313D Troubleshooting and System Diagnosis 3 — Questions and Answers
Question 1: A technician measures a compressor drawing high amperage with low suction and high discharge pressure. What is the most likely diagnosis?
- Refrigerant overcharge (Correct answer)
- Failed condenser fan motor
- Liquid slugging
- Refrigerant undercharge
Correct answer: Refrigerant overcharge
Refrigerant overcharge increases system pressures and compressor workload, resulting in elevated amperage draw.
Question 2: A homeowner complains of short cycling. The technician measures that the compressor runs for less than 2 minutes per cycle. Which component should be tested first?
- Capacitor and contactor
- Thermostat anticipator or setting
- High and low pressure safety switches (Correct answer)
- Condenser coil cleanliness
Correct answer: High and low pressure safety switches
Short cycling is frequently caused by pressure switches that trip due to abnormal operating pressures, making them the first item to test.
Question 3: A technician notices the system cools adequately in the morning but loses capacity significantly in the afternoon heat. Which diagnosis is most consistent with this behavior?
- Dirty evaporator coil
- Intermittent thermostat fault
- Dirty or blocked condenser coil (Correct answer)
- Low refrigerant charge
Correct answer: Dirty or blocked condenser coil
A partially blocked condenser coil can handle lighter loads in cooler morning temperatures but cannot reject adequate heat when ambient temperatures peak in the afternoon.
Question 4: While troubleshooting a residential system, a technician finds oil stains near the compressor service valves. What does this indicate?
- Normal compressor lubrication weeping
- A refrigerant and oil leak at that location (Correct answer)
- Condenser coil corrosion
- Excess refrigerant charge
Correct answer: A refrigerant and oil leak at that location
Oil stains near refrigerant connections indicate a refrigerant leak, since oil travels with refrigerant and deposits at leak points.
Question 5: What is the purpose of measuring the temperature difference across a filter-drier (liquid line drier) during operation?
- To verify proper superheat
- To detect a restriction inside the drier (Correct answer)
- To confirm correct refrigerant charge
- To measure subcooling accurately
Correct answer: To detect a restriction inside the drier
A notable temperature drop across a filter-drier (more than 1–2°F) indicates the drier is restricted and should be replaced.
Question 6: A technician is troubleshooting a system with intermittent cooling. The contactor closes normally but the compressor hums and does not start. What is the most likely cause?
- Open high-pressure switch
- Failed run capacitor (Correct answer)
- Stuck reversing valve
- Defective thermostat
Correct answer: Failed run capacitor
A failed or weak run capacitor prevents the compressor from developing enough starting torque, causing it to hum without starting.
Question 7: When a residential air conditioner produces adequate cooling but the homeowner reports unusually high humidity indoors, what should the technician evaluate?
- Refrigerant charge level
- Evaporator coil temperature and airflow rate (Correct answer)
- Condenser fan blade pitch
- Compressor valve efficiency
Correct answer: Evaporator coil temperature and airflow rate
Excessive airflow or elevated evaporator coil temperature reduces the system's ability to condense and drain moisture from the air.
313D Troubleshooting and System Diagnosis 3
A technician measures a compressor drawing high amperage with low suction and high discharge pressure.
What is the most likely diagnosis?