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. The steady-state comparison demonstrates a 7.5% deviation of air-side COP compared to the experimental data. The (transient) pull-down performance comparison shows maximum of 2.4% and 1.8% deviations of accumulated compressor power and accumulated capacity compared to the experimental data, respectively. The cabin temperature profile predicted by the model during the pull-down condition is within 1K of experimental data. Finally the validated model was used to study the effects of coolant charge amount and glycol concentration on the cabin thermal comfort. It was concluded that the charge variation of up to 2 liters will not significantly affect the thermal comfort, while a less than 30% glycol concentration is preferred.