Air-Conditioning Performance Testing

Performance testing provides a measure of A/C system operating efficiency. A manifold pressure gauge set is used to determine both high and low pressures in the refrigeration system. The desired pressure readings will vary according to temperature. Use temperature/pressure charts as a guide to determine the proper pressures. At the same time, a thermometer is used to determine air discharge temperature in the passenger compartment.

SERVICE VALVES
System service valves provide the attachment point for the manifold pressure gauge set. They are located in the low and high sides of most A/C systems. Two basic types of service valves were used on R-12 systems: stem and Schrader. R-134a systems have quick-connect fittings. The fittings for the gauges are connected to the system’s fittings. The gauge’s fitting has a knob that is turned in the clockwise direction to open the valve. No refrigerant should leak out when the gauge set is connected.

A thermometer is used to determine air discharge temperature in the passenger compartment.

Stem Valves. The stem valve was sometimes used on twocylinder reciprocating-piston compressors. The service valves are mounted on the compressor head. These valves can be used to
isolate the rest of the A/C system from the compressor when the compressor is being serviced. These valves have a stem under a cap with the hose connection directly opposite it.

Schrader Valves. R-12 systems without a stem service valve have Schrader service valves. Closely resembling a tire valve, Schrader valves are usually located in the high-pressure line and in the low-pressure line. All test hoses have a Schrader core depressor in them. As the ose is threaded into the service port, the pin in the center of the valve is depressed, allowing refrigerant to flow to the manifold gauge set. The valve closes automatically when the hose is removed.

Schrader service valves for a R-134a system.

QUICK-CONNECT FITTINGS
R-134a systems use two different sized quick-connect fittings; the low-pressure fitting being the smaller of the two. This is to prevent connecting the service hoses incorrectly. The service hose fittings may have a manual valve that threads through the fitting to open the service port connection. This manual valve is used to reduce the amount of refrigerant released when the service hoses are disconnected from the system.

PURITY TEST
When you are not sure of the refrigerant used in a system or if you suspect that a mixing of refrigerants has occurred, you should run a purity test and/or use a refrigerant identifier. Knowing what refrigerant is in the system, or what condition it is in, will help you determine what steps you need to take to properly service the system. It is recommended that the system be tested for sealant identification and/or contamination before conducting the purity test.

MANIFOLD PRESSURE GAUGE
The manifold gauge set is one of the most important A/C tools. It is used when discharging, charging, and evacuating the system; it also is used for diagnosing the system. With the new legislation on handling refrigerants, all gauge sets are required to have a valve device to close off the end of the hose so that the fitting not in use is automatically shut.

A manifold gauge set for R-134a.

The low-pressure gauge is graduated into pounds of pressure from 1 to 120 (with cushion to 250) in 1 - pound graduations, and, in the opposite direction, in inches of vacuum from 0 to 30. This is the gauge that should always be used in checking pressure on the low-pressure side of the system. The gauge at the right is graduated from 0 to 500 pounds pressure in 10 - pound graduations. This is the high-pressure gauge that is used for checking pressure on the high-pressure side of the system.

The center manifold fitting is common to both the low and the high sides and is for removing or adding refrigerant to the system. When this fitting is not being used, it should be capped. A test hose connected to the fitting directly under the low-side gauge is used to connect the low side of the test manifold to the low side of the system. A similar connection is found on the high side.

The gauge set has hand valves that isolate the low and high sides from the central portion of the manifold. During all tests, both the low- and high-side hand valves are in the closed position (turned inward until the valve is seated).

The test connectors with shutoff valves for an R-134a system.

Because R-134a is not interchangeable with R-12, separate sets of hoses, gauges, and other equipment are required to service vehicles. Manifold gauge sets for R-134a can be identified by one or all of the following: Labeled “FOR USE WITH R-134a,” labeled “HFC-134” or “R-134a,” and/or have a light blue color on the face of the gauges. Also, R-134a service hoses have a black stripe along their length and are clearly labeled “SAE J2196/R-134a.” The low- pressure hose is blue with a black stripe. The highpressure hose is red with black stripe, and the center service hose is yellow with a black stripe. Service hoses for one type of refrigerant will not connect into the wrong system, because the fittings for an R-134a system are different from those used in an R-12 system.

Connecting the Gauge Set. Identify the type of refrigerant used in the systems and obtain the appropriate gauge set. Locate the high- and low-side service valves. Make sure the valves on the gauge set are fully closed. Remove the protective cap from the low-side service valve and connect the low-side service hose to the service valve. Then remove the protective cap from the high-side service valve and connect the high-side service hose to the valve.