# front radiator fan runs when climate control is on?



## boyfriendmechanic (Aug 2, 2007)

It is cold outside (20 F) but when I start the car (2002 C5 A6/Allroad w/2.7T) one of the radiator fans starts running fast and loud. This is new. There is the fan driven by the fan-belt with the fan clutch, that spins at idle (what are the symptoms of a failing fan clutch because it seems to squeak and whine a bit?). There is an electric fan between the engine and the radiator, which stays off most of the time as far as I can tell. 
Then there is a bigger electric fan between the bumper and the radiator, which is the reason I am posting.








Sometimes this outer electric fan turns on when I start the car, sometimes it doesn't. Usually it stays on for the whole drive. When it does turn on as I start the car, I turn the climate control off by slowing the fan for the cabin air lower and lower (disengaging the auto setting) until the display goes dark and then I can hear the fan (out in front of the radiator) spin down. A few clicks on the climate control, to bring the cabin fan up, and the radiator fan starts to boogie again. Fine, but it is winter, and I need the heat!
What is the outer fan on the radiator for? Is it associated to the A/C? Why is it doing this and how can I stop this new behavior? I thought it might be as simple as a bad temperature sensor?


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## frankinstyn (Oct 11, 2004)

*Re: front radiator fan runs when climate control is on? (boyfriendmechanic)*

It is connected to the a/c. Keep in mind that if your defrost is on, the ac compressor will turn on to dry the air. That means that the fan will run when the defrost is on.


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## boyfriendmechanic (Aug 2, 2007)

*Re: front radiator fan runs when climate control is on? (frankinstyn)*

Thank you for your post http://****************.com/smile/emthup.gif 
These are my assumptions:
The A/C assists in the defrogger when you push the defrost button regardless of temperature inside or outside the car. check.
The A/C comes on when the selected cabin air temp is lower than outside. check.
The A/C is off when simply using the heat on the auto settings to make the cabin warmer than outside.
Curious: My radiator assist fan has recently started coming on immediately upon startup when the climate control is on auto and the cabin fan is barely turning. The engine is cold, there is no heat to push, so the cabin fan (smart) doesn't start blowing cold air. Yet the outside electric fan is blasting!


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## boyfriendmechanic (Aug 2, 2007)

*Re: front radiator fan runs when climate control is on? (boyfriendmechanic)*

it is 30 degrees outside. I start the car and the this electric fan turns on. The cabin air temp was set to auto and 69 degrees. I turn the cabin air fan off, the outside fan turns off. 
The defroster is not on, the cabin air temp is set well above the outside temp, so the A/C is not on. Why is this fan coming on?


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## ECS Tuning - Audi (Oct 12, 2009)

*Re: front radiator fan runs when climate control is on? (boyfriendmechanic)*


_Quote, originally posted by *boyfriendmechanic* »_The A/C assists in the de*frogger* when you push the defrost button regardless of temperature inside or outside the car. check.

1st - lol at having a little green problem








2nd - I've been following this thread since you started it and I have a theory.
3rd - Bad fan switches are popular on these cars and as such, typically replaced with one of these, click here for new fan switches.
4th - I know you are the husbandmechanic (congrats btw) so maybe you can answer this. Have you replaced a coolant temp sensor? If so, they are the same style connection as the fan switch and many times get confused. A CTS in the place of a fan switch will cause the fan to always be on.
So check to make sure you put a CTS in the CTS spot and not a fan spot. If that all checks out, this sounds like a classic bad fan switch issue.


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