OK guys, it seems odd replying to my own post
but very useful nevertheless :-)
What they say is that the JE G9T diesels with air con have massive cooling systems
And the Dci's are so effective, that very little energy actually is left to heat the engine/coolant. Thats also why they have a coolant-heater installed, to assist the coolant in reaching a temperature, where it can be used for creature comfort inside.
(Unfortunately, I don't think there is a simple way to modify this engine-heater to work without the engine running as it is very integrated into the ECU software ... it would have been a great feature for those of us living in cold countries.)
Turning the AC on requires airflow to the condenser - which is why it turns the fan on. However this only verifies that the fan works, not that it will cut in at a certain coolant temperature - here you need the neat trick your Renault dealer did by injecting false temperature values, and you probably also should also check that the sensor sends correct data. Does anybody know what the different bars on the temp-gauge roughly corresponds to in degrees Celcius ?
/Lennart