310T HVAC and Cab Systems Questions and Answers — Questions and Answers
Question 1: A technician connects a manifold gauge set to a truck's A/C system and finds the static pressure (engine off) is 30 PSI on both high and low sides, with an ambient temperature of 25°C (77°F). When the A/C is turned on, the compressor clutch does not engage. What is the most likely cause?
- A faulty compressor clutch relay.
- An overcharged refrigerant system.
- A significant refrigerant leak. (Correct answer)
- A clogged thermal expansion valve (TXV).
Correct answer: A significant refrigerant leak.
A static pressure of 30 PSI is far too low for an ambient temperature of 25°C, where it should be approximately 70-80 PSI. This indicates that most of the refrigerant has leaked out. The system's low-pressure cutout switch will prevent the compressor clutch from engaging to protect the compressor from damage due to lack of lubrication.
Question 2: Which of the following is a key characteristic of R-1234yf refrigerant compared to the older R-134a refrigerant?
- It has a significantly higher Global Warming Potential (GWP).
- It is classified as mildly flammable and requires specific service equipment. (Correct answer)
- It is a direct drop-in replacement for R-134a systems without modification.
- It operates at much higher pressures than R-134a.
Correct answer: It is classified as mildly flammable and requires specific service equipment.
R-1234yf was introduced to replace R-134a due to environmental concerns. Its primary benefit is a very low Global Warming Potential (GWP) of around 4, compared to R-134a's GWP of about 1430. However, R-1234yf is classified as an A2L refrigerant, meaning it is mildly flammable and requires technicians to use specific SAE-certified equipment for recovery, recycling, and charging to handle it safely.
Question 3: A driver complains of no heat from the vents. The technician confirms the engine reaches normal operating temperature and the blower motor functions correctly on all speeds. Which of the following is the LEAST likely cause of this problem?
- A heater control valve that is stuck closed.
- A blend door actuator that has failed in the cold position.
- A faulty blower motor resistor. (Correct answer)
- A completely clogged heater core.
Correct answer: A faulty blower motor resistor.
The question states that the blower motor functions correctly on all speeds. A faulty blower motor resistor would typically cause the blower to work only on the highest speed setting or not at all on lower speeds. Since the fan is working properly, the resistor is not the cause. A stuck heater valve, failed blend door, or clogged core are all plausible reasons why hot coolant or heated air cannot reach the cabin vents.
Question 4: What is the primary function of a receiver/drier in a commercial vehicle A/C system equipped with a thermal expansion valve (TXV)?
- To meter refrigerant into the evaporator based on heat load.
- To store liquid refrigerant and remove moisture using a desiccant. (Correct answer)
- To protect the compressor from liquid refrigerant slugging.
- To cycle the compressor on and off based on system pressure.
Correct answer: To store liquid refrigerant and remove moisture using a desiccant.
The receiver/drier is located on the high-pressure liquid line between the condenser and the expansion valve. Its name describes its two main functions: it acts as a storage vessel (receiver) for liquid refrigerant and contains a desiccant bag to absorb any moisture (drier) that may have entered the system. Metering is done by the TXV, and protecting the compressor from liquid is the function of an accumulator (used in orifice tube systems).
Question 5: A driver reports a persistent whistling noise coming from the top of the passenger-side window at highway speeds. The window appears to be fully closed. Which of the following is the most effective and logical first diagnostic step?
- Immediately replace the door's weatherstripping.
- Use a smoke machine to pressurize the cab and look for leaks.
- Apply masking tape over the exterior window and door seals, then road test. (Correct answer)
- Disassemble the door panel to inspect the window regulator alignment.
Correct answer: Apply masking tape over the exterior window and door seals, then road test.
Applying masking tape over the suspected leak areas is a simple, non-invasive, and highly effective way to pinpoint the exact source of wind noise. By taping a section and road testing, a technician can confirm if the noise disappears. If it does, the leak is under that piece of tape. This allows for accurate diagnosis before replacing parts or performing more complex procedures like pressurizing the cab or disassembling the door.
310T HVAC and Cab Systems Questions and Answers
A technician connects a manifold gauge set to a truck's A/C system and finds the static pressure (engine off) is 30 PSI on both high and low sides, with an ambient temperature of 25°C (77°F).
When the A/C is turned on, the compressor clutch does not engage.
What is the most likely cause?