System Performance Test

Fig. 1 A/C system pressure/temperature relationship (Typical). Equivalent to 1750 RPM (30 mph):

The system should be operated for at least 15 minutes to allow sufficient time for all parts to become completely stabilized. Determine if the system is fully charged by the use of test gauges and sight glass. Head pressure will read from 180 psi to 220 psi or higher, depending upon ambient temperature and the type unit being tested, and the sight glass should be free of bubbles. Low side pressures should read approximately 15 psi to 30 psi, again depending on the ambient temperature and the unit being tested. It is not feasible to give a definite reading for all types of systems used, as the type control and component installation used on a particular system will directly influence the pressure readings on the high and low sides, Fig. 1.

The high side pressure will definitely be affected by the ambient or outside air temperature. A system that is operating normally will indicate a high side gauge reading between 150 to170 psi with an 80°F ambient temperature. The same system will register 210 to 230 psi with an ambient temperature of 100°F. No two systems will register exactly the same, which requires that allowance for variations in head pressures must be considered. Following are the most important normal readings likely to be encountered during the season.

RELATIVE TEMPERATURE OF HIGH & LOW SIDES
The high side of the system should be uniformly hot to the touch throughout. A difference in temperature will indicate a partial blockage of liquid or gas at this point.
The low side of the system should be uniformly cool to the touch with no excessive sweating of the suction line or low side service valve. Excessive sweating or frosting of the low side service valve usually indicates an expansion valve is allowing an excessive amount of refrigerant into the evaporator.

EVAPORATOR OUTPUT
At this point, provided all other inspection tests have been performed, and components have been found to operate as they should, a rapid cooling down of the interior of the vehicle should result. The use of a thermometer is not necessary to determine evaporator output. Bringing all units to the correct operating specifications will insure that the evaporator performs as intended.