Your system may be working correctly -- I have been looking into this the last week on my 560sl. If you do a search you will find much information.
The system in the 560 did not turn the fan on when the ac was first turned on -- it only turns on if pressure is high in the ac system or the water temp is too high.
If the ac is a little low on gas the pressures never get high enough to trip the switch. This is what they feel is my problem -- I had it converted a few years ago -- I have made an appointment next week. I do not really think that this is the best layout.
The tech was saying if everything is right the fan will rarely go on -- the engine driven fan on this model is very large and powerfull and does a good job of cooling the system. He was saying that the high water temp cut in is very rare and could indicate something else up -- unless you were in very hot weather
If you bypass the pressure switch the fan will run all the time - even with the ac off! So this simple change will not solve the problem.