I have been trying to troubleshoot an overheating problem with my A6 and have determined that the electric fan is not coming on. After about 10 minutes of driving the coolant begins to boil and a healthy amount appears to bleed out once I come to a rest. I tested the fan by bridging the connectors on the wire that goes to the auxilary fan switch on the lower radiator hose wih the ignition switch on. I have replaced the fan and the cooler fan switch in the lower radiator hose along with upper and lower radiator hoses. I have checked all of the fuses that relate to the cooling system which are located behind the storage compartment under the display panel on the drivers side. The fan still doesn't work and the only two items left that I haven't replaced are the Auxilary Fan Control Unit and the Auxilary fan resistor.

Is there any trick to determining the source of the problem? I would rather not replace the control unit and find out that it is not the cause since it is $200 for the part alone. Am I wrong in assuming that the auxilary fan should come on while just idling and is the lack of this fan capable of creating the boiling radiator fluid. The fan on the fan clutch is still turning, albeit limited to the rotation of the serpintine belt.

Any advice would be greatly appreciated?

 

First off, I'm guessing that you have ac? If so, check if the radiator (for the coolant, behind the condenser) is clean. Looking from above there should be a few centimeters of free space between the radiator and condenser, It very often gets blocked by insects and stuff, so very little air can pass through. The mechanical fan could also be at fault, if the visco coupling is damaged. When the car is very hot and you rev the engine, you should clearly hear the woosh of air when it engages.