ASE P2 Test Prep: Hvac

ASE P2 Test Prep: Hvac

Information to help parts professonals pass the ASE P2 test.

The compressor is a pump that circulates the refrigerant through the A/C system. It is belt driven, mounted on the engine and is engaged by a "magnetic clutch" that cycles it on and off as needed to control cooling. Most compressor failures are caused by loss of lubricant and/or refrigerant, but 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, condenser and evaporator with refrigerant or an approved A/C system solvent to remove contaminants. 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 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.

Replacement compressors for older vehicles that still use R12 refrigerant require mineral oil. Compressors for older vehicles that have been retrofitted to R134a refrigerant must use a POE oil or the type of PAG oil specified by the compressor manufacturer. Compressors for newer (1994 and up) vehicles must use the type of PAG oil specified by the vehicle or compressor manufacturer.

The condenser is a heat exchanger in the A/C system for cooling the high-pressure refrigerant after it leaves the compressor. This allows the refrigerant vapor to condense into a liquid. The condenser is usually mounted in front of the radiator and often has its own separate cooling fan, which should run when the A/C is on.

The condenser may have to be replaced if it has been damaged, is leaking or has been contaminated by sludge or debris from a compressor failure. If an older vehicle is being converted to R134a refrigerant and does not cool well, installing a larger, more efficient condenser may be necessary to improve cooling performance.

The second heat exchanger in the A/C system is the evaporator that cools the air as it enters the passenger compartment. The evaporator is usually located inside the HVAC assembly inside the vehicle. Replacement may be necessary if internal corrosion is causing the unit to leak refrigerant. Evaporator odor can sometimes be a problem when moisture allows molds and bacteria to grow on the surface of the unit. Spraying it with a biocide will usually kill the odor-causing germs. Replacement evaporators may have a special surface coating that inhibits mold growth and odors.

This component serves as a filter and reservoir for the A/C system. It is usually located in the engine compartment near the firewall or evaporator connections. Inside it is a bag of moisture-absorbing "desiccant" crystals that help protect against the formation of acids and sludge that could damage the system. Replacement is recommended when the A/C system is opened for repairs, if the A/C system has a serious refrigerant leak or following a compressor failure.

The orifice tube is a metering device in the A/C system that 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. As soon as the refrigerant gets past this hole, it expands and absorbs heat, chilling the air around the evaporator. Orifice tubes can become plugged with sludge and debris. The resulting blockage will prevent cooling and may prevent lubricant from circulating to the compressor, causing it to fail, too. The orifice tube should be replaced if the compressor has failed, or sludge or contamination is found inside the A/C system. Aftermarket "variable orifice" tubes can provide improved, low-speed cooling over a fixed orifice tube.

A metering device, called the expansion valve, is used in some A/C systems and regulates the flow of refrigerant using a temperature-sensing capillary bulb. It is mounted on or near the evaporator.

Air conditioning hoses are used to connect the compressor, condenser and evaporator so that refrigerant can travel in a loop. The "suction hose" is located between the evaporator and condenser. The "high-pressure hose" is located between the compressor and condenser. Newer vehicles with R134a A/C systems require "barrier" (nylon lined) hoses to prevent loss of refrigerant.

Located between the compressor and hose fittings, seals and o-rings should be replaced if the connection is leaking. If a system is being retrofitted, the older seals may not be compatible with R134a and may have to be replaced with ones that are.

R12 was the refrigerant used in older A/C systems until 1994. R12 is a pressurized gas that changes "phase" (from a liquid to a vapor and back again) to carry away heat and provide a chilling effect. R12 was taken out of production because it contains chlorine, which scientists say can damage the ozone layer (which reduces the amount of harmful ultraviolet light that reaches ground level.) R12 must be recovered and recycled when vehicles are serviced, and it can only be sold to "certified" professionals (not DIYers). Prices are high due to scarcity and federal excise taxes.

R134a is the new "ozone-safe" refrigerant that is used in all vehicles since 1995. It has similar cooling characteristics to R12 but contains no chlorine. It must not be intermixed with R12. Most older vehicles can be retrofitted to R134a if all the R12 is first removed from the A/C system. Retrofits also require changing the compressor lubricant to POE oil or a specified type of PAG oil. Some vehicles may require additional modifications, and some older compressors are not compatible with R134a.

All refrigerants other than R134a that are approved for use in older R12 A/C systems are called alternative refrigerants. The EPA has a list of approved alternatives, but the vehicle manufacturers say only R134a should be used for retrofits. Alternative refrigerants require special service connections and a separate recycling machine to prevent refrigerant cross-contamination. Any refrigerant that contains flammable hydrocarbons, propane or butane is illegal and must not be used in an automotive A/C system.

Mineral oil is required in older (1994 and older) R12 A/C systems. Newer vehicles with R134a A/C systems require some type of PAG oil. POE oil is most commonly used for retrofits. The amount of oil in an A/C system is critical both for compressor longevity and proper cooling performance.

Many HVAC systems in newer vehicles have an activated carbon cabin air filter to stop dust, pollen and odors from entering the passenger compartment. The filter may be located behind the glove box, in the HVAC plenum or in the HVAC intake. A plugged filter can restrict airflow and reduce ventilation, heating and air conditioning performance. The recommended replacement interval is typically 12,000 to 15,000 miles or once a year.

The heat exchanger that uses engine coolant to heat air in the passenger compartment is the heater core. Located inside the HVAC assembly, the heater core may have to be replaced if it is leaking.

The heater control valve is the valve that regulates flow of coolant through heater core. May be located in one of the heater hoses that connects the heater core to the engine.

The blower motor is a fan located inside the HVAC assembly for blowing air into the passenger compartment in all modes of operation (heat, defrost, A/C and ventilation). The motor is often powered through a relay that routes power through a "resistor block" to vary blower speed. If the blower doesn’t work, the control switch, relay, wiring or blower motor itself may be defective.

You May Also Like

MAF and MAP Sensors

These small-but-mighty components play an outsized role in keeping fuel-injected engines running smoothly.

MAF and MAP Sensors

While it might not sound like it to the untrained ear, the orchestration of components to achieve the ideal combustion cycle is nothing short of a symphony.

For fuel-injected engines, two important instruments in this precise arrangement are the mass airflow (MAF) sensor and the manifold absolute-pressure (MAP) sensor.

Electronic Parking Brakes

Safety, convenience and holding power – what’s not to love?

Electronic Parking Brake
Driveshaft Dynamics

Don’t let the terminology trip you up.

Serpentine Belts Have a Strong Supporting Cast

Tensioners, balancers and pulleys are working behind the scenes to maintain harmony under the hood.

Tensioners and Pulleys
Artificial Intelligence in the Automotive Aftermarket

The applications for AI are endless, but hurdles still remain.

AI Aftermarket

Other Posts

Interpreting Dashboard Warning Lights

Navigating the neon jungle isn’t as easy as it used to be.

Dashboard Diagnostics
Stopping Power: Brake Master Cylinders

The brake master cylinder is the hydraulic hero behind every safe stop.

Brake Master Cylinder
Active vs. Passive Wheel-Speed Sensors

Both have the same job, but they differ in how they do it.

Winter Vehicle Prep

Keep your customers safe with these preventative-maintenance tips.