Bimmerfest - BMW Forums - View Single Post - Replacing the final stage unit/blower motor resistor (E39)
View Single Post
Old 04-30-2008, 04:43 PM
bluebee's Avatar
bluebee bluebee is offline
Seek to understand,^Value
Location: San Jose, California
Join Date: Mar 2008
Posts: 25,236
Mein Auto: 02 BMW 525i M54 auto 130K
The BMW FSU can be tested but it will require an EE to tell us how

Good to know anyway.
I agree! Sorry I didn't chime in sooner. My FSU seemed to have "fixed" itself (except for the early morning wheezing) so I didn't pursue this further.

But I did send your BMW FSU wiring diagram to an electrical engineer friend in the silicon valley who wrote back the following that might help the next DIY.

> Does this electrical BMW FSU thingey look easy to debug?

Well, if you have access to the wires in the car, I guess it shouldn't be too bad.
You can see that the terminals are:
1,5) blower
2) +12
3) control input (in spite of being drawn as the collector of a bipolar transistor!)
4) ground

So I would:
A) spray WD40 on the contacts
B) check terminal 2 for voltage
C) put a two channel scope on terminals 3 and 1, and then 3 and 5

Now the question is, is that much effort worth more than
the $100 or so to just replace the unit? Consider your
options. Either the unit is bad, so replacing it is required.
Or, the control unit is bad. That's going to cost a bundle.
So maybe spending $100 is a reasonable thing to do.

Last edited by bluebee; 04-30-2008 at 04:47 PM.
Reply With Quote