The present invention involves a drive unit, specifically a drive unit for a motor vehicle having an internal combustion engine, a cooling system with fan, a transmission and a hydrodynamic retarder with a stator, rotor and housing. The retarder rotor is mounted on either the engine shaft or a crankshaft journal which is coaxial with the engine shaft. The retarder is located within a space bounded on one side by the front of the crankcase, on a second side by the fan shaft which runs parallel to the engine shaft, and on a third side by the rear edge of the fan wheel. Viewed from the front, the retarder is substantially located within the fly circle of the fan wheel whereby the air flow generated by the fan wheel removes heat, generated by braking actions, from the housing of the retarder.