HVAC System Components and Operation
The loop, in order
A truck A/C system is a sealed loop with four states of refrigerant. The compressor raises pressure and temperature, pushing high-pressure gas to the condenser, where airflow removes heat and the refrigerant condenses to liquid. Liquid passes the receiver-drier (or accumulator on orifice-tube/cycling-clutch designs), which stores refrigerant and removes moisture, then through the expansion device - a TXV or an orifice tube - where pressure drops sharply and the refrigerant flashes to a cold low-pressure mix. In the evaporator, that mixture absorbs heat from cab air and boils back to vapor before returning to the compressor.
Know the control side too: the clutch engages the compressor, the low- and high-pressure switches protect the loop, the blower moves air across the evaporator, and the blend door mixes heated and cooled air. A cycling-clutch orifice-tube system uses an accumulator and a low-pressure cycling switch; a TXV system uses a receiver-drier and regulates by superheat.
Traps
Questions love the difference between receiver-drier and accumulator, and between TXV and orifice tube. Another trap: which component is high-pressure liquid versus low-pressure liquid. Remember the expansion device is the pressure boundary. Also know that a condenser mounted ahead of the radiator depends on airflow and fan operation - a weak fan can look like a charge problem.
Worked example
If the suction line is warm and the vent air is cool but not cold, with normal high-side pressure, suspect an undercharge or a blend door not fully closing. Rule of thumb: hot gas off the compressor, warm liquid into the expansion device, cold vapor back to the compressor - trace the loop and the fault localizes quickly.