I have a very untrusting nature and this just smells of "opportunism". I would want it proven to me that the cause is not the thermal switch...it WOULD cause the very same thing.
When I looked up your fan, I found that it did indeed have a built in control unit (very different from the year before your car). That is the one that you (they) could order for $420 complete...identical OEM Audi unit.
But again, I'd be suspicious of their diagnosis and price...REALLY don't get the high labor charge for such a simple job(?)

 

My first guess would be the thermal switch also. Have you had the system scanned for DTC's? It may or may not show a fault though. It sounds like the fans are in default mode (high speed). It CANNOT be the thermostat and the coolant fan control module is not a common failure. In most cases, when the module fails you have NO coolant fans. Also, depending on what part has failed, you may need to clear all system DTC's after the repair or the fans will still be in default. You have to "tell" the car it's fixed first.

On the battery, when replaced you should use the battery replacement procedure on the scan tool and enter the serial number of the new battery if you use a genuine Audi battery (HIGHLY recommended). Basically, as with the fan deal you have to tell the car it has a new battery or the MMI display will be incorrect as well as the energy management system will not function properly. Just a little advise anyways.

Also, if you are careful, you can hook a battery maintainer up to the jump start posts while swapping the battery. Just be careful with the positive cable. This way, the electrical system is never powered down and you don't lose all the basic settings and adaptations. You do still need to do the serial number thing though.

 

Im in agreement with aspen79 and silverd2, i don't have much experience with the D3 cooling fan system (since i haven't seen one fail) but i CAN tell you that it will be more advanced than the cheaper models. The A4 and A6 won't usually store a fault code for a cooling fan issue, they are usually just simple units. The A8 on the other hand will literally store a fault code for everything so i would be asking your mechanic what the VAG COM says. (NOT THE SNAP ON ONE, AND NOT THE MAC GENISIS TOOL) If he is using anything besides a factory scanner or a vag com, he CAN'T diagnose this car correctly.
If you want to shot gun it, throw in a thermo switch first.

 

they have no codes show.I would like to try the thermal switch first.What is the part number?...Any DIY?

 

cough, thats not what i asked you. Is this an audi shop? Or a euro repair shop that specializes in VW/Audi?

 

Oh and according to this web site the fans CAN be purchased seperately. Universal though.
http://www.ecstuning.com/Audi-D3_A8-...e/Cooling/Fan/

 

it is euro shop specialize on vw audi porsche mercedes bmw

 

i cannot find the thermal switch for my car anywhere.Somebody have the info?

 

Its a little two wire sensor located on the radiator somewhere near the bottom of it, not unlike the ECM's coolant temp sensor behind the engine. Looks like this.
http://www.ecstuning.com/Audi-D3_A8-...ne/Electrical/
Scroll to the bottom of the page, i would get the genuine audi part which costs the most.

 

Not to contradict...actually got me curious. But not having a manual or diagrams on D3, just going on the D2 version. Mine (D2) has the 3 pole switch (not called a sensor in the manual), because it sends signals for low and high fan speed , depending on coolant temp. It's attached to the radiator, right next to the lower coolant hose, passenger side.
Can't find a part # anywhere for this switch for a D3, just various "temp senders"...genuinevwaudiparts.com lists a temp sensor on the same page as coolant fan, under the cooling system heading for 2004 A8, but doesn't have part # or description.
Is it possible that the D3 uses a different system which uses a temp sender like you said (auditech79), like the ECM version and then control unit translates the signal to switch on fan...instead of the D2 system that uses a simple 2 temp on/off switch?

Anyway, to "gu9cci" : If the sensor (or switch) is mounted in a similar place on the radiator, it would require draining the radiator to change the switch...possible removing the belly pan under the engine to get good access (like on a D2). If you find it, I would still try unplugging it for a test...but like aspen79 said, the system may need a Vagcom reset to change back to normal, so I don't know if this would be a fair test.

Possibly aspen79 can tell us the exact location of the sensor (or switch) and if it is a 3 pole switch like D2's or temp sender (like the ECM type) ...possibly mounted somewhere else. I can't seem to find that info anywhere.