Method and control unit for operating a vehicle drive train

A method for operating a vehicle drive train (1) that comprises a prime mover (2) comprising an internal combustion engine, a transmission (5) in which multiple ratios (“1” through “9”, “R”) are implementable via selective engagement and disengagement of shift elements (A through F), a hydrodynamic torque converter (3) comprising a torque converter lockup clutch (4), and a driven end (6), the method comprising:

• initiating a downshift in the transmission (5) from an actual ratio (i_ist) presently engaged in the transmission (5) towards a demanded desired ratio (i_soll);
• during the downshift, disengaging at least one shift element, which is engaged in a power flow (K) of the transmission (5) in order to implement the actual ratio (i_ist) presently engaged in the transmission (5), from the power flow (K) in order to interrupt the power flow (K);
• adjusting a power transmission capacity of the torque converter lockup clutch (4) to a level at which the torque converter lockup clutch (4) is in a non-slip operating condition during a positive engine override, the rotational speed (n2) of the prime mover (2) adjusted from the synchronous speed (n2_sync(i_ist)) of the actual ratio (i_ist) towards the synchronous speed (n2_sync(i_soll)) of the desired ratio (i_soll) during the positive engine override;
• and
• no later than a point in time (T3) of the downshift at which the rotational speed (n2) of the prime mover (2) is equal to the synchronous speed (n_sync(i_soll)) of the desired ratio (i_soll), adjusting the power transmission capacity of the torque converter lockup clutch (4) to a level at which the torque converter lockup clutch (4) transfers into a continuous slip operation due to torque which is transmittable via the transmission (5).