HVAC and Cab Systems Flashcards
7 cards from real 310T practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 HVAC and Cab Systems flashcards as text
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?
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.
Which refrigerant is currently approved for use in heavy-duty truck and coach A/C systems in the United States?
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.
During cab pressurization testing on a coach, excessive cab pressure drop indicates:
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.
What causes a compressor to short-cycle (turn on and off rapidly) in a truck A/C system?
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.
A technician finds oil residue around an A/C hose fitting on a truck. This most likely indicates:
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.
What is the function of the fresh air/recirculation door in a truck cab HVAC system?
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.
A truck heater core is leaking coolant into the cab. In addition to replacing the heater core, what else must the technician do?
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.