Issue on why I replaced the viscous clutch my 350SDL was pretty straightforward ... was operating fine (temps staying always in 82-85C range), but the bearing was shot ... started wobbling and getting too close to the cowl ... that was how I first noticed it. I think the waterpump bearing was also going as it was still a bit wobbly (not as bad, though) after I replaced the clutch.
Like I said, I always look for changes to occur after changes have been made ... and, mine immediately starting to run at a new temperature "profile" of 82-85C under light load and 100C plus under heavy load (high speeds in warm weather (80F) with AC on or in traffic with AC on or off) ... my change has been very noticable.
Wish I had saved the old clutch ... would put it back in and then see if it changed the profile.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg