Heating, Ventilation, and Air Conditioning
Scope
The G1 covers A/C inspection and routine service, heater performance, and air distribution. You must know the refrigerant circuit: compressor, condenser, receiver-drier or accumulator, expansion device, evaporator, and the controls that move air.
Concepts you must own
- Refrigerants: R-134a and R-1234yf are not interchangeable. R-1234yf is mildly flammable and needs dedicated recovery equipment and fittings. Never vent refrigerant to atmosphere.
- Manifold gauge interpretation pairs with ambient temperature. Both gauges low suggests an undercharge; both high suggests overcharge, poor condenser airflow, or an inoperative fan; high side very high with a low or normal low side suggests a restriction.
- Expansion devices: an orifice tube is a fixed restriction; a thermal expansion valve modulates. Restrictions show as a temperature difference across the component.
- Heater performance depends on coolant flow, blend door operation, and a properly burped cooling system.
- Cabin air filter restriction reduces airflow without changing vent temperature.
- Leak detection: UV dye, electronic sniffers, and pressure testing after recovery.
Traps
A/C that cools poorly at idle but well at speed usually points to condenser airflow, not charge. A compressor that will not engage may be disabled by a low-pressure switch, a bad clutch, or a control module command; verify the command before replacing the compressor. Do not assume a warm blend door is a refrigerant fault; check coolant level and heater hoses first.
Worked example
At 30 C ambient, a system shows low side 22 psi and high side 180 psi with poor cooling. Both gauges run low, so recover, evacuate, and recharge by weight to the underhood label rather than guessing, then verify pressures and vent temperature.
Rule of thumb
Charge by weight, never by gauge watching; approximate healthy R-134a readings near 30 C ambient are roughly 30 to 45 psi low side and 200 to 280 psi high side.