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 264 Replies

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ecodrive 12 years ago

the FSR /fsu is a variable resistor that will control the fan speed; consequently at the lower speed of the fan the FSR will have the highest resistance > hence it will heat the most, and the bad thing: there is not enough air flow to cool it down (around 200W i suppose) newer replacement FSR units have more aluminium bars to increase the heat dissipation area. maybe the worst thing of all is that the FSR will eat up fuel for nothing (just to produce heat by electricity) something in the range of 100..200W and since the electricity is made with the efficiency of a gasoline engine (from

44MJ/kg), it is ~4times more expensive as the same electric load in the house

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WW 12 years ago

Someone had scoped the whole board, and it was DC voltage everywhere (according to that reference). It's pretty clear there is no PWM.

The 16-pin surface mount chip seems to be a automotive temperature compensated voltage regulator with a huge voltage range, according to a lookup of the part number on it.

Here is the Elmos 10901D chip of my FSU as I cut it open today.

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I wish there was a way to get rid of that heavy fibrous plastic goop! (What is that black fibrous tough stuff anyway?)

I have had good luck removing the stuff used on motorcycle voltage regulators that were potted soaking in MEK Methyl ethyl ketone (spelling?). Potent stuff. Use outdoors and keep your hands out of it. WW

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Ralph Mowery 12 years ago

I don't know what it actually is but at work we desolved a potted firing system for a boat motor with something called OCP. Really stinks. Don't think I would want it in a car after it was used.

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Paul Drahn 12 years ago

My electronic assembly company built the electronic control unit for BMC Chopper of Bend, Oregon. Now defunked. We used two-part black epoxy that took 24 hours to completely cure. When applied it was water thin. Had to prep the box with RTV to keep the potting from running out around the connectors.

Paul

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