I have a 2000 a6 2.7 turbo so when driving with the ac on and the temp under 88 degree the temp gauge is normal at half once the temp get over 88 degree with the ac on the temp gauge goes over half the fan kick on and it start to over heat but only does it at idle or when I am driving under 30 mph once I am on the highway it fine it only does this with the ac on any one ever had this problem was thinkin about changing the thermostatus and changing the coolant .

 

The source of the issue can be anything at this point. I would check the fan clutch first. Then if the clutch is good, then flush the system and see what happens. Changing a thermostat on your motor is not an east task.

 

Well the fan is working fun whenever the temp goes over half the fan turn on maybe I have air in the cooling system

 

Not the electric fan... Check the mechanical fan attached to the engine. The clutch is designed to engage after certain temperature. When the engine is cold, you should be able to slow the fan with a roll of newspaper and hold the fan still while the motor is running. When the engine is warmed up, you shouldn't be able to. Once warmed up, turn the motor off, and see if you can spin the fan with your hand. If it spins, then you need a new fan clutch.

 

You talking about the cooling fan ? That cool the engine off . I got two fan a cooling fan and a ac fan you talking about the engine cooling fan right ?

 

My mistake there 3 fan two on the inside under the hood on in front the radiator ?

 

So I took a look at the fans under my hood today and I notice the one on the driver side run but the one on the passage side don't come on and when I put the ac on the one on in front the radiator come on so is the fan on the passage side the cooling fan ? I don't see it running should it be running ?

 

Just let the car sit and run with the ac on and wait until the coolant guage goes past center. If all the fans are not running then, you may have found the problem. Both inside fans should run at the same time as far as I know.

 

Only one the inside fan running the one on the driver side and the on in front the radiator . So the passage side fan should be running too when the temp gauge go pass half right ?

 

The one on passenger side behind the radiator is aux cooling fan. It's supposed run when a/c is on, plus something other conditions.
If it does not starts thus the overheating.

It can be the fan dies, the fuse, the relay by the ECU, the fan module on top of driver side bigger fan or the temp sensor located at the bottom part of radiator.

Search the forums to find out more info re "aux fan not turn on". I had a noisy aux fan, and not yet died, so I am pretty sure my case is need change the fan.