Diagnosing AC Concerns

An undercharged or overcharged system mimics almost every other AC fault. Connect your gauge set. Read the pressures. Compare to specifications for the ambient temperature. If the system is low — find the leak before adding refrigerant. Adding refrigerant to a leaking system is not a repair.

The most common reason the compressor clutch does not engage is low refrigerant charge. The low-pressure switch prevents the compressor from running when the system is undercharged to protect it from running without lubrication. Check charge first. If charge is correct — check for voltage at the compressor clutch connector. No voltage — trace the control circuit through the relay, pressure switches, and PCM command.

Both sides high — the condenser cannot reject heat. Check fan operation and condenser airflow. Both sides low — system is undercharged. Low refrigerant from a leak. Low side high and high side low with pressures equalizing — the compressor is not pumping. Low side very low and high side very high — restriction between the compressor output and the evaporator input. Feel the high-side line — the temperature changes dramatically at the restriction point.

Verify the charge is correct first. If charge is correct — check condenser airflow, check for a blend door that is not fully closing to max cool, check cabin air filter for restriction reducing airflow through the evaporator. On vehicles with dual-zone climate, one side may blow warm if the blend door actuator on that side has failed.