bribenz - on my 560sel viscous fan was semi -freewheeling when engine was cold - not pumping much air - fan speed increased slightly as you revved engine but was still not locked up. When engine was hot - same thing and engine temp climbed to over 105C. I put a new fan viscous unit in. Now when engine is hot - the fan locks up and pulls maximum air. I don't know for sure - but I believe the air temp the fan pulls through the fan produces the lock up. From what I have read - the viscous fan was used as means to increase gas mileage with less torque required from the pulley when the fan is not needed. If my understanding is wrong - can someone please explain why the viscous fan is used instead of a solid mount fan that is locked at all times?