It isn't possible to test a visco clutch by spinning it by hand. The "clutch" is actually a two speed fluid coupling, sort of like a torque converter.
The bimetalic spring rotates the pin, and the pin then operates the stator in the fluid coupling, changing from high slip at low temp to low slip at high temp by changing the angle of the blades. GM had auto transmissions many years ago that worked this way.
The easy way to test a visco clutch is to allow the car to idle and get warm. At the temp where the clutch should engage (220F or so), it should start to roar and blow large volumes of air. Sounds like a jet engine on the highway.
If you them shut off the engine, the fan should stop rotating within one revolution after the engine stops. If it spins more or less freely for many revolutions, the visco clutch isn't locking up.
The two things that go wrong are for the fluid to leak out (spins freely all the time) and for the guts to freeze (never locks up). You can sometimes fix the latter by rotating the pin back and forth by hand. Never pull on it, you will break something or disconnect the stator, in which case it won't lock up anymore. Occasionally, the clutch will freeze, and the fan will run at engine speed. Very annoying, very noisy.
The cure for a visco clutch that won't lock up is to replace it. Can be expensive, but much cheaper than a cracked heat from overheating.
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!
Last edited by psfred; 09-09-2002 at 11:23 PM.