310T HVAC and Cab Systems 5 — Questions and Answers
Question 1: On a coach with an automatic temperature control (ATC) system, the cabin temperature does not reach the set point. After verifying refrigerant charge is correct, what should the technician check next?
- The compressor belt tension only
- ATC sensor calibration, blend door actuator operation, and control module inputs/outputs (Correct answer)
- The condenser coil fin spacing
- Refrigerant line insulation
Correct answer: ATC sensor calibration, blend door actuator operation, and control module inputs/outputs
ATC systems rely on accurate sensor feedback and properly functioning actuators; a miscalibrated sensor or failed actuator will prevent the system from reaching setpoint even with correct charge.
Question 2: When flushing a contaminated truck A/C system, what component must NOT be flushed and should be replaced instead?
- Condenser
- Receiver-drier or accumulator (Correct answer)
- Evaporator
- Liquid line
Correct answer: Receiver-drier or accumulator
Receiver-driers and accumulators contain desiccant that is destroyed by flushing solvents and must always be replaced when the system is opened or flushed.
Question 3: A truck cab window fogs repeatedly on the inside despite the defroster being on. What is the most likely HVAC-related cause?
- Excessively cold outside air entering the cab
- A leaking heater core introducing moisture into the cabin airstream (Correct answer)
- Overcharged A/C system
- Blocked evaporator drain tube
Correct answer: A leaking heater core introducing moisture into the cabin airstream
A leaking heater core introduces coolant steam into the cabin, significantly increasing humidity and causing persistent interior fogging that defrosting alone cannot overcome.
Question 4: What is the correct procedure for leak-testing a truck A/C system after repair?
- Charge with refrigerant and check with a sniffer only
- Pressure-test with dry nitrogen, then evacuate, then charge and verify with an electronic leak detector (Correct answer)
- Use compressed air to pressurize the system
- Check for leaks only at ambient temperature with no pressure
Correct answer: Pressure-test with dry nitrogen, then evacuate, then charge and verify with an electronic leak detector
Nitrogen pressure testing verifies mechanical integrity without introducing refrigerant, followed by evacuation to remove moisture and air before charging and final leak verification.
Question 5: A truck's cab air suspension seat is not adjusting properly and the air compressor runs continuously. What should the technician suspect?
- Faulty height control valve
- A leak in the seat air bladder or supply line causing the compressor to run without building pressure (Correct answer)
- Incorrect seat weight setting
- Failed HVAC blower motor affecting air supply
Correct answer: A leak in the seat air bladder or supply line causing the compressor to run without building pressure
A continuously running compressor that cannot build pressure indicates an air leak in the seat's bladder, lines, or fittings that prevents the system from reaching the target height.
Question 6: In a coach roof-mounted A/C unit, water is dripping into the passenger compartment instead of draining outside. What is the most likely cause?
- Overcharged refrigerant system
- Blocked or frozen evaporator condensate drain (Correct answer)
- Faulty condenser fan drawing moisture inside
- Cracked evaporator coil
Correct answer: Blocked or frozen evaporator condensate drain
Condensate from the evaporator normally drains outside through drain tubes; if these are blocked or frozen, water backs up and leaks into the coach interior.
Question 7: A 310T technician is evacuating a truck A/C system before recharging. The system will not pull down below 500 microns. What does this indicate?
- The vacuum pump is working correctly and the system is ready to charge
- A leak in the system or vacuum pump, or moisture present, preventing a deep vacuum (Correct answer)
- The refrigerant is still partially in the system
- The system requires R-134a instead of HFO-1234yf
Correct answer: A leak in the system or vacuum pump, or moisture present, preventing a deep vacuum
Inability to reach deep vacuum (below 500 microns) indicates a leak that allows air ingress, a faulty vacuum pump, or moisture that keeps vaporizing and raising pressure.
310T HVAC and Cab Systems 5
On a coach with an automatic temperature control (ATC) system, the cabin temperature does not reach the set point.
After verifying refrigerant charge is correct, what should the technician check next?