System Diagnosis, Leak Detection and Fault Finding
System diagnosis, leak detection and fault finding
This domain tests systematic fault finding rather than parts guessing. It covers complaint analysis, pressure-temperature interpretation, superheat and subcooling, airflow checks and leak detection methods.
Core rules
- Confirm the complaint and check history, visual condition, drive belt, cooling fans, cabin filter, blower and blend doors before condemning the refrigeration circuit.
- Compare running pressures with a pressure-temperature chart for the actual refrigerant. Static pressure approximates saturation pressure at ambient. Low-side and high-side readings must be read together with vent temperature, ambient temperature and airflow.
- Superheat at the evaporator outlet and subcooling at the condenser outlet help identify undercharge, restriction and condenser performance.
- Leak detection: electronic detector, UV dye, bubbles, ultrasonic and nitrogen pressure or vacuum decay. Check service ports, Schrader valves, hose crimps, condenser, evaporator, O-rings and compressor seals. Use equipment suitable for A2L refrigerants.
Worked example
An R134a vehicle at 30 C ambient has vent temperature 16 C, low side 4.5 barg and high side 9 barg, with the clutch engaged and a noisy compressor. A healthy system would normally show a much higher high-side pressure. Both sides being low, with poor performance and noise, points to a weak compressor rather than a simple undercharge. Adding refrigerant would not fix the root cause.
Traps
- Diagnosing from pressures alone.
- Adding refrigerant when the high side is low.
- Ignoring airflow, blend doors and cabin filter.
- Using a leak detector not rated for the refrigerant.
- Missing service-port and Schrader leaks.