I agree... very rare for fan clutch to go bad. Other than radiator... the
temperature gauge do go bad and thereby reading high temp when in fact is
not.

You really need a touchless infrared temperature gauge... I just got one and
it registered my car car in normal operating range of 80 when my instrument
cluster reads 95 degree. I pointed that thing at the brass fitting of the
temperature sensor.

On various Mercedes forums there is quite a bit of discussion RE the
fan clutch.  There are instructions how to refill the assembly with a
very thick oil (available from Toyota dealer).  There are quite a few
folks who seem to have problems with the fan clutch.

If the clutch is stored in a horizontal position the oil will leak out
(had one of those- the oil will be all over the unit in the box).

A local repair shop who specializes in older MB seems to think that the
viscous-fluid clutch fans are not reliable and think that the fan
clutch failure often leads to overheating/cracked heads.  They do like
the electrically activated fans on some of the MB's.

My fan will 'free wheel' even when the engine is very warm.  If the fan
is working correctly then it only activates (locks) when the engines is
really really hot.

I also agree that the temp gauges are not reliable.

Bear in mind that reliablility is relative.  I have two MB's, one a
commercial truck with over 1 million miles (never had the engine down,
never replaced the clutch, never even replaced a u-joint).  My 300sdl
has 325,000 miles on it and has had only routine maintenance.  Most
cars would have been crushed long ago lol.  I just want to preserve the
engine and would gladly sacrifice a mpg on the car to have the security
of KNOWING that my fan clutch is cooling the engine.

Thanks ...

I am sure that it is true that the radiator is not working at 100% of
designed effeciency in most cases.  I am sure that mine is not.  A new
radiator is approx. $400, a new fan clutch (with plastic fan) is
approx. $250.

If I may say a word on the topic:
I own MB C200 Elegance 1996 and only after 145000 km I had to replace fan
clutch,. The old one simply gave up and even very hot radiator wasn't able
to make it work. Visco simply wore out. After the temperature rose a few
times to 120"Celsius I replaced it to new one (not geniune orignal, but
sitll very good - Hans Pries Topran). New one works good and engine never
reaches more than 95"Celsius.
I asked my mechanician about changing only oil and visco in old clutch, but
he said, that it is very delicate matter, there are very few people, if any,
who can make it right, and even so, regenerated clutches seem to fail much
sooner, than new replaced. So if you have the clutch regenerated, maybe it
failed so soon because people that vere doing it simply didnt do it good
enough.
Besides - new clutches make no problem with storing in vertical or
horizontal position, as old ones were. So if you can, replace it, and make
sure they'll fiot NEW one, not regenerated or "good" one pulled out from an
old car.
About locking the clutch - if you want to do it the hard, "hammer" way,
three good, durable screws and drilling machine will do the job. Three years
ago i was on the vacation in Croatia (I'm form Poland) and our visco broke
there. One mechanician fixed it mentioned way..... Well, we made way home
without any problems, but engine rumbling a litte more after exceeding 120
km/h. That way it will always cool with maximal power, and engine will sound
more sporty, but you have to remember, that it also will take out a few
horsepower. One more - you'll newer have to worry about changing clutch
again :)
Btw, idea of checking radiator is a good one, as it may be the cause of
problem. Also check cooling liquid pump for correct working and thermostat,
as if it is broken, it can also make problems.
Well, that's it.....
Andrzej