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Old 08-20-2009, 03:04 PM
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bluebee bluebee is offline
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Mein Auto: 02 BMW 525i M54 auto 130K
Originally Posted by holmesg View Post
it gets so hot it would burn your hand ... can anybody tell me if this is normal.
Does anyone actually know how this FSU/FSR basically works?
And how to test if it's bad? (other than just replacing it w/o thinking ... which is what most of us did)

The best I can glean from the wiring diagrams and threads where people took it apart and actually fixed the bad solder connections is the following (perhaps incorrect) summary.

Can someone take this FSU-test-procedure from here and explain how this FSR actually works & how to test it?

How the fsu is wired:
Pin 1 = (bottom left) to wiring diagram #31; then to battery negative/ground (-)
Pin 2 = (bottom middle) to fuse F76 (40 amp); then to #30, then to battery positive (+)
Pin 3 = (bottom right) input to fsu from heat/ac module, #15
Pin 4 = (top left) output power to the blower motor? (speed 1?)
Pin 5 = (top right) output power to the blower motor (speed 2?), #I5, Ignition power (RUN & START)

Test procedure, from this thread ...
Pins 1 (SW/GN wire) and (5 RT/GN wire) are the negative and positive to the blower motor (right???)
- If you ground pin 1 and apply 12 volts to pin 5, the blower motor should operate; otherwise the blower motor is bad.

- If you test resistance between pin 1 and the metal chassis fins, you should get zero ohms (right???)

- Test if pin 2 (the fuse 76 circuit) has 12 V when the HVAC button is switched on.

Last edited by bluebee; 08-20-2009 at 03:16 PM.
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