Device for and method of operating a hydrodynamical retarder

A hydrodynamic retarder (1) has a stator (2), a rotor (3) and an oil reservoir (9) operated by an electric motor by means of an actuator (7). A piston (8) or a membrane is used to operate the oil reservoir. A heat exchanger (5) links to a vehicle's coolant circuit (6). An independent claim is also included for a device for operating a hydrodynamic retarder.