A thermodynamic methodology of TDC determination in IC engines based on a motoring pressure-time diagram is presented. This method consists in entropy calculation and temperature-entropy diagram analysis. When the TDC position is well calibrated, compression and expansion strokes under motoring conditions are symmetrical with respect to the peak temperature in the (T,S) diagram. Moreover, in case of error on the TDC position, a loop appears, which has no thermodynamic significance.Hence, an easy methodology has been conceived to obtain the actual position of TDC. This methodology is applied to motoring measurements in order to present its performance, which are compared to usual methods.