I was driving along tonight with the ventilation system set to fan speed 4, set to cool, only the middle slider over (blow air at you, not feet). All of a sudden, I hear a huge "CRACK" noise and then the fan stopped blowing! I turned off the blower and then put it back on; it started back up but it sounded quieter than normal, and it made weird noises when set to 3 or 4 position (some kind of buzzing). What on earth could this be?!?! The fan still works, and after the incident, I turned it off and on a couple of times and eventually the noise went away when turned to 4. Is this the sound of imminent failure?
Might be - for me, I've found the fan-blade shaft that runs through the blower motor eventually bores through the shaft bearings, and when I start to hear the motor squeaking (the motor rotor starts rubbing against the stators), I know it's on its last legs. With that kind of noise, sounds like something broke. Guess I'd start by seeing if the slide controls are doing what they're supposed to; after that, pull the access panel at the back of the engine bay, take off the blower cover, and see what's going on in there. I don't think you have to for basic access, but if you get into disconnecting the recirculating vent flaps, be really cautious, those and the connectors to the motor are easily broken. Not sure where to suggest to look after that; never had a blower switch itself crack, but I suppose anything's possible.
I don't think I've had a problem with the motor squeaking. Does this make a difference? Either way, I'll try to gain access to a part of it from the engine bay. Haven't done that before, so perhaps tomorrow after I get my VANOS problem fixed. It would be painfully obvious if something was wrong in there, right? I don't know what I'm looking for...
You'll want to pull off this thing, on the firewall immediately below the front window. http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=64_0087&hg=64&fg=99 Hopefully, the S50 stuffed in there doesn't limit access to the panel enough to be a problem; if you have a stressbar, you may need to pull it, or if it's not a 1-piece, pull 1 bolt and swivel it up out of the way. Once you pull it off, you'll see the the cover & retaining straps shown here, labeled "1": http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=64_0480&hg=64&fg=15 Parts breakout: http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=64_0514&hg=64&fg=15 Push down on the tension straps to unsnap them from the lower part of the strap, and pop off the cover (7). Spin the blower blades manually (carefully, as not to break any), see if it's spinning freely with minimal resistance. Turn on the blower and see what it's doing at different speeds. http://www.realoem.com/bmw/partgrp.do?model=1254&mospid=47308&hg=64&fg=15 Here's what things look like if you pulled the blower motor AND it's surround (part 3) - if you have to replace the blower, you don't have to pull any of that out; that's the part that has the easily-broken recirculating flaps. If you're not getting all 4 fan speeds, you might need a new blower resistor (part 19). If you pull the blower motor, it's a good time to replace that if necessary. Of course, access isn't necessarily that bad (for a stock motor setup, anyway) if that comes up later. http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=64_0525&hg=64&fg=55 If you do end up pulling out the blower motor, note its orientation, it sits in the housing in one specific position - it's possible to fit it in and snap the clip over it without it being properly seated; don't do that, make sure it's seated. Also, I've gotten blower motors that were the correct part, but the blades were too far in or out on the shaft. If you accidentally push the shaft through the motor, you'll ruin it, so don't do that, and you can't push on the blades with the necessary force without risking breaking the blades (or pushing the shaft through the motor). So if you get a replacement blower motor, and the blades aren't spaced properly to fit without rubbing, make sure to support the shaft if you use something like a socket to push on the hub of the fan blades; and all that's at your own peril. But it's possible. Part 8 is the blower motor retaining clip - it comes out completely, so don't let that sproing off into unobtainableville if you pull the blower motor out - http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=64_0514&hg=64&fg=15 If the blower motor & blades appear to be ok and spinning freely, it still might be worth popping it out to look at the resistor; I'm just speculating that if that broke for some reason, it might have yielded your loud crack noise - only a guess. Bentley manual should detail the procedure for you - might be worthwhile to measure the electrical values of things while you're in there if nothing else is obvious.
You will know if the resistor broke because the fan speeds will not work properly. If the fan speeds still work correctly, the resistor can not be broken (well, nothing is impossible). Let me restate. If fan speeds work correctly, I believe it to be unlikely the resistor broke apart causing the "CRACK" sound.
