Method for calibrating a coupling system of a summation mechanical transmission, which is configured to couple a first drive shaft coupled to a first hydraulic motor and a second drive shaft coupled with a second hydraulic motor, 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, wherein said pressure regulating device comprises activating means, which are configured to regulate the pressure inside said hydraulic cylinder based on a current (i C) provided to said activating means; wherein the method comprises the following step: estimating a function over the temperature for a current (i_kp) provided to said activating means, which is requested for reaching the kiss point, characterised in that said function is estimated by 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. Measuring the current (i_kp) provided to said activating means at which the kiss point during said coupling process of step a. has been reached; c. Averaging the value of said measured current (i_kp); 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 measured current (i_kp) obtained for said at least two different temperatures an interpolating function is estimated by interpolating the averaged values of the current (i_kp) at which the kiss point has been reached obtained for said at least two temperatures (T L, T H), wherein said interpolating function is a function representing said current (i_kp) over the temperature. wherein said pressure regulating device comprises activating means, which are configured to regulate the pressure inside said hydraulic cylinder based on a current (i C) provided to said activating means; wherein the method comprises the following step: estimating a function over the temperature for a current (i_kp) provided to said activating means, which is requested for reaching the kiss point, characterised in that said function is estimated by 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. Measuring the current (i_kp) provided to said activating means at which the kiss point during said coupling process of step a. has been reached; c. Averaging the value of said measured current (i_kp); estimating a function over the temperature for a current (i_kp) provided to said activating means, which is requested for reaching the kiss point, characterised in that said function is estimated by 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. Measuring the current (i_kp) provided to said activating means at which the kiss point during said coupling process of step a. has been reached; c. Averaging the value of said measured current (i_kp); 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. Measuring the current (i_kp) provided to said activating means at which the kiss point during said coupling process of step a. has been reached; c. Averaging the value of said measured current (i_kp); 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 measured current (i_kp) obtained for said at least two different temperatures an interpolating function is estimated by interpolating the averaged values of the current (i_kp) at which the kiss point has been reached obtained for said at least two temperatures (T L, T H), wherein said interpolating function is a function representing said current (i_kp) over the temperature.