Cheap and cheerful visco clutch checks:

First, sound. Should roar when first started as the fan is dragging, sound will vanish in a few seconds as the fluid pumps out.

Second: on startup, you shoudn't be able to hold the fan stationary with some folded newspaper or a flyswatter. DO NOT HOLD THE FAN BY HAND! If you can easily prevent it rotating, the clutch is bad (no fluid in it).

Third: when engine gets up to 90C or so, the fan should engage, producing considerable air movement and noise. May be a turbinelike whine or just the sound of rushing air, but will be noticalble. If the engine is shut off at this temp, the fan MUST stop within one revolution when the the engine stops -- takes two people to check. If it spins more than that, it's bad, replace (or refill) the visco 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!