On Wednesday, November 29, 2017 at 6:30:32 AM UTC-5, wrote:
On Wednesday, March 20, 2013 at 11:08:09 PM UTC-4, 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

I'm going to test a theory, that few if any FSU units are "fried" what is happening is corrosion on the plugs and pins cause erratic behavior depending on which are corroded. One pin will cause it to run when the vehicle is off and another might let it run only on high. Being analog systems this type of problem was common to "that" I have 3 units and I will interchange after cleaning. The pins appear to be a poor quality metal that is reacting to the copper female plug.

I don't doubt that some problems are due to that. I've seen it once myself..
However I doubt most of them are due to it. The draining current
when the system is off mode for example. It's basically power transistors
in a module and unless they have some very bizarre design, there is no
way removing or degrading a connection can turn it on. Are you sure those
female connector pins are copper? That would be very unusual. For the
times I thought it was a connection issue, I slightly bent the pins in
various directions, then connected it, figuring the increased pressure
would help. I think that might have worked one time, but it could also
be that cracks develop inside the circuit on the module and fiddling
with it restores it temporarily. If you think you have a connection
problem, there is electrical contact cleaner. That followed by dielectric
grease which would prevent it from recurring could be worth a try.