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ASE PS2 Test Preparation Guide: Heating, Ventilation & Air Conditioning (HVAC)


● COMPRESSOR — Pumps refrigerant to the condenser and evaporator to cool the passenger compartment, and pulls refrigerant vapor from the evaporator to compress it. Most compressors are belt-driven (a few hybrids have electric compressors). A “magnetic clutch” on the compressor pulley cycles the compressor on and off as needed to control cooling. Some compressors run continuously and use variable displacement to vary cooling output.


Most compressor failures are caused by loss of lubricant and/or refrigerant. Repeat failures are usually due to system contamination (sludge or metallic particles from a previous compressor failure).

Customers who are replacing a failed compressor should be advised to flush out the refrigerant hoses and evaporator with refrigerant or an approved flushing chemical to remove contaminants. Serpentine style condensers can be flushed, but parallel flow condensers have to be replaced if contaminated. The accumulator or receiver/drier should also be replaced, along with the orifice tube. Installing an inline filter in the compressor liquid line is also recommended to trap any residual contaminants.


 Replacement compressors may contain the proper PAG oil lubricant for the vehicle application, but some may contain a temporary shipping oil that must be drained prior to installation. Others are shipped dry.
Replacement compressors for older vehicles with R-12 A/C systems that have been retrofitted to R-134a refrigerant must use a POE oil or the type of PAG oil specified by the compressor manufacturer. POA oils are another option. Compressors for 1995 and newer vehicles mostly use one of three different types of PAG oil. Pre-1995 compressors used mineral oil. Using the wrong type of oil can cause compressor failure.


● CONDENSER — A heat exchanger that receives high-pressure refrigerant after it leaves the compressor. The condenser cools the refrigerant, allowing the vapor to condense into a liquid. The condenser is usually mounted ahead of the radiator, and may have its own cooling fan. Some condensers have a subcooler to increase cooling efficiency.

● EVAPORATOR — A second heat exchanger in the A/C system that cools and dehumidifies air entering the passenger compartment. It is usually located inside the HVAC assembly inside the vehicle. Some minivans and SUVs may have a second evaporator for cooling the rear seating area. Evaporator leaks due to internal corrosion or electrolysis can sometimes be temporarily plugged by adding a sealer product to the refrigerant, but replacement is the best repair.


Evaporator odor can sometimes be a problem when moisture allows mold and bacteria to grow on the surface of the unit. Spraying the unit with a biocide deodorizer usually solves the problem. Replacement evaporators may have a special surface coating that inhibits mold growth and odors.

● ACCUMULATOR or RECEIVER/DRIER — Stores, filters and dehumidifies refrigerant. It is usually located in the engine compartment near the firewall or evaporator connections. It contains moisture-absorbing “desiccant” crystals to remove moisture contamination that could cause acids and sludge to form. Replacement is recommended after a compressor failure, when the A/C system is opened for repairs, or if the A/C system has a leak.


● ORIFICE TUBE — Controls the flow of refrigerant into the evaporator. Located in the liquid line just ahead of the evaporator, the orifice tube has a small calibrated hole that allows a certain amount of refrigerant to flow through it. Orifice tubes can become plugged with sludge and debris, causing a blockage that prevents cooling. The orifice tube should be replaced if the compressor has failed, or sludge or contamination are found inside the A/C system. Aftermarket “variable orifice” tubes can provide improved low speed cooling over a fixed orifice tube.

● EXPANSION VALVE — A metering device used in place or an orifice tube to regulate the flow of refrigerant. It uses a temperature-sensing capillary bulb, and is mounted on or near the evaporator.


● A/C HOSES — Carry refrigerant between the compressor, condenser and evaporator. The “suction hose” is located between evaporator and condenser. The “high-pressure hose” is located between compressor and condenser. R-134a A/C systems require “barrier” (nylon lined) hoses to prevent seepage of refrigerant with crimped end fittings. Older-style hoses used barbed end connections.

● SEALS & O-RINGS — Are used between the compressor and various hose fittings. O-rings should be replaced if the connection is leaking or parts are being replaced. If an older R-12 system has been retrofitted to R-134a, the seals must be compatible with the new refrigerant.


● R-12 — An obsolete refrigerant that was used until 1994. R-12 is a chlorofluorocarbon (CFC) that is damaging to the Earth’s protective ozone layer, so it is no longer produced in the U.S. Sales are restricted to “certified” professionals (no DIY sales). Older vehicles with R-12 A/C systems can be retrofitted to R-134a or other EPA approved alternative refrigerants.

● R-134a — Is the standard refrigerant for all 1995 and newer vehicles (for now). R-134a is “ozone-safe” and has similar cooling characteristics to R-12 but contains no ozone damaging chlorine. R-134a is a “greenhouse gas” that contributes to global warming. A new refrigerant “HFO-1234yf” may eventually replace it in future vehicles.


● ALTERNATIVE REFRIGERANTS — Are refrigerants that have been EPA approved to replace R-12 in older vehicles with R-12 A/C systems (not newer vehicles with R-134a systems). Alternative refrigerants require special service connections and should NOT be mixed with other refrigerants in an A/C system. Flammable refrigerants are illegal.

● CABIN AIR FILTER — Used in the HVAC systems of many newer vehicles to filter air entering the passenger compartment. Some are dust filters, others are dust and odor filters. Usually located behind the glove box, in the HVAC plenum or intake in the cowl area of the windshield. Recommended replacement interval is typically once a year for odor filters, every two to three years for dust filters.


● HEATER CORE — A heat exchanger that uses engine coolant to heat air entering the passenger compartment. Located inside the HVAC assembly, the heater core may have to be replaced if leaking.

● BLOWER MOTOR — A fan located inside the HVAC assembly that blows air through the heater core and A/C evaporator for heating and cooling.


Cooling System

Electrical System

Exhaust Parts

Ignition System

Manual Transmission/Transaxle Parts

Suspension & Steering Parts

Fuel System

Engine Parts

Emission Controls

Driveline Components

Heating, Ventilation & Air Conditioning (HVAC)


Automatic Transmission/Transaxle

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