Free 313D Troubleshooting and System Diagnosis Questions and Answers 1 — Questions and Answers
Question 1: A technician is called for a no-cool complaint and finds the suction pressure is unusually low, near 0 psig, and the head pressure is also significantly lower than normal. The suction line at the outdoor unit feels warm to the touch. What is the most likely cause?
- Failed condenser fan motor
- Non-condensables in the system
- Severe refrigerant leak or empty system (Correct answer)
- A major system overcharge
Correct answer: Severe refrigerant leak or empty system
When a system has lost most or all of its refrigerant, there is very little medium to absorb heat in the evaporator or reject it in the condenser. This results in extremely low pressures on both the high and low sides. The lack of refrigerant means no cooling is occurring, leading to a warm suction line.
Question 2: A technician is troubleshooting a compressor that hums but fails to start. After safely discharging the capacitors, a multimeter with a capacitance function is used to test the dual-run capacitor, which is rated for 45/5 MFD. The reading between the 'C' (Common) and 'HERM' (Hermetic) terminals is 22 MFD. What is the correct course of action?
- Replace the dual-run capacitor (Correct answer)
- Add a hard start kit to assist the compressor
- Replace the compressor contactor
- Check for a grounded compressor winding
Correct answer: Replace the dual-run capacitor
The 'HERM' terminal on a dual-run capacitor serves the compressor's start winding. The measured value of 22 MFD is significantly below the required 45 MFD, falling far outside the typical +/- 6% tolerance. This weak capacitor cannot provide the necessary phase shift and starting torque for the compressor motor, causing it to hum without starting. The capacitor has failed and must be replaced.
Question 3: During a maintenance check, a technician observes frost forming on the evaporator coil, and the suction line is sweating heavily all the way back to the compressor. Manifold gauge readings show normal head pressure but higher-than-normal suction pressure. Which of the following is the most probable cause?
- Low refrigerant charge
- A restricted liquid line filter drier
- A stuck-open metering device
- Poor indoor airflow across the evaporator (Correct answer)
Correct answer: Poor indoor airflow across the evaporator
Insufficient airflow over the evaporator coil, often caused by a clogged filter or dirty coil, reduces the amount of heat being absorbed by the refrigerant. This causes the refrigerant's saturation temperature to drop below freezing, leading to frost. The reduced heat load also means the refrigerant doesn't boil off as quickly, resulting in a higher-than-normal suction pressure and low superheat (flooding).
Question 4: A technician suspects that a system has been contaminated with non-condensables, such as air. Which of the following is the most reliable method to confirm this diagnosis?
- Observing an unusually high superheat reading at the evaporator outlet.
- Comparing the condenser's saturation temperature (from the gauge) to the ambient temperature after the system has been off for several hours. (Correct answer)
- Measuring a very low amp draw on the compressor while the unit is running.
- Noticing that the suction line is colder than usual.
Correct answer: Comparing the condenser's saturation temperature (from the gauge) to the ambient temperature after the system has been off for several hours.
After a system has been off long enough for its temperature to equalize with the surrounding air, the pressure inside should correspond to the saturation pressure of the pure refrigerant at that ambient temperature. Non-condensables exert their own partial pressure, resulting in a total system pressure that is higher than the refrigerant's saturation pressure for that ambient temperature. This discrepancy is a definitive sign of contamination.
Question 5: A customer reports their AC is blowing air, but it is not cool. The technician verifies the indoor blower and outdoor condenser fan are running, but the compressor is not. A distinct 'clicking' sound is heard from the outdoor unit every few minutes. The 'clicking' is most likely caused by which component?
- The thermostat cycling on and off
- A faulty condenser fan motor relay
- The compressor's internal overload protector (Correct answer)
- The main power disconnect
Correct answer: The compressor's internal overload protector
The symptoms describe a compressor that is attempting to start but failing, causing a high amperage draw and significant heat buildup. The 'clicking' sound is characteristic of the thermal overload protector opening the circuit to protect the motor. After a few minutes, it cools, resets ('clicks' closed), and the compressor tries to start again, repeating the cycle.
Question 6: While diagnosing a system with a TXV, a technician measures a consistently high superheat and low suction pressure. The subcooling measured at the condenser outlet is found to be normal to high. Which of the following is the most probable fault?
- Inefficient compressor valves
- The system is overcharged with refrigerant
- The TXV sensing bulb has lost its charge (Correct answer)
- The condenser coil is dirty and restricted
Correct answer: The TXV sensing bulb has lost its charge
If the TXV's sensing bulb loses its refrigerant charge, the opening force on the valve's diaphragm is lost. The valve's spring pressure will then overcome the evaporator pressure and force the valve to a nearly closed position. This starves the evaporator of refrigerant, causing low suction pressure and high superheat. The refrigerant that cannot enter the evaporator backs up in the condenser, leading to normal or high subcooling.
Free 313D Troubleshooting and System Diagnosis Questions and Answers 1
A technician is called for a no-cool complaint and finds the suction pressure is unusually low, near 0 psig, and the head pressure is also significantly lower than normal.
The suction line at the outdoor unit feels warm to the touch.
What is the most likely cause?