Although the elaboration of the case representation is the key problem of the case-based reasoning system conception, there exists no proven methodology targeted to this task. This paper is a contribution to fill this gap in the maintenance domain, more precisely in the equipments diagnostic and repair help. A methodology to elaborate the case representation is proposed based on knowledge management techniques and existing engineering analytical tools that are used in the industry. Different ontological models are proposed to take into the account similarity and adaptability aspects of the case representation and to optimize the case base size.