Phillip:

Actually, while the fan spins all the time, this is the way to check:

It should roar for a few seconds when you first start it since the viscous oil is all stitting at the bottom of the mechanism. It will then slow down as the oil runs out of the gap between the disk that drives the fan and the slot it is in. It stays up in the "reservior" so long as the hole in it is covered by a plate pushed in by the bimetallic strip.

When the bimetallic strip gets hot enough, it pops out, releasing the plate (actually a simple valve) and the viscous oil runs back down into the working area and the fan speeds up.

The fan never sits completely still, and it never actually runs at engine speed, either -- close at lower speeds, but never as fast. At high speeds, the oil runs out too fast to drag the fan at engine speed, and the drag of the fan is too great for it to speed up more.

When engaged, the fan will be running near engine speed from about 900 rpm to around 3000 rpm -- above that, it usually won't go much faster than 3000 rpm no matter how fast the engien runs.

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!