Visco clutch should drag the fan up to engine speed when it gets to something like 90C or so. You will hear it, usually roars when it's engaged.

It should also roar for a second or two when first started. All the fluid will collect in the bottom of the housing and cause the fan to engage until it spins out again.

Best way to check is to get engine hot enough that the fan should be engaged, then check for air flow at idle. Should be substantially higher than when cool. Next, shut the engine off while watching the fan. If it makes more than one revolution after the engine stops, it's not engaging, usually because all the oil has leaked out. This normally leaves a sticky, filthy mess on the fan clutch.

Peter

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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!