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ASE P2 Test Preparation Guide: HVAC

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Sample Review Questions:
1. A/C compressors in most late model vehicles require what type of lubricant?
a. PAG oil
b. POE oil
c. Mineral oil
d. Motor oil

2. All of the following statements about A/C condensers are true EXCEPT:
a. It is usually located in front of the radiator
b. It receives high-pressure refrigerant gas from the compressor
c. It cools the air entering the passenger compartment
d. It cools the refrigerant so the gas will condense into a liquid

3. The orifice tube in an A/C system is located where?
a. In the suction hose
b. Between the evaporator and compressor inlet
c. Between the compressor and condenser
d. Between the condenser and evaporator

4. Counterman A says the accumulator should always be replaced if an A/C system has been opened up for repairs. Counterman B says a plugged orifice tube is a common cause of compressor failure.  Who is right?
a. Counterman A only
b. Counterman B only
c. Both Counterman A and B
d. Neither one

ANSWER KEY
1A, 2C, 3D, 4C

EXPLANATIONS:
1. Mineral oil is used to lubricate compressors on older R-12 applications (pre-1994 vehicles). If an older R-12 A/C systems has been converted (retrofitted) to R-134a refrigerant, POE (polyester) oil can be used, or the type of PAG specified by the compressor manufacturer. The reason why mineral oil should not be used with R-134a is because it does not mix with the refrigerant. This prevents the oil from circulating through the system normally, which may cause the compressor to fail.

Almost all late model (1995 and up) vehicles with R-134a A/C systems specify PAG oil. Using the wrong viscosity of PAG oil may also a compressor to fail.

2. The condenser is a large heat exchanger located in front of the radiator. It receives hot compressed refrigerant gas from the compressor. Air flowing through the cooling fins in the condenser absorbs heat and cools the refrigerant, allowing the refrigerant to condense into a liquid. This is necessary so the refrigerant can provide a cooling effect when it flows to the evaporator and is allowed to revaporize. A dirty condenser will reduce the cooling efficiency of the A/C system. A larger or more efficient condenser can improve cooling, and may be recommended if the original condenser is being replaced or is not cooling well in an older vehicle that has been converted to R-134a. If the A/C system is contaminated internally with sludge or debris from a compressor failure, the condenser should be cleaned or replaced.

3. The orifice tube is located in the high-pressure liquid line between the compressor and evaporator (usually at the evaporator inlet). The orifice tube has a small opening that meters how much refrigerant enters the evaporator. This restriction separates the high- and low-pressure sides of the A/C system, and allows the refrigerant to expand and cool inside the evaporator. If the orifice tube becomes clogged, it will block the flow of refrigerant and cause a loss of cooling.  The orifice tube should always be replaced following a compressor failure.

4. The accumulator (also called a Receiver Drier) is a small metal canister that stores excess refrigerant and also contains a bag of “desiccant” crystals to absorb moisture. This prevents the formation of harmful acids and sludge that could damage the A/C system. The accumulator should be replaced when changing a compressor, or if the A/C system has been open and exposed to air for a long period of time. If the small opening in the orifice tube becomes plugged with sludge, it can block the flow of refrigerant and compressor oil through the system.

Sections covered:

Automatic Transmission

Batteries

Brakes

Cooling System

Drivetrain

Emissions

Engine Mechanical Parts

Exhaust

Fuel System

HVAC

Gaskets

Ignition System

Manual Transmission

Suspension and Steering

Management

 

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