Abstract: A pulley assembly comprising a locking assembly connected to an input shaft of a water pump, a pulley mounted in a idle condition with respect to the shaft, an armature which is axially mobile in order to selectively cooperate by friction with the pulley, an electromagnet to displace the armature arranged on the axially opposite side of the pulley with respect to the armature, and an elastic element preloaded to maintain the armature in contact against the pulley when the electromagnet is not energised and having a substantially flat shape in its non-preloaded or non-deformed configuration.