Here is my two bits worth. The aux fan on my 99 528 came on nearly all of the time when I first bought my car and I just thought this might be normal. As it ultimately turned out the reason it was coming on all of the time was that it was defaulting to the radiator temp sensor. But, when I overhauled my cooling system I found that the sensor on the bottom radiator hose was unplugged. So, I plugged it in and then the fan would not come on at all. I replaced the sensor with a new one and still no fan; no matter how warm the engine got or how high the pressure in the AC. After driving myself nuts in every way trying to figure it out, I finally took it to a high quality indy shop. They hooked it up to their super scanner and got a message that an aux fan code was missing in the control module. They disconnected the battery for ten minutes, reset the codes and now the fan works just fine.
