R134a, a vehicle refrigerant used in the vehicle heat pump system, is regulated according to the Montreal Protocol. Refrigerants such as R1234yf, R744, and R290 in vehicle heat pump systems are being investigated to identify their alternatives. Because developing a new system exclusively for new refrigerants is costly, an empirical test was conducted on the R1234yf refrigerant in a heat pump system designed for the R134a refrigerant in an actual vehicle system. The heating, cooling, and battery-cooling modes were tested for the amount of refrigerant charge, and operability tests were conducted for the compressor load; heating, ventilation, air conditioning (HVAC) air flow rate; coolant temperature; and flow rate of each mode. The optimal refrigerant charge in heating mode was 0.7 kg, and the optimal refrigerant charge in the cooling and battery-cooling modes was 0.9 kg. To yield the highest coefficient of performance of the system, the compressor load was 50%, the HVAC fan was 12 V, and the coolant flow rate was 10 LPM. The most efficient system operation was possible at a coolant temperature of 30 °C in the cooling and heating modes and at 20 °C in battery-cooling mode.