I got a 1999 A6 2.8 quattro and the car started overheating recently. At first I though that it might be the thermostat stuck closed or the water pump is bad, but I just realized that the radiator fan doesnt engage. It just free spins on the pulley, and I can stop it with my hands. Anyone know why the clutch is not engaging? If I spin the fan by hand it does around half a turn and stops, so it should be good right? I am clueless right now.

 

Bad fan clutch!

Does the electric fan work with the a/c on?

 

ya the electric fan works fine, its just the one the on the belt that doesnt engage. the fan seems fine because when i spin it by hand, it stops right away after like half a turn. I was told that there is a coolant temp sensor on the radiator that controls when the fan turns on, but i just cant find it!

 

That fan that you are referring to is controlled by a fan clutch. Once it heats up to certain temperature, then the clutch engages and spins the fan. It is normal for the fan to spin freely when cold. To test it, drive the car until it starts to barely over heat. Get out and with the car still on, try to stop the fan with a roll of newspaper. If you are able to stop the fan, then the clutch is bad. If not, then better look at something else.

 

^^^ very good advice.

 

The fan clutch doesnt engage even when the car is overheating. I am still leaning towards the thermostat being stuck closed and not letting hot water go through the rad to let the fan clutch detect the heat. previous owner said that the timing belt was replaced a few thousand miles ago, but there was no paperwork to go with that, and there is no indication of it being done. I would rather play it safe and just do the timing belt job, if the car still overheats after that, I will look into the fan clutch more, but I am pretty sure that it is good.

 

"....I was told that there is a coolant temp sensor on the radiator that controls when the fan turns on, but i just cant find it!"

I think the temp sensor is located at the bottom of radiator on the passenger side.

 

Also do what I'm about to do and get a failsafe thermostat ( unless there is a reason out there I don't know about ) you can get one from Amazon.com delivered for less than $9.