An excellent way to determine if the AC system is operating correctly is to conduct a performance test. This provides useful data that can be used to diagnose problems in the system. This section provides detailed instructions on how to perform the test and what expected outcomes should be.
Setting Up for Performance Testing
1. Connect a manifold set to the service ports of the AC system. The valves at the end of the service hoses should be turned clockwise after connecting them in order to open the check valves on the service ports.
2. Turn the blue knob and red knob on the manifold set clockwise to close off these valves so the pressure can be read accurately. The pressure that is present at this point before the AC system is turned on is known as static pressure.
3. Install a thermometer into the center duct of the vent.
4. Start the vehicle and allow the engine to rise to normal operating temperature.
5. Set the HVAC controls to the coldest setting with the blower on the highest setting. Set the control to recirculated air and the location at the vent.
The blue gauge is used to measure the pressure on the low side of the AC system. This gauge has a range from a vacuum up to about 140
The red gauge is used to measure the pressure on the high side of the AC system. This gauge has a range from 0 to 500
The red manifold service hose and fitting is connected to the high side service port of the AC system. The round valve needs to be turned clockwise to push the Schrader valve down, which allows refrigerant to pass into the red hose and the high side
The blue manifold service hose and fitting is connected to the low side service port of the AC system. These fittings use a quick-connect coupling that is similar to an air coupling used on air-powered tools. The round valve needs to be turned clockwise to push the Schrader valve, which allows refrigerant to pass into the blue hose and low side
The blue valve at the manifold set should be turned clockwise to block any refrigerant from entering the low side connection when the pressures are being
The red valve at the manifold set should be turned clockwise to keep any refrigerant from entering the high side connection when the pressures are being
6. Increase the engine to about 1,200 rpm for a couple of minutes.
7. Inspect the system pressures on the low side as well as on the high side.
8. Inspect the duct air temperature near the end of the test period.
Static pressures should be equal and very close to the same as the ambient temperature since pressure will rise and fall as the temperature changes because of the pressure/temperature relationship of
The engine should be brought to normal operating temperature prior to running a performance test on the AC
The engine speed needs to be brought to about 1,200 rpm during the test in order to attain maximum performance and
A reliable thermometer needs to be installed into the center duct to measure the duct temperature during the performance test. The ambient air temperature was approximately 80°F on the day of this performance
The HVAC controls need to be set on coldest setting with the recirculated air coming out at the vent location. The blower motor needs to be set on high
The low side pressure should be checked during the performance test. Keep in mind that the pressure should be noted just before the compressor cycles off when doing this test on a cycling
The high side system pressure should be checked during the performance test. Keep in mind that the pressure should be noted just before the compressor cycles off when doing this test on a cycling
Expected Results
1. The duct temperature should be 40 to 50°F.
2. The low side refrigerant pressure should be in the 20 to 40 psi range.
3. The high side refrigerant pressure should be two to three times the ambient temperature, depending on the humidity level. With low to moderate humidity, the high side pressure will be around twice the ambient temperature. With higher humidity levels, the high side pressure will be up to three times the ambient temperature.
4. The suction line will be cold and have water droplets dripping from it.
The duct temperature at the center duct location should be checked to see if the expected temperature was achieved during the
