With an increase in the number of electrical systems installed in vehicles, the wire harness design has become increasingly complicated and, accordingly, it has become essential that it is validated. Simulation is a good method of validation but it requires models of the installed electrical systems as well as wire harness design tools to support the simulation function. An electronic control unit (ECU) is a particularly important electrical system because it is connected to many other electrical systems through the wire harness and controls their operations. In this study, a method for modelling ECUs is developed, using the Capital Harness system design tool. The model parameters of the ECU are categorized into system parameters, external parameters, network parameters, and power parameters. The electrical structure of an ECU is built on the basis of the types and default states of the parameters. The behaviour of the ECU is defined in terms of simplified combinational logics in a state-machine-based structure since the target of the modelling is to simulate the logical connection conformity of the wire harness. A power-seat ECU model is developed, using the proposed method, and the feasibility of the simulation for wire harness validation is described.