Old before you replace it. Make sure the radiator is getting hot all around the fins if the middle of radiator is cool the fan wont kick on. Its trigger by a bimetalic spring sensor that measures the temp of the radiator.Theres a kinda liquid paste that heats up and makes the clutch engage. Its normal when the engine is cold you can hold the fan blades. That fan clutch kicks in about 95 C.
The electrical fans kick on about 110. Just make sure before you go spend a few hundred dollars/to double check that thermal effeciency of the radiator.
Factory Cert Tech MB