The OP started this thread 9 years ago but I have written the following so as to help other members who will have much the same issue with their AC system.
I can guarantee that the system has little to no gas in it and thus the low pressure switch has been activated so that the compressor can not run, thus protecting the system, mainly the compressor. One will find that the gas has leaked out at one of 3 locations so look at these for oil dampness,
1, at the TX valve which is located under the black plastic cover at the base of the windscreen
2, at the hose connector block on the compressor
3, the "P" clip that holds the AC pipe on to the rad support panel behind the bumper on the passenger side. The Ally pipe will rot out inside the plastic clip liner as it constantly gets wet.
This (point 3) is the most common fail point in the system. The pipe that runs from the TX valve down the chassis rail to the Receiver drier will have to be replaced. These are available now in a two piece kit to make it easy to install while the engine is in place, something it isn't when the pipe was originally installed during the vehicles assembly.
Make sure that you replace any "O rings", do not under any circumstances reuse old ones as when the system gets re-gassed "IT WILL LEAK".
Once the system is re-gassed the thermo engine fan will come on as soon as the AC button is activated and the compressor is running regardless whether it has Climate Control which all Calais and Berlina models have or standard AC in Execs etc. The AC system overrides the coolant temperature function where the fan cycles in and out due to engine temperature rising and falling.
Cheers.
