Thermal or Viscous? Clutch fan.

Wheeler Dealers showed how to test a viscous clutch. Ed China (original tech) grabbed the fan blade with a thick leather glove while the engine was idling. I think he said if the viscous clutch was good he wouldn't be able to hold on. Perhaps the metric is to count fingers before and after. Less fingers after means "good clutch"? A thermal type should have more drag as the air from the radiator gets warmer. Even with the thermal sensor, the fluid (silicone?) might get more viscous as it gets hotter. Perhaps a better method is to turn off the engine while hot and see how easy you can spin the fan. If hard to turn, the clutch is likely fine. If it freewheels, definitely bad. Perhaps some mechanics watch the fan blade vs engine damper as they turn off the engine to judge.

Some people on my M-B diesel forum have fussed with the clutch-fans. BTW, those fans (1982-85 300D or SD, probably other years and engines) will bolt to a Mopar water pump (same hole diameter and 4 mounting holes). It is much thinner, so I thought of using on my 1964 225 engine, but still not enough room before the radiator core. The nylon fan blade is nice. Another thin clutch-fan is on older Jaguars (I recall some here have used). Anyway, those have a flat thermal spring which acts on a little valve underneath, which you see when you pop the flat spring off. Some guys found the proper silicone fluid to refill the reservoir w/ syringe since it can leak out (perhaps stored flat like someone here said). I haven't read of a Mopar owner doing similar, probably since new clutches are easy and cheap. Likely there is a way since "if built by man, a man can fix it" ("just a question of price").