Method for calibrating a coupling system of a summation mechanical transmission which is configured to couple a first drive shaft and a second drive shaft, the coupling system comprising an hydraulic coupling with two releasable coupling sections (28, 30), wherein a first coupling section is connected to the first drive shaft and a second coupling section is connected to the second drive shaft, the coupling system further comprising an hydraulic cylinder which is connected to one of the first and the second coupling sections (28, 30) and is configured to engage and to disengage said first coupling section with said second coupling section based on a pressure inside said hydraulic cylinder, wherein the pressure inside said hydraulic cylinder can be regulated by means of a pressure regulating device, the method being characterized by comprising the following steps: a. Coupling and decoupling the first drive shaft with the second drive shaft for a predetermined number of times, wherein for each of said number of times the temperature of a pressure medium, which is the pressure medium introduced into said hydraulic cylinder during the coupling or during the uncoupling process by means of said pressure regulating device, is substantially the same; b. During the coupling process measuring a first time interval (t 1), which is the time requested for reaching a predetermined pressure value inside said hydraulic cylinder, wherein said predetermined pressure is a pressure at which a kiss point between said first coupling section and said second coupling section has still not been reached; c. Averaging the value of said first time interval (t 1) obtained in each coupling process; wherein said steps a. to c. are repeated for at least two different temperatures (T L, T H) of the pressure medium, which is introduced into said hydraulic cylinder during the coupling or during the uncoupling process of step a.; wherein based on the averaged values of said first time interval (t 1) obtained for said at least two different temperatures (T L, T H) an interpolating function is estimated by interpolating the values of said first time interval (t 1) obtained for said at least two temperatures (T L, T H), wherein said interpolating function is a function representing said first time interval (t 1) over the temperature. a. Coupling and decoupling the first drive shaft with the second drive shaft for a predetermined number of times, wherein for each of said number of times the temperature of a pressure medium, which is the pressure medium introduced into said hydraulic cylinder during the coupling or during the uncoupling process by means of said pressure regulating device, is substantially the same; b. During the coupling process measuring a first time interval (t 1), which is the time requested for reaching a predetermined pressure value inside said hydraulic cylinder, wherein said predetermined pressure is a pressure at which a kiss point between said first coupling section and said second coupling section has still not been reached; c. Averaging the value of said first time interval (t 1) obtained in each coupling process; wherein said steps a. to c. are repeated for at least two different temperatures (T L, T H) of the pressure medium, which is introduced into said hydraulic cylinder during the coupling or during the uncoupling process of step a.; wherein based on the averaged values of said first time interval (t 1) obtained for said at least two different temperatures (T L, T H) an interpolating function is estimated by interpolating the values of said first time interval (t 1) obtained for said at least two temperatures (T L, T H), wherein said interpolating function is a function representing said first time interval (t 1) over the temperature.