A motor vehicle drive train including a hydrodynamic retarder (1) that comprises a revolving bladed impeller (10) and a stationary or counter-rotating bladed turbine (11) which jointly form a working chamber (12) that can be filled with a working medium to switch on the retarder (1) is disclosed. The hydrodynamic retarder (1) can be mechanically disengaged from the drive train by a disconnect clutch (2). A fill level monitoring device (14) can detect the current fill level of the working medium in the working chamber (12), while a disconnect clutch-blocking device (9) is effectively connected in a communicating or mechanical manner to the fill level monitoring device (14) and prevents the disconnect clutch (2) from engaging in accordance with the detected fill level.