I don't think they are supposed to lock up below 110 C. Should lock up at the mark between 80 C and the red zone on the temp guage. Aux fan kicks in at 100 C.
I looked at the replacement clutch I have for the 300 TE I've never gotten around to replacing (overheating problem last summer that turned out the be a bad radiator cap), and there is a strap type spring. The Volvo has a coiled spring that rotates a pin in the center, so do most American visco clutches I've seen. I'd expect the MB type to be more reliable -- less chance of crud making the pin stick!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!