An arrangement and a method for a drive unit of a vehicle. The drive unit incorporates an engine, a mechanical stepped gearbox and a connecting device which is designed to transmit rotary motion from the engine to the stepped gearbox. An electric rotor machine acts upon the connecting device such that substantially no torque is transmitted from the engine to the gearbox during gear changing. A control unit connected to the rotor machine may be responsive to the output shaft from the gearbox or to the speed of the engine.