Heating and Air Conditioning A7 Flashcards

A technician is checking refrigerant type before servicing a vehicle's A/C system. The system identifier shows the refrigerant is contaminated with air (non-condensable gases). How does air contamination affect A/C system operation?

Answer: Air raises high-side pressure and reduces cooling efficiency without being detectable by smell or color

Air is a non-condensable gas — it does not condense at normal A/C operating temperatures and pressures. Air accumulates in the high side of the system, raising high-side pressure above normal for the ambient temperature. This elevated pressure causes the compressor to work harder and reduces system efficiency.

An A/C system has normal pressures when tested with a manifold gauge set but produces insufficient cooling. The evaporator is cold and the refrigerant charge is correct. What should be checked NEXT?

Answer: The blend door actuator and mode door function in the HVAC box

When the refrigerant system pressures and charge are correct and the evaporator is cold (confirmed cooling is occurring), the problem is in the air distribution system, not the refrigerant circuit. A stuck or mispositioned blend door or mode door prevents cold air from reaching the cabin.

R-1234yf refrigerant is used in late-model vehicles. What is the PRIMARY reason the industry changed from R-134a to R-1234yf?

Answer: R-1234yf has a much lower global warming potential (GWP) than R-134a

R-1234yf was adopted as a replacement for R-134a primarily due to its dramatically lower Global Warming Potential. R-134a has a GWP of 1,430 (1,430 times more potent than CO2 as a greenhouse gas over 100 years), while R-1234yf has a GWP of approximately 4, satisfying new environmental regulations.

A technician performing A/C service must recover refrigerant from a system before disassembly. The refrigerant recovery machine stops before the system is fully recovered, showing only partial vacuum. What is the MOST likely reason?

Answer: The recovery tank is full and must be replaced or emptied before continuing

Recovery machines stop when the recovery tank reaches its maximum safe filling level, which is a safety requirement. A full recovery tank cannot accept more refrigerant and the machine shuts off or stops pumping. The tank must be replaced with an empty tank or have its contents transferred before recovery can be completed.

After an A/C system repair and refrigerant charge, the system is tested and the suction line becomes very cold (frosted) between the evaporator and compressor. What does this indicate?

Answer: The expansion valve is stuck open, flooding the evaporator and suction line with liquid refrigerant

The suction line should be cool but not frosted under normal operation. Frosting of the suction line extending back toward the compressor indicates the evaporator is flooded — liquid refrigerant is not fully evaporating in the evaporator and is flowing as liquid into the suction line, which can damage the compressor.

A customer reports that the A/C produces adequate cooling but the air from the vents smells musty when the A/C is first turned on. The smell goes away after a few minutes. What causes this and what is the recommended service?

Answer: Microbial growth (mold/mildew) on the evaporator core from condensation, requiring evaporator cleaning and disinfecting

Musty odor when A/C first activates that fades during operation is caused by mold and mildew growing on the moist evaporator surface and in the HVAC box. Condensation during A/C operation provides moisture for microbial growth. Treatment involves disinfecting the evaporator and HVAC box with an antimicrobial agent.