310T HVAC and Cab Systems 4 — Questions and Answers
Question 1: A 310T technician measures A/C high-side pressure of 350 psi and low-side pressure of 55 psi on a hot day. What does this indicate?
- Normal system operation
- Overcharged system or condenser airflow restriction (Correct answer)
- Undercharged system
- Compressor valve failure
Correct answer: Overcharged system or condenser airflow restriction
Abnormally high high-side pressure with elevated low-side suggests excess refrigerant or heat not being dissipated, often from a blocked or dirty condenser.
Question 2: Which refrigerant is currently approved for use in heavy-duty truck and coach A/C systems in the United States?
- R-12
- R-134a or HFO-1234yf depending on system design (Correct answer)
- R-22
- R-410A
Correct answer: R-134a or HFO-1234yf depending on system design
R-134a has been the standard for heavy-duty mobile A/C, with HFO-1234yf being adopted in newer systems as regulations tighten on global warming potential.
Question 3: During cab pressurization testing on a coach, excessive cab pressure drop indicates:
- The HVAC blower is too powerful
- Air leaks in door seals, windows, or cab penetrations (Correct answer)
- A faulty fresh air damper
- Normal operation during highway driving
Correct answer: Air leaks in door seals, windows, or cab penetrations
Rapid pressure loss during a cab pressurization test points to unsealed gaps where pressurized air escapes, compromising dust and noise isolation.
Question 4: What causes a compressor to short-cycle (turn on and off rapidly) in a truck A/C system?
- Dirty evaporator coil
- Low refrigerant charge causing the low-pressure cutout switch to trip repeatedly (Correct answer)
- Excessive cabin temperature
- Faulty condenser fan motor
Correct answer: Low refrigerant charge causing the low-pressure cutout switch to trip repeatedly
Low refrigerant causes suction pressure to drop below the low-pressure switch threshold, cycling the compressor off; when pressure recovers slightly, it cycles on again.
Question 5: A technician finds oil residue around an A/C hose fitting on a truck. This most likely indicates:
- Normal compressor lubrication overflow
- A refrigerant leak at that fitting, since refrigerant oil escapes with refrigerant (Correct answer)
- Condensation from the evaporator
- Engine oil contaminating the A/C circuit
Correct answer: A refrigerant leak at that fitting, since refrigerant oil escapes with refrigerant
Refrigerant carries compressor oil with it; oil residue at a joint is a reliable indicator of a refrigerant leak at that location.
Question 6: What is the function of the fresh air/recirculation door in a truck cab HVAC system?
- To control coolant flow through the heater core
- To select between drawing outside air or recirculating cabin air through the HVAC system (Correct answer)
- To regulate refrigerant flow to the evaporator
- To open and close the condenser airflow path
Correct answer: To select between drawing outside air or recirculating cabin air through the HVAC system
The fresh air/recirculation door controls the air source, allowing the driver to recirculate cabin air for faster cooling or defog, or draw in outside air.
Question 7: A truck heater core is leaking coolant into the cab. In addition to replacing the heater core, what else must the technician do?
- Replace the entire HVAC box assembly
- Flush the cooling system, refill with correct coolant mixture, and check for other leaks (Correct answer)
- Replace the thermostat and water pump
- Recharge the A/C system with additional refrigerant
Correct answer: Flush the cooling system, refill with correct coolant mixture, and check for other leaks
After replacing a leaking heater core, the system must be flushed to remove any debris or scale, refilled with the correct coolant mixture, and bled of air.
310T HVAC and Cab Systems 4
A 310T technician measures A/C high-side pressure of 350 psi and low-side pressure of 55 psi on a hot day.
What does this indicate?