Interesting Thing After FSU Replacement

So today i replaced the FSU on my 97 540iA. Until now, with the AC on, with the blower setting all the way at max, there would be a decent amount of cold air in the cabin. (i never had the ac working at full capacity up until now as im finding out today, and i didnt even realize). So up until now, the aux fan wouldnt always come on while the AC was on, it would only come on for a few mins spinning very fast and then shut off, and so on. After changing the FSU today, the aux fan is on almost all the time (i say almost, because the rest of the time im driving and cant really tell) while the AC is on, and the amount of air into the cabin is at least double what i used to get before. Needless to say, the blower no longer has a mind of its own. Is this just a coincidence or does the FSU somehow also control the aux fan along with the temp switch and other items? Just wanted to share what i found today.

The aux fan should come on when the A/C is on, at the low speed. If the engine gets hotter, the fan will speed up (it has 3 speeds on your MY).

The FSU has no control over the aux fan - it's strictly to control the HVAC blower.

Originally Posted by mrpumpk1n

So today i replaced the FSU on my 97 540iA. Until now, with the AC on, with the blower setting all the way at max, there would be a decent amount of cold air in the cabin. (i never had the ac working at full capacity up until now as im finding out today, and i didnt even realize). So up until now, the aux fan wouldnt always come on while the AC was on, it would only come on for a few mins spinning very fast and then shut off, and so on. After changing the FSU today, the aux fan is on almost all the time (i say almost, because the rest of the time im driving and cant really tell) while the AC is on, and the amount of air into the cabin is at least double what i used to get before. Needless to say, the blower no longer has a mind of its own. Is this just a coincidence or does the FSU somehow also control the aux fan along with the temp switch and other items? Just wanted to share what i found today.

that's what my car does as well,which I think is how it shud work,cyclic.to come on only when needed ie when the condenser temps increases

With your AC dumping more cold air into the cabin the heat load is expelled into the condenser coil in front of the radiator, this makes the radiator hotter which in turn keeps the aux fan running longer/faster depending on how slow you are driving.

where did you purchase the FSU from? just curious and it has nothing to do with your results or at least, my asking doesnt