Method for operating a vehicle drive train

A method of operating a vehicle drive-train (5) which comprises a drive machine (2) and a transmission device (3), and during a gearshift in the transmission device (3) at least one interlocking shifting element (F) is disengaged and at least one frictional shifting element (B) is engaged, the method comprising the steps of:

• initiating monitoring, at a time (T1) of a shift command, of a shifting time and either a transmission input speed or a rotational speed equivalent to the transmission input speed;
• recognizing that the interlocking shifting element (F) is in a disengaged operating condition, if the monitored transmission input speed or the rotational speed equivalent thereto deviates from a synchronous speed equivalent to a gear ratio (“4”) to be disengaged in the transmission device (3);
• adjusting a current drive torque (m_mot) of the drive machine (2) if an engaged operating condition of the interlocking shifting element (F) to be disengaged is detected after a lapse of a pre-defined time interval (t7) that begins at the time (T1) of the shift command, by motor actuation (me) to a torque level that corresponds to a difference between either a current transmission input torque or a torque value equivalent thereto and a torque that is transmittable by the frictional shifting element (B) to be engaged, operating with slip, plus a torque offset value, and at which the interlocking shifting element to be disengaged changes, at least for a time, to an at least approximately load-free operating condition such that a transmission capacity of the frictional shifting element (B) to be engaged, during traction operation of the vehicle drive-train (5), is brought to a traction torque value level (m_mot(zug)) equivalent to the transmission input torque;
• adjusting the transmission capacity of the frictional shifting element (B) to be engaged, if a load change takes place in the vehicle drive-train (5) during the gearshift, to a thrust torque value level (m_mot(sub)) equivalent to the current transmission input torque;
• and
• operating the frictional shifting element (B) with slip at least while the interlocking shifting element (F) is engaged.