package. Another approach is to improve
the heat-sinking, maybe by adding metal to the existing fins. Or, cut
off the existing fins, bolt it to a huge slab of metal, and relocate the
entire thing away from the blower duct.
As a crutch, you could drop the voltage to the FSU a little bit. This
would slow down the blower, but also might tend to keep the voltages
and currents down to what the transistors can handle. You would need
to know the maximum current you would expect the FSU to draw; this
probably happens when the charging system voltage is at is maximum,
the blower motor is stone cold, and you turn it from "off" to "max".
Then, buy a big rectifier diode with a rating of a few amps more than
that, and splice it in to the power wire to the FSU. This will drop
the voltage by a volt or two all the time. Or, you could put a power
resistor in line instead; this will cause a variable voltage drop
depending on how much power the FSU and blower is drawing at the time.
Keep in mind that in the winter, keeping the windshield clear is a
safety function, so don't drop the blower speed too much.
Also, does anyone have an idea HOW TO TEST a "repaired" FSU?
The tricky part depends on the nature of the control signal to the FSU.
If it's a simple analog voltage, that is easy to generate on the bench
with a potentiometer. If it's some kind of digital bus (CAN?), it is
*possible* to generate that on the bench, but it's probably easier to
get the dashboard heater control out of a junked car and let it generate
the signal.
To load the FSU, you can either use a power resistor that draws about
the same amount of current as the blower motor on "high" (a headlight
lamp might qualify), or an actual blower motor. The resistor will be
"better behaved" than a real motor.
For a power supply, it depends on how much current the blower motor
needs. You can get relatively inexpensive 13.8-volt power supplies in
ranges up to several amps, designed for running "12 V" equipment on the
bench. Samlex is one manufacturer but there are others. If it needs
more than 10 A or so, it's probably cheaper to just use a real car
battery and charge it when it's not being used.
You should probably arrange it so that there is some air blowing on the
FSU under test. If you are using a real blower motor, you can make a
duct out of cardboard. If not, use something like a 12 V computer case
fan to move a little air across the FSU.
It may also be interesting to have some kind of thermometer on the FSU
case while it is under test.
A good way to figure out what the FSU is actually doing is to probe a
working car with an oscilloscope. This will show you immediately how
the FSU is controlling the blower motor speed, as well as what the
control signal looks like. You can stick a straight pin *through* the
wire insulation as a test point, and then seal up the hole with
electrical tape or silicone sealant.
As has been mentioned, it would be also interesting to cut one of the
blower wires and measure the current drawn by the blower motor. For
extra credit, do this on a new car (or a newly-installed blower motor)
and then compare to a blower motor in a car that has just had its FSU
fail.
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.
On one hand, you would like stuff not to break. On the other hand,
spending $100 every two or three years on a car that starts around
$36,000 new is maybe not out of the realm of possibility. (Perspective:
that's one tire or 25 to 30 gallons of gas.)
These cars are apparently sold worldwide. If the FSUs sell for cheaper
in a lower-cost country, enough to offset shipping and taxes, import a
box full of them and make money.
Matt Roberds
