System Diagnosis and Performance Testing
Core concepts
Performance testing ties everything together: measure ambient temperature and humidity, engine/compressor speed, vent temperature, and both gauge pressures, then compare to a performance chart. Understand superheat and subcooling as the primary charge/restriction indicators, and how ambient conditions change expected pressures. Know the sequence: verify complaint, visual inspection, performance test, then targeted diagnosis. Recognize that bus systems with multiple evaporators require checking each circuit.
Diagnostic method
Use a logical flow: confirm controls and airflow first, then electrical (clutch command, sensors), then mechanical (compressor, expansion device), then charge. Recovery/recharge only after ruling out airflow, electrical, and restriction faults. Understand pressure-pattern diagnosis: both low → undercharge/leak/restriction; both high → overcharge/airflow/overcharge or non-condensables; high low-side with low high-side → compressor wear. Leak detection (electronic, dye, pressure) matters because slow leaks are common.
Traps and a worked rule
Traps: charging by gauge alone without superheat/subcooling; skipping airflow checks; and misreading normal pressures under high ambient/humidity as a fault. Rule of thumb: vent temperature should be well below ambient (often 30–40°F below) at steady state; if not, check airflow and charge before condemning components.
Worked example
Vent air only 15°F below ambient, low side high, high side low, clutch engaged. Pattern suggests compressor wear or internal leak - verify with superheat and a cylinder/performance test before replacement.