auxillary fan - when A/C is on, when is it supposed to turn on?

[Cobra Jet]

The electric auxillary fan (in front of A/C condensor) - does this fan turn on at a predetermined temp controlled by the DME when the A/C is turned on OR should this fan turn on the same time (instantly) when the A/C is switched to "on"?

Also, since on the subject of the aux. fan - does this fan only operate when the A/C is being used?

[Wise Old Dog]

Immediately on low speed with the AC on. 2 speed thermo switch turns it on when the engine gets hot.

[Cobra Jet]

So, regardless of engine operating temp, once the A/C switch in the car is turned "on", the aux. fan should also be "on", but operating at low speed - is this correct?  

I turned my A/C on today to test it out and the aux. fan was not on at all.  Vehicle was at normal operating temp, A/C compressor was cycling and cold air was coming out of the vents.  I let it sit for about 2 mins or so w/ the A/C on and I watched the aux. fan at the front of the vehicle and it did not spring to life, even after putting the interior fan control on max speed.

Is the thermo switch replaceable?



Is it possible the low speed is "out" on my aux. fan?

[Wise Old Dog]

Yes it is replaceable, but the whole system is problematic. There are a number of problems that could cause the fan not to work. Number 1 is the fan itself. Start with it and work back to the switch. Kinda tricky, especially since the Bentley wiring diagram for this system is not there. A lot of people here just rip that s**t out and put an after market system in. It's been covered pretty well previously.

[D. Clay]

There is a lo/hi speed resistor on the fan shroud. It's a ceramic block about 2" X .5". When it goes bad the fan will only run on hi speed. The A/C will not turn it on because the A/C only activates the low speed setting. The hi/lo temp sensor in the radiator will not turn it on until the motor is in danger of overheating. Run 12 volts directly to the fan resistor - first to one side of the resistor and then the other. If the fan is good it will come on hi speed but not low. Here's a pic:
http://www.realoem.com/bmw/showparts.do?model=AF93&mospid=47305&btnr=64_0516&hg=64&fg=55

[batsbats]

There is a lo/hi speed resistor on the fan shroud. It's a ceramic block about 2" X .5". When it goes bad the fan will only run on hi speed. The A/C will not turn it on because the A/C only activates the low speed setting. The hi/lo temp sensor in the radiator will not turn it on until the motor is in danger of overheating. Run 12 volts directly to the fan resistor - first to one side of the resistor and then the other. If the fan is good it will come on hi speed but not low. Here's a pic:
http://www.realoem.com/bmw/showparts.do?model=AF93&mospid=47305&btnr=64_0516&hg=64&fg=55

I guess I have the opposite.  My aux fan turns on when I turn on the A/C.  I've let it idle and sit a little beyond the middle of the 1/2 to 3/4 hash marks.  It has never turned on without me using the A/C.  

So I guess I have access only to the low speed, but when should the hi speed activate?

Oh and my clutch fan failed the newspaper test.

I was wondering if I swap in an electric fan with a new thermo 80/88C switch, should everything be honky dory?

[monty23psk]

^ You could have a bad temp switch or aux resistor. If the fan turns on, then those are your 2 areas that might be bad. If you have the oem aux temp switch, the temps for the switch turning on is 91 / 99 degree celcius. I need to get a new one as i checked it in hot water. Only one of mine is working. So only if the temp gets to those degrees, would the fan turn on, but if your switch is ok.