310T HVAC and Cab Systems 3 — Questions and Answers
Question 1: A truck's cab heater produces warm air only at idle but blows cool air at highway speeds. What is the most likely cause?
- Low coolant level causing air pockets at higher RPM (Correct answer)
- Faulty blower motor resistor
- Clogged cabin air filter
- Defective compressor clutch
Correct answer: Low coolant level causing air pockets at higher RPM
Low coolant can cause air pockets that disrupt heater core flow under the increased demand of highway operation.
Question 2: When recovering refrigerant from a 310T-compliant truck A/C system, the technician must:
- Vent the refrigerant outside the shop
- Use EPA-certified recovery equipment and store refrigerant in approved containers (Correct answer)
- Mix recovered refrigerant with new refrigerant before reuse
- Only recover refrigerant if the system pressure exceeds 200 psi
Correct answer: Use EPA-certified recovery equipment and store refrigerant in approved containers
EPA Section 608 regulations require certified recovery equipment and approved containers when servicing any stationary or mobile A/C system.
Question 3: A coach A/C system uses a thermostatic expansion valve (TXV). The evaporator is frosting over. What is the most probable cause?
- Overcharged system
- TXV stuck closed restricting refrigerant flow (Correct answer)
- Condenser fan running too fast
- High ambient temperature
Correct answer: TXV stuck closed restricting refrigerant flow
A TXV stuck closed starves the evaporator of refrigerant, lowering evaporator pressure and temperature below freezing, causing frost buildup.
Question 4: What does a sight glass with bubbles during A/C system operation typically indicate?
- The system is fully charged
- The system is low on refrigerant (Correct answer)
- The compressor is overloaded
- The condenser is blocked
Correct answer: The system is low on refrigerant
Bubbles or foam in the sight glass during normal operation indicate insufficient refrigerant charge in the system.
Question 5: A truck cab blower motor runs only on high speed. Which component is most likely faulty?
- Blower motor itself
- Blower motor resistor or speed control module (Correct answer)
- HVAC control head
- Evaporator temperature sensor
Correct answer: Blower motor resistor or speed control module
The blower resistor (or PWM control module) provides resistance for lower speeds; if it fails open, only the direct high-speed circuit works.
Question 6: On a coach with dual-zone HVAC, the rear zone blows warm air while the front zone cools correctly. What should the technician check first?
- Compressor output pressure
- Rear zone blend door actuator and control valve (Correct answer)
- Refrigerant charge level
- Condenser coil cleanliness
Correct answer: Rear zone blend door actuator and control valve
In dual-zone systems, independent blend door actuators control each zone; a stuck or failed actuator can prevent the rear zone from receiving cooled air.
Question 7: What is the purpose of a receiver-drier in a truck A/C system?
- To increase refrigerant pressure before the compressor
- To store liquid refrigerant and remove moisture and contaminants (Correct answer)
- To regulate evaporator pressure
- To cool refrigerant after it leaves the condenser
Correct answer: To store liquid refrigerant and remove moisture and contaminants
The receiver-drier stores a reserve of liquid refrigerant and contains desiccant to absorb moisture that could damage the system.
310T HVAC and Cab Systems 3
A truck's cab heater produces warm air only at idle but blows cool air at highway speeds.
What is the most likely cause?