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ASE Test Prep: HVAC System


10/25/2006

There will be three questions on the ASE P2 test that deal with the vehicle heating, ventilation and air conditioning system.
 

The A/C compressor is the heart of the refrigeration circuit. It pumps and pressurizes the refrigerant. The compressor is belt-driven by the engine, and most have a “magnetic clutch” that cycles the compressor on and off.

Compressor failures are often caused by loss of lubrication, which in turn, may be due to a blockage in the orifice tube or expansion valve. Most compressors do not hold much oil and rely on oil circulating with the refrigerant for lubrication. If the A/C system has a leak and loses refrigerant, it will also lose oil.

Different types of compressors require specific types of oil. Older R-12 A/C systems require mineral oil, while newer R-134a systems use mostly PAG oil. Older R-12 systems that have been retrofitted to R-12 can use POE oil or PAG oil. Using the wrong lubricant can cause compressor failure.

Replacement compressors may contain the proper lubricant for the vehicle application, but some may contain a temporary shipping oil that must be drained out prior to installation. Others are shipped dry. Anyone who is replacing a compressor may also need a new drive belt.

The accumulator or receiver/drier is a device that serves as a refrigerant reservoir and a system filter. It contains a bag of moisture-absorbing crystals called desiccant. A new accumulator or receiver/drier should be installed if the compressor, condenser or evaporator are being replaced, or if the system has been open and exposed to air for more than a day.

A new orifice tube should also be installed following a compressor failure, or if the system is contaminated with sludge. Aftermarket “variable displacement” orifice tubes can improve low speed cooling on older vehicles that have been retrofitted to R-134a. Some newer vehicles have variable orifice tubes from the factory.

Other parts that may need to be replaced include hoses, O-rings and seals. The “suction hose” is located between evaporator and condenser. The “high-pressure hose” is located between the compressor and condenser. Newer vehicles with R-134a A/C systems all require nylon-lined “barrier” style hoses with pre-formed end fittings.

Many A/C systems have a low pressure cutout switch to protect the compressor should a leak allow the refrigerant to be lost. When pressure drops below a certain point, the switch prevents the compressor clutch from engaging. Many systems also have a high-pressure cutout switch that turns the compressor off if pressure gets too high (which can occur during extremely high load, high temperature conditions).

A/C systems require one of two types of refrigerant: R-12 (Freon) for most 1993 and older vehicles, or R-134a for most 1994 and newer vehicles. The two different refrigerants should not be intermixed. Only certified professionals can purchase R-12 legally, but anyone can purchase R-134a. Most older R-12 systems can be converted to R-134a with minimal changes, but on some Ford and Japanese cars the compressor must also be replaced because the seals are not compatible.

The heater is not part of the refrigeration system and uses engine coolant to provide warmth to the passenger compartment. Hot water from the engine circulates through the heater core, which is connected to the engine and water pump with hoses. Heater output depends on engine temperature (which requires a good thermostat and a full coolant level) and air routing through the Heating Ventilation and A/C (HVAC) system. Blend air control doors direct incoming air through the heater (for heating), A/C evaporator (for cooling) or both (in defrost mode to dehumidify the air so the windows don’t steam over).

Other parts that affect the operation of the A/C and heating system include the heater blower motor, heater control valve (restricts coolant flow to the heater core), radiator/condenser cooling fan(s), and automatic temperature control system.













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