Listen to the fan when you start the engine cold...the fan should roar. It should quiet down in about a minute or so...that indicates the fan is now slipping. It roars during startup because it is engaged due to the internal fluid running to the bottom of the clutch.

After awhile, maybe 10 minutes or so depending on the ambient temperature, the fan will start to roar again, indicating the clutch is engaging and pulling air through the engine. It will roar for awhile and then quiet down. The conclusion is that the clutch is working.

The cooling system on the 740turbo is reasonably good if you use good coolant and change it every two years. The plastic radiators do die after about 10 years or so when the plastic neck gets brittle...or if you run the car low on coolant or overheat it some other way (there are failed thermostats too!). You can replace the radiator with an all metal three row from IPD which has worked well for me.