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As a potential replacement to traditional automotive R134a direct expansion (DX) systems, a secondary-loop system allows for the usage of flammable but low-GWP refrigerants such as propane (R290). However, as the secondary-loop system has an additional layer of thermal resistance, the cycle's transient behavior and cabin thermal comfort during pull-down and various driving cycles may be different from traditional DX systems. This paper presents a Modelica-based model to simulate both steady-state and transient operation of automotive secondary-loop systems. The model includes a lumped cabin component and a secondary-loop automotive air-conditioning system component. The air-conditioning system component consists of a condenser, a compressor, an expansion device, a coolant plate type heat exchanger, a coolant to air heat exchanger and a coolant pump. The developed model was validated against both steady-state and transient experimental data for an R290 secondary-loop system.