Hello,
I think i might know the answer to your problem.
My native language is not english but i think you mean
the engine cooling fans in front of the radiator.
I had the problem that mine did not turn on at high temps (between about 90 and 105)
and only the second stage of the fans cut in at about 105°C
in summer. The first stage did not work anymore. The problem was simple
between the fans you see a big fuse or i think its called resistor in english.
This thing can fail over the years and make problems. The electric fans have two operating modes
on the w140 one mode turns it on when the temp is something over 95°C or so and the second stage
where the things really turn up is at a 105°C.
You can see the engine cool water temps in the secret menu on the climatic display, press the "REST" button
for 5sek and select menu point 7, i think, with the auto buttons. Your problem of a permanent running fan could
be caused by a faulty elektric resistor between the fans.
This electric resistor was replaced according to mercedes with the old model which is a little bit bigger than the one in the car.
It took about 20min to install it. Placed between the fans like i said. Your problem of a stedy running fan coult be a faulty resistor,
this part was not expensive to replace under 100 if i remember correclty.
EDIT: The part i changed was about 10cm high and slim like a fat marker typ of pen, the new replaced part from mercedes look like this,
i think they said they changed this part on the newer w140 to the ones like the old pre facelift models had it. My S320 in which the mistake occured is from ´95.
http://3.static-hood.de/img1/big/4592/45921303.jpg
Greets