Mercedes® Fan Clutch, 1977-1981 (123)

To check the fan clutch, use a fender cover or piece of cardboard to block the air flow through the radiator. Start & run the engine until the engine temperature reaches above 100 degrees Celsius. Shut off the engine & see how easily you can spin the engine-cooling fan. If the fan turns freely or without much drag, the clutch needs to be replaced. If the fan is hard to turn, but will turn by hand pressure, its ok. Some people see lateral play in the fan clutch with the engine stopped & think the clutch is bad. As long as the play at the outer ends of the fan blade is a quarter inch of less & it passes the above test, the clutch is not faulty. Also check the fan clutch bearing bracket for excessive play and any cracks in the housing.