HVAC System Inspection, Diagnosis, Testing, and Repair
From complaint to verified repair
Begin with a performance test: engine at operating temperature, doors open, blower on high, max A/C, recirculation on, thermometer in the center vent. Record ambient temperature, humidity, and vent temperature. Then connect gauges and compare against a pressure-temperature chart. R-134a at 80 F has about 88 psi static. Running low side is typically 25-45 psi, and high side roughly 2.2-2.8 times ambient in degrees F, varying with conditions.
Interpret patterns. Both gauges low suggests low charge, a restriction, or a weak compressor. Low side high with high side high suggests overcharge, poor condenser airflow, or wrong refrigerant. Very high high side with normal low side points to condenser airflow. Low side in vacuum means a restriction. Fast cycling suggests low charge or a pressure switch.
Leak detection combines electronic detectors, UV dye, soap bubbles, and a look at the evaporator drain and service ports. After repair, evacuate with a micron gauge, hold vacuum, charge by weight, add oil as needed, and repeat the performance test.
Traps
Charging to make gauges look right without finding the leak; ignoring airflow; trusting a sight glass on R-134a; skipping the vacuum hold; leaving the blend door in the wrong position during a temperature test; and condemning a compressor without checking clutch, mounts, and charge.
Rule of thumb: if vent temperature is 30-40 F below ambient at 80 F and both pressures are in range, the refrigeration side is probably fine. Look at blend doors, heater control, and airflow instead.