aux. fan not working

wow, really?

For the cross-linked record, today, JimLev provided nice aux fan test info:
- E39 (1997 - 2003) > Aux Fan Help

Originally Posted by JimLev

The fan is controlled by the dual temp switch on the side of the radiator and the HVAC system.
Stage 1 is low speed from the HVAC panel or the 91 deg switch on the radiator and relay K21.
Stage 2 is controlled by the AC high pressure switch and relay K22.
Stage 3 (high) is controlled by the 104 C deg switch on the side of the radiator and relay K201, which gets power from fuse F75 (50 amp) behind the glove box. F20 and F105 powers K201.

You said the fan comes on when you turn the AC on so Stage 1 (low) is working which means the 2 resistors are good.
Pull the connector off the dual temp switch and jump it to see if you can get.
Jumping pins 1 to 2 should make it run at high speed and pins 1 to 3 for low speed.

For the crosslinked record, QSilver7 posted some nice aux fan information over here today:
-> E39 (1997 - 2003) > please explain!!!

Originally Posted by QSilver7

It depends on which engine you have...in your case...it's the M52TU...

I'm having a problem with my aux fan too. My aux turns on and goes off but when it kicks on it makes a very loud noise and sounds like it is straining and the car doesn't over heat. Does anyone have any idea what it could be or what I should do???

Originally Posted by akillies

I'm having a problem with my aux fan too. My aux turns on and goes off but when it kicks on it makes a very loud noise and sounds like it is straining and the car doesn't over heat. Does anyone have any idea what it could be or what I should do???

Mine did that just before it died, mine would turn on, but would stay on the highest fan speed setting. Sounds ridiculously loud right? The cheapest OEM fan i've found is from ecstuning, or just get a used one for cheap.

Ok, can I replace the motor itself to maybe save some money??

For the crosslinked record, JimLev offers good advice here for discerning aux fan types:

Originally Posted by JimLev

Did your fan motor look like this with these color wires coming out of it?
If so, you have a PWM fan.
The older fans are 3 speed that only have 2 wires ans use relays and resistors to switch speeds.
The resistors are mounted to the fan frame.

See also:
- DIY for how to replace the auxiliary fan (1) (2) (3) & how to test the cooling system auxiliary electrical fan (1) (2) & the infamous fuse 75 (1) & the aux fan relay (1) & an exploding aux fan noise situation (1)