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ASE P2: Heating, Ventilation & Air Conditioning (HVAC)


Most air conditioning parts are sold for service work, often to repair a refrigerant leak or to replace a failed component such as a compressor, evaporator or condenser.

Belt-driven compressors have a magnetic clutch on the input shaft that cycles the compressor on and off to control cooling. The clutch can be replaced separately if bad. If the compressor has failed, the replacement unit may or may not come with a new clutch installed. A new clutch is recommended if not

Compressor failures may be caused by sludge in the system (from moisture contamination) or refrigerant and oil loss. Compressors require special PAG oil lubricants that vary by manufacturer. The wrong PAG oil or not enough oil may cause a repeat failure. Some compressors come prefilled with the required PAG oil while others do not.

A compressor failure also may require replacing other parts such as the condenser (the large heat exchanger in front of the radiator) and orifice tube (a metering valve in the high pressure line) if sludge or debris is found inside the A/C system. A serpentine condenser may be cleaned by flushing with refrigerant or a recommended flushing chemical, but parallel flow condensers cannot be cleaned and must be replaced if contaminated. The orifice tube, and accumulator or receiver/drier (components that store refrigerant and trap moisture) should also be replaced when a compressor is changed.

Hoses carry refrigerant between the compressor, condenser and evaporator. The “suction hose” is between the evaporator and condenser. The “high-pressure hose” is between the compressor and condenser. Late-model vehicles with R-134a A/C systems require “barrier” (nylon lined) hoses with crimped end fittings. Older (1994 and back) vehicles with R-12 systems can use unlined hoses with barbed end connections.

New o-rings or seals should always be used when hoses are replaced.

The evaporator is the heat exchanger inside the HVAC unit that cools air entering the passenger compartment. Internal corrosion can cause an evaporator to leak. Replacement usually requires considerable labor and expense, so one option if a leak is minor is to add a sealer product to the A/C system.

R134a refrigerant is used in all 1995 and newer vehicles. It can be sold to any customer, but only those who have passed an EPA-approved certification course can purchase R12. Alternative refrigerants are available to replace R-12 in older vehicles if the A/C system cannot be easily converted to R134a. CM

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