Refrigerant System Diagnosis and Repair
Reading pressures
Diagnosis starts with static and running pressure readings compared to ambient temperature, plus vent temperature and clutch behavior. Low side too low and high side too low points to an undercharge or restriction; both sides too high with poor cooling points to overcharge, airflow loss at the condenser, or a weak compressor. A restriction (often at the expansion device or a kinked line) shows a large temperature drop across the restriction and low suction.
Leak and charge service
Use electronic leak detection, dye, or pressure testing; never rely on a single method. Evacuate to a deep vacuum and hold it to confirm no leaks and no moisture before charging. Charge by weight whenever possible; sight-glass bubbles are not a reliable charge indicator on all systems. Confirm the correct refrigerant - R-134a versus R-1234yf - and never mix them. Verify oil type and quantity when replacing components.
Traps
Questions test that a compressor that will not engage may be a pressure-switch, clutch-gap, or control issue rather than a failed compressor. Another trap: high-side pressure that climbs only at idle suggests condenser airflow, not charge. Do not overlook that some systems use a variable-displacement compressor controlled by a valve, so clutch cycling is not expected.
Worked example
Both gauges read low, vents are cool at speed but warm at idle, and the clutch cycles rapidly. That pattern fits a low charge: add refrigerant by weight to spec, then recheck pressures at idle and at 1,500 rpm before condemning parts.