Most of the time engineers focus on the design of physical products and on their interactions with others objects and this is why technical services are not considered very early during the design process. On the other hand, some PSS methodologies still exist but are focused on the system and do not sufficiently specify engineering product criteria. Indeed, to achieve the development of consistent Product-Service Systems, a methodology is required to support engineering designers during the development process. PSSs are composed of physical objects and service units that relate each others. To have a competitive PSS, the designers must consider carefully and early in the design phase the interactions between those elements. The aim of the proposed methodology is to provide engineering designers with technical engineering specifications in relation with the whole system's requirements as precise as possible for the development of the physical objects involved in those systems. The paper describes the context of PSS development and the current methods used to develop such systems. Then, the tools and formalism used in the proposed methodology based on a function-oriented description and an activities related description are explained. Finally, an industrial example of a helium-based refrigeration unit illustrates the proposed methodology.