2001 540i A/C Will Not Start Auxiliary Fan

[greh3595]

This is for a 2001 E39 540i with about 83,000 miles. A/C is all original as was the cooling system until very recently.

Last week I lost the A/C blower function, i.e., the A/C system comes on and the compressor kicks in but their is no forced air in the system. Being short of time, in a fit of stupidity I took the car to the Stealer who charged me a ridiculous amount of money to replace the IHKA Final Stage unit. Picked the car up from the Stealer and the A/C seemed to be just fine - I wasn't paying attention to whether the auxiliary fan was working or not at this point.

The next day I am driving down the Interstate and the car overheats - spewing coolant from the radiator adjacent to the inlet hose, a classic 540 failure point. Anyhow, I have replaced the radiator and other components and the coolant system is working as it should, including turning on the auxiliary fan when things get a little too toasty for the M62 engine.

However, as a result of paying attention to everything under the hood now, I realize that the auxiliary cooling fan is not coming on at all when the A/C is turned on. This is true under any and all operating conditions - hot, cold, or otherwise. I understand that the fan should, at a minimum, run in low speed mode whenever the A/C is on. The A/C otherwise blows relatively cold air, so I believe it is operating properly - I don't leave it on to long though for fear of building excessive pressure in the system and blowing it up.

Does anyone have an idea as to why the auxiliary fan is not coming on, and if so how to fix it?

Many Thanks,

[witeshark]

The thermostat switch (relay) may not be working. Has the fan itself been tested? Nearly all cars start both fans immediately when the A/C is turned on regardless of the engine temp.

[greh3595]

Coolant Temperature sensor has been replaced. Wiring to fan and fuses has been checked. I have read that on the 2001 E39 540i the auxiliary cooling fan is controlled by the ECM. It sends pulses to the fan that tell it to run and how fast to run based upon inputs from the coolant temperature sensor, IKHA A/C control unit and vehicle speed.

My feeling is that the fan is functioning fine. It does run and runs at varying speeds from slow and very quiet to high and noticeably noisy. The fan, when running, seems perfectly normal. It just doesn't seem to run anymore when the A/C is turned on.

[witeshark]

The replacement of that final stage unit seems like the most central factor to have a look at, it would seem.