This work presents a thorough modelling study of Electro-Rheological (ER) Dampers used in vehicle suspension systems. The main purpose is to consider the overall behaviour of these ER dampers, which basically, present a resistance against shearing that varies according to a controlled electric field. This study is a twofold: i) the first part is an analytical approach towards the dynamical modelling of the ER damper force, wherein a conclusive para-metric model is obtained; ii) the second part aims to analyze and model the possibility of faults upon the dampers which is of paramount importance for suspension system diagnosis and reliability. Throughout this study, simulation and experimental validation results from a real mechatronic test bench are presented. The overall results assess the ability and the accuracy of the proposed model to characterize the force delivered by ER dampers in both healthy and faulty situations.