Free Air Conditioning Repair System Diagnosis and Troubleshooting Questions and Answers — Questions and Answers
Question 1: A technician connects gauges to an operating air conditioning system and observes that both the suction (low-side) and head (high-side) pressures are significantly lower than normal. Which of the following is the most likely cause?
- A dirty condenser coil
- A severe refrigerant overcharge
- Non-condensables in the system
- A low refrigerant charge (Correct answer)
Correct answer: A low refrigerant charge
A low refrigerant charge is the most common cause for both low-side and high-side pressures to be low. There is not enough refrigerant mass flowing through the system to absorb a normal amount of heat in the evaporator (resulting in low suction pressure) and subsequently reject that heat in the condenser (resulting in low head pressure).
Question 2: When diagnosing a system with a thermostatic expansion valve (TXV), a technician measures a high condenser subcooling and a normal to low superheat. What is the most likely diagnosis?
- Refrigerant overcharge (Correct answer)
- Failed compressor valves
- Low refrigerant charge
- Restriction in the liquid line
Correct answer: Refrigerant overcharge
A refrigerant overcharge will cause excess refrigerant to back up in the condenser, leading to a high subcooling reading. [3, 8] This surplus refrigerant can also flood the evaporator, causing the TXV to close down but still resulting in a superheat that is normal or lower than specified. [9, 10]
Question 3: A technician is troubleshooting a compressor that hums for a few seconds and then trips the thermal overload. After safely disconnecting power and discharging the capacitor, a multimeter check shows the capacitor's microfarad (µF) reading is well below its rating. What is the most appropriate next step?
- Ohm out the compressor windings.
- Replace the run capacitor. (Correct answer)
- Add refrigerant to the system.
- Replace the contactor.
Correct answer: Replace the run capacitor.
The symptoms of humming followed by tripping on overload are classic signs of a failed run capacitor. [20, 24, 29] The capacitor provides the necessary phase-shifted voltage to create starting torque for the compressor motor. Since the capacitor test confirmed it is faulty, the next logical step is to replace it.
Question 4: A customer with a heat pump complains that it is blowing warm air when the thermostat is set to 'cool'. A technician on-site confirms the outdoor unit is running, but the larger, insulated refrigerant line (the suction line) is hot. Which of the following is the most probable cause?
- A failed condenser fan motor
- An undercharged system
- A reversing valve stuck in heat mode (Correct answer)
- A clogged filter-drier
Correct answer: A reversing valve stuck in heat mode
When a heat pump is stuck in heating mode, the reversing valve fails to switch the flow of refrigerant. [2] The system operates as if it's heating, making the outdoor coil the evaporator and the indoor coil the condenser. This causes the suction line, which is connected to the outdoor coil, to become a hot gas line, resulting in warm air being delivered indoors despite the call for cooling. [15, 16]
Question 5: To accurately evaluate total system airflow and diagnose potential restrictions in ductwork, which of the following measurements is most essential?
- Temperature drop across the evaporator coil
- Velocity pressure in the return duct
- Amperage draw of the indoor blower motor
- Total external static pressure (Correct answer)
Correct answer: Total external static pressure
Total external static pressure is the measurement of resistance (pressure) the blower must work against in both the supply and return ducts. A reading higher than the manufacturer's specification indicates a restriction to airflow, such as a dirty filter, clogged coil, or undersized ducts.
Question 6: A technician uses a megohmmeter to test a compressor's windings to ground. The meter displays a reading of 0.5 MΩ (megohms). What does this result indicate?
- The compressor windings are open.
- The compressor's crankcase heater is working.
- The motor insulation is breaking down and shorted to ground. (Correct answer)
- The compressor is acceptable for service.
Correct answer: The motor insulation is breaking down and shorted to ground.
A megohmmeter checks for high electrical resistance between the motor windings and the compressor casing (ground). A very high reading (typically over 100 MΩ) is considered good. [13] A low reading, such as 0.5 MΩ, indicates that the winding's insulation has failed, allowing electrical current to leak to ground, which is a dangerous fault. [14, 23]
Free Air Conditioning Repair System Diagnosis and Troubleshooting Questions and Answers
A technician connects gauges to an operating air conditioning system and observes that both the suction (low-side) and head (high-side) pressures are significantly lower than normal.
Which of the following is the most likely cause?