Within the last years, the number of electric vehicles in Europe is increasing faster than the number of charging stations. Based on this, the reliability of the charging process takes on greater significance. The communication between the electric vehicle (EV) and their charging station is a prerequisite to transfer the charging power. Using a universal charging communication test system allows an observation of the whole charging communication according to ISO 15118 and IEC 61851 standards. It is important to know whether the communication between the EV and the charging station functions accordingly, in order to ensure functionality. Furthermore, tolerances must comply with the standard. In a first step, the basic communication according to IEC 61851 is considered. Preliminary investigations of the pulse width modulation (PWM) signal have shown that some EVs are tolerant of the standards. Which means, that a charging process is still possible, although the communication is not conform to the standard. Therefore, the FKFS is presenting a Man-in-the-middle system which allows a systematic manipulation of the charging process. Based on the total interruption of the communication, a regulation of the edge steepness and the frequencies of the PWM signal are possible. This allows further investigations of the limits of charging process, which are necessary to guarantee a reliable charging process. The investigations focus on the start of the charging process under different conditions as well as an observation on how the charging process is affected, if parameters change while charging. Additionally, the tolerance of timeouts can be evaluated. The modularity of the chosen test system makes it possible to investigate the low-level as well as the high-level communication.