190E engine fan only engages when engine cold

[Rocambolesque]

I have this problem on my 1992 190E 2.3: the clutch fan is only engaged when the engine is cold. When I start the car, it is engaged. I tried the newspaper test and I cannot stop the fan. However, when I get the engine to operating temp the fan disengages. The temp slowly creeps up and the fan clutch still doesn't lock. When temperatures hit 100C, the electric fan come on.
I tried disconnecting the blue temperature switch on top of the engine and when I do this the electric fan comes on. I guess this switch is the one for the electric fan... Where is the switch for the engine fan located? Why would the fan engage when the engine is cold en disengages when hot?