310S Heating, Ventilation and Air Conditioning (HVAC) 2 — Questions and Answers
Question 1: When diagnosing an A/C system with manifold gauges, what should be done FIRST to ensure accurate readings?
- Recover all refrigerant from the system
- Run the engine and A/C system for 10 minutes to stabilize conditions (Correct answer)
- Check the system for leaks using an electronic detector
- Add refrigerant to bring pressures up to specification
Correct answer: Run the engine and A/C system for 10 minutes to stabilize conditions
Running the engine and A/C for 10 minutes allows the system to reach stable operating conditions, ensuring accurate and comparable manifold gauge readings.
Question 2: Both the high-side and low-side pressures are higher than normal specifications. This MOST likely indicates:
- A faulty compressor with worn reed valves
- Low refrigerant charge
- System overcharge or restricted condenser airflow (Correct answer)
- A blocked expansion device
Correct answer: System overcharge or restricted condenser airflow
High pressures on both sides indicate the condenser cannot dissipate heat efficiently due to blocked airflow, damaged fins, or the system is overcharged with refrigerant.
Question 3: Before opening an A/C system component for replacement, the technician MUST:
- Run the engine until refrigerant pressure drops to zero
- Recover the refrigerant using certified recovery equipment (Correct answer)
- Discharge refrigerant to atmosphere to relieve pressure safely
- Flush the system with nitrogen to clear the refrigerant
Correct answer: Recover the refrigerant using certified recovery equipment
Federal and provincial regulations require all refrigerant to be recovered using certified equipment before opening any A/C system component.
Question 4: After replacing an A/C system component, what is the main purpose of evacuating the system with a vacuum pump?
- To test the system for leaks before adding refrigerant
- To remove air and moisture from the system (Correct answer)
- To circulate oil throughout the new component
- To verify compressor operation before charging
Correct answer: To remove air and moisture from the system
Evacuation removes non-condensable air and moisture, which could cause corrosion, acid formation, and ice blockage in the expansion device.
Question 5: What vacuum level should be achieved when evacuating an A/C system to ensure adequate moisture removal?
- 25 in Hg held for 5 minutes
- 500 microns (0.5 mm Hg) or lower (Correct answer)
- Any vacuum that holds stable is acceptable
- 15 in Hg for a minimum of 30 minutes
Correct answer: 500 microns (0.5 mm Hg) or lower
Pulling the system to 500 microns or below causes moisture to boil off at ambient temperature and be removed by the vacuum pump, preventing contamination.
Question 6: An A/C compressor clutch that rapidly cycles on and off during operation is MOST likely caused by:
- An overcharged system activating the high-pressure cut-out switch
- Low refrigerant charge activating the low-pressure protection switch (Correct answer)
- A faulty blower motor causing the evaporator to freeze over
- Excessive compressor oil reducing system efficiency
Correct answer: Low refrigerant charge activating the low-pressure protection switch
Low refrigerant charge causes low suction pressure, repeatedly triggering the low-pressure protection switch and causing rapid compressor clutch cycling.
Question 7: What type of oil must be used when servicing an R-134a automotive A/C system compressor?
- Mineral oil
- Ester oil
- PAG (Polyalkylene Glycol) oil (Correct answer)
- ATF (Automatic Transmission Fluid)
Correct answer: PAG (Polyalkylene Glycol) oil
PAG oil is required for R-134a compressors because it is miscible with R-134a and provides proper lubrication; mineral oil used with R-12 is not compatible.
310S Heating, Ventilation and Air Conditioning (HVAC) 2
When diagnosing an A/C system with manifold gauges, what should be done FIRST to ensure accurate readings?