A multidisciplinary toolset integrating ANSYS FLUENT CFD solver and GM in-house thermal system design tool - e-Thermal has been developed to design automotive HVAC systems. This toolset utilizes COM software interface standard of MS Windows for inter-process communication at simulation run-time to synchronize the two applications and to exchange data. In this report, first, the implementation of this fully transient, coupled method between FLUENT CFD and e-Thermal is introduced. We then apply this integrated tool to simulate a transient A/C operating cycle including hot-soak and cool-down of a cabin. The coupled simulation consists of an A/C and an Air-Handling (HVAC module) system models, and a cabin CFD model. It demonstrates that the coupled method can simulate fully transient HVAC system operations in a vehicle. This coupled method enables right-sizing HVAC systems against realistic transient operations instead of peak operations to reduce compressor load, thus to achieve overall fuel economy improvement. On the other hand, the CFD model in this report is part of a coupled system simulation involving multiple 1-D or 3-D modeling components. A rapid 3-D coarse CFD calculation is employed based on simplified cabin geometry to predict overall temperature distribution. In addition to transient mass, momentum and energy solution, it also solves for transient species transport of dry air and water vapor mixture for the humidity effect of the cabin air. The CFD prediction of the peak air temperature during the hot-soak is validated against a published test result.