The fan is supposed to turn on at low speed whenever the A/C is running. There is a problem if yours is not.

The fan came on due to the engine temperature, for backup cooling, regardless of A/C operation. You should check the operation of the fan clutch on the mechanical fan. With the engine stopped, the fan should move smoothly, as if it's in molasses. If it moves more easily, it's dead, and s/b replaced.

THe electric fan may be bad, if it does not reliably turn on due to engine heat.

BTW, the fan is not supposed to run when the engine is off.

 

It is a 1997 528i, so you are lucky because EuroDavid on roadfly addressed this in detail:

Original thread is listed below. Scroll down to see the reply by "EuroDavid" on June 03, 2010; it is written in great detail (resistor pack):
http://bimmer.roadfly.com/bmw/forums/e39/9510410-1.html

PS: At low speed (when AC is on at idle), the Aux Fan is barely audible. So it is hard to hear it.
Trick: start the car, and start the AC. Using a drinking straw (the straw from McDonald's soft drink) and stick it in there to check if the blade is rotating.

 

re: auxillary fan issue

I just got through working on my car. I first done the obvious stuff I checked the fuses all was fine. Someone mention additional 2 fuses is 75 and 76 underside the glove compartment I found just one and that was a 50amp and yes it was blown so I replaced it. I then decided to change thermostat and flush radioator. I also cleaned the condeser coil which had a lot of debree. I put everthing back together top off raiator started car and a/c on low. The aux. fan did come on so I decided to take it for a drive within 10min. on the road. It started to gradually get hot I made it back home open hood and fan was not running. Im thinking it could be the cooling fan sensor..I dont think it is sensing the temperture to keep running does this sound like my problem? If so, how many is there just one on top radiator back towards passenger side? I dont believe its a head gasket issue because I just changed oil last week and I did pop the top and look inside as well as pull dipstick all is well there.

Mordly

 

You just changed the thermostat.

Bleed and bleed and bleed air out!!!

http://forums.bimmerforums.com/forum/showthread.php?t=1423821

 

I think the bleeding done the trick...thanks so much for your help


Mordly

 

Good to hear you had positive results after bleeding.
I have had aux. fan issues on my 2000 528i and now that I think about it, my mechanic replaced the water pump when I first bought the car and I dont think he properly bled the system. Could this be the problem?
EDIT: I did replace the switch (or sensor) on the lower radiator hose with no difference. I do know the fan turns on sometimes and it sometimes stays on even after shutting off the car until I unplug it and plug it back in again.

 

What happens if you unplug the bottom hose sensor? When I bought my 99 528 the bottom sensor had been unplugged and I didn't discover it until I overhauled my cooling system. With it unplugged, the fan didn't come on until the car was completely warmed up, but, then it came on high speed and stayed on most all of the time that the car was running. Long, long story short, I replaced the sensor and it then never came on even at high temp and high condensor pressure. Finally, an indi shop diagnosed it as a missing code in the control module. They cleared and reset the codes and it now works as it should.