The car is a 1977 450 SL. The engine temperature<br> gradually increases and tends to the high end of <br> safe operating range when the car is ascending long grades, and in city taffic. The car has a new mercedes radiator. The safety fan has been tested and works to specs. The fan clutch is <br> a rebuild, two years old. I do not hear the main fan kick in as temperature increases. How can I tell if the fan clutch is working? Any other ideas appreciated.