CRST Troubleshooting & Repair Techniques 2 — Questions and Answers
Question 1: A refrigeration system is short-cycling on the low-pressure control. What is the MOST likely cause?
- Refrigerant overcharge
- Low refrigerant charge (Correct answer)
- Condenser fan motor failure
- Plugged liquid line filter-drier
Correct answer: Low refrigerant charge
A low refrigerant charge causes suction pressure to drop below the low-pressure control cutout setpoint, causing repeated short cycling.
Question 2: A technician measures a 15°F superheat at the evaporator outlet and 80°F subcooling at the condenser outlet. What condition does this indicate?
- Normal operation
- Refrigerant overcharge (Correct answer)
- TXV stuck open
- Evaporator fan failure
Correct answer: Refrigerant overcharge
Excessive subcooling combined with normal or low superheat is a classic sign of refrigerant overcharge in the system.
Question 3: When checking a hermetic compressor for an open motor winding, which instrument should be used?
- Manometer
- Clamp-on ammeter
- Ohmmeter (Correct answer)
- Megohmmeter
Correct answer: Ohmmeter
An ohmmeter measures resistance between terminals to identify an open winding, which shows infinite or very high resistance.
Question 4: A walk-in cooler is not maintaining setpoint even though the compressor runs continuously. The evaporator coil is heavily frosted. What is the FIRST action to take?
- Add refrigerant
- Replace the thermostat
- Defrost the evaporator coil (Correct answer)
- Check the condenser fan motor
Correct answer: Defrost the evaporator coil
Heavy frost blocks airflow across the evaporator, reducing heat transfer efficiency; defrosting restores normal operation before further diagnosis.
Question 5: Oil logging in the evaporator of a low-temperature system is MOST likely caused by:
- High discharge pressure
- Insufficient refrigerant velocity to return oil (Correct answer)
- Overcharge of refrigerant
- Excessive superheat setting
Correct answer: Insufficient refrigerant velocity to return oil
At low temperatures, refrigerant vapor density decreases, and if velocity is too low, oil cannot be carried back to the compressor crankcase.
Question 6: A system has high head pressure and low suction pressure. The superheat is normal and subcooling is low. What is the MOST probable cause?
- Refrigerant overcharge
- Non-condensables in the system
- Restricted metering device (Correct answer)
- Condenser coil fouled
Correct answer: Restricted metering device
A restricted metering device causes high head pressure upstream and starves the evaporator, resulting in low suction pressure and low subcooling.
Question 7: A technician finds the compressor discharge temperature is 280°F. What is the MOST likely cause?
- Low superheat
- High subcooling
- High superheat due to low suction pressure (Correct answer)
- Condenser flooding
Correct answer: High superheat due to low suction pressure
Excessive superheat entering the compressor raises compression temperature dramatically, often caused by low refrigerant or high evaporator load.
CRST Troubleshooting & Repair Techniques 2
A refrigeration system is short-cycling on the low-pressure control.
What is the MOST likely cause?