Method for operating a vehicle drive train with a drive motor, an output and a transmission which is arranged between the drive motor and the output

A method for operating a vehicle drivetrain (1), the vehicle drivetrain (1) having a drive machine (2), a drive output (3) and a gearbox (4), the gearbox (4) arranged in power flow between the drive machine (2) and the drive output (3), the gearbox (4) having at least one positively engaging shift element (A, F) and a plurality of frictionally engaging shift elements (B, C, D, E), the at least one positively engaging shift element (A, F) and the plurality of frictionally engaging shift elements (B, C, D, E) selectively closable in order to engage different transmission ratios (“1” to “R”) of the gearbox (4), only some of the transmission ratios (“1” to “R”) engageable with the at least one positively engaging shift element (A, F), the vehicle drivetrain (1) configured such that, during a demand for activation of an engine start-stop function of the vehicle drivetrain (1), the drive machine (2) is decoupled from the drive output (3), the drive machine (2) is shut down, and the at least one positively engaging shift element (A, F) is transferred into or kept in a closed operating state of the at least one positively engaging shift element (A, F), the method comprising:

• when there is the demand for activation of the engine start-stop function of the vehicle drivetrain (1) and a simultaneously shut-down drive machine (2), operating the vehicle drivetrain (1) with a sailing operating function of the vehicle drivetrain (1) active, with the drive machine (2) decoupled from the drive output (3), with the positively engaging shift element (F) open, and with a rotational speed (n_ab) of the drive output (3) greater than a defined rotational speed at which a rotational speed difference between shift element halves of the positively engaging shift element (F) lies within a rotational speed range within which the positively engaging shift element (F) is transferrable into the closed operating state;
• and
• actuating the positively engaging shift element (F) in a closing direction no later than when the defined rotational speed is reached.