HVAC Systems
HVAC fundamentals
Air conditioning moves heat from the cabin to the outside. The compressor raises refrigerant pressure and temperature; the condenser rejects heat to outside air; the expansion valve or orifice tube drops pressure; the evaporator absorbs cabin heat. The accumulator or receiver-drier stores refrigerant and removes moisture. The heater uses engine coolant through the heater core, and blend doors control temperature.
Traps: gauge pressures depend on ambient temperature and airflow, so do not judge charge by one gauge alone. Overcharging raises both pressures and reduces cooling. A restricted condenser or inoperative cooling fan causes high head pressure. A failed blend door can cause no heat or no cooling even when the system is charged. Refrigerant must be recovered and recycled; venting is illegal. Use the specified oil and refrigerant; R-134a and R-1234yf are not interchangeable without conversion. Never open the system without PPE and proper training.
Rule of thumb: on a properly charged system, vent temperature is typically about 35-45°F at 60-80°F ambient, depending on humidity and vehicle.
Worked example: At 90°F ambient, vent temperature is 55°F. Low-side pressure is high and high-side pressure is high. Suspect overcharge, poor condenser airflow, or a failed cooling fan. Verify fan operation, clean the condenser, recover the charge, and recharge to the exact specification by weight.