On 03/20/2013 09:13 PM, jim beam wrote:
On 03/20/2013 08:08 PM, Bimmer Owner wrote:
Does anyone have insight into what is the root cause (and repair) of the
FSU failure that plagues almost every 1997 to 2003 BMW?

http://www.bimmerfest.com/forums/att...0&d=1194115994


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.
http://www.bimmerfest.com/forums/sho...d.php?t=143393

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.
http://www.bimmerfest.com/forums/sho...d.php?t=528566

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?
http://www.bimmerfest.com/forums/sho...d.php?t=309399

Here is the best (admittedly sketchy) wiring diagram we have so far:
http://www1.picturepush.com/photo/a/...g/12467819.png


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

well, you live and learn. apparently the reason they use a linear
controller is because it allows the fan to run near silently at low
speed. with pwm control the fluctuating magnetic fields in the motor
coils cause it to vibrate and make a humming noise at the pwm control
frequency.

that doesn't of course get around the fact that the unit in question
here is apparently badly under rated, but the above does at least
explain why it's used.



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.

http://www.flickr.com/photos/38636024@N00/5068043855
http://arduino.cc/en/Tutorial/PWM

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.




--
fact check required