Air Conditioning Service & Diagnostics
System charging, superheat/subcooling diagnostics, pressure analysis, coil icing, short cycling, and matched systems.
- Diagnose system problems using suction pressure, head pressure, superheat, and subcooling patterns
- Identify common causes of high and low system pressures
- Explain the causes and solutions for evaporator coil icing
- Describe the function of accumulators, crankcase heaters, and low-ambient controls
- Explain why matched systems are important for efficiency and reliability
Leçon 1
System Pressures & Diagnostic Patterns
Reading System Pressures
The manifold gauge set provides two critical readings that form the foundation of refrigeration diagnostics:
- Suction pressure (low side) - reflects evaporator conditions
- Head pressure (high side) - reflects condenser conditions
Normal operating pressures depend on the refrigerant type, ambient temperature, and indoor conditions. For R-410A at a typical 95 degrees F outdoor / 75 degrees F indoor condition:
Pressure Diagnostic Patterns
Low Suction / Low Head
Cause: Undercharge or low load
Refrigerant leak depletes charge
Both pressures drop together
High superheat, low subcooling
High Suction / High Head
Cause: Overcharge or compressor issue
Too much refrigerant in system
Both pressures elevated
Low superheat, high subcooling
Normal Suction / High Head
Cause: Dirty condenser or airflow issue
Condenser cannot reject heat
High condensing temperature
High subcooling possible
Low Suction / Normal Head
Cause: Low indoor airflow or metering restriction
Evaporator cannot absorb heat
Possible coil icing
High superheat
High-Pressure Safety Switch
The high-pressure switch opens the circuit to the compressor if discharge/head pressure exceeds a safe limit. This protects the compressor from damage due to excessive pressure. Common causes of high-pressure switch trips include dirty condenser coils, blocked condenser airflow, overcharge, or non-condensables in the system.
Low suction / low head pressures typically indicate undercharge or low load. Normal suction / high head indicates a condenser problem (dirty coil or airflow restriction). The high-pressure switch protects the compressor by opening the circuit when head pressure exceeds safe limits.