Root cause insight into the common BMW blower motor resistor failures

Bimmer Owner · 2013-03-21T03:08:09+00:00

Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW? 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. 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. 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? Here is the best (admittedly sketchy) wiring diagram we have so far:

Mar 21, 2013 183 Replies

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therivetman 8 years ago

The blower resistor heat sink sits in the main vent coming from the blower. As the blower is set to a lower and lower speed the unit gets hotter and h otter absorbing the energy the fan is not using. At slower speeds there is less air flowing over the heat sink to absorb the additional heat and so at any fan setting below 6 the unit becomes hot enough to fry the electronics . The solution is to "never" run the fan below 6

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