Heating and Air Conditioning: Description and Operation

Fig. 1 Pressure-temperature relationship (Typical). Conditions equivalent to 30 mph or 1750 engine RPM:

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 if one is installed on system. Head pressure will read from 180 psi to 220 psi or higher, depending upon ambient temperature and the type unit being tested. The sight glass should be free of bubbles if a glass is used in the system. 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---170 psi with an 80°F ambient temperature. The same system will register 210---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.

Air Conditioning Compressor And Mounting Bracket:

COMPRESSOR OPERATION
The compressor is a five piston pump that compresses the refrigerant to a relatively high pressure state. Refrigerant in a high pressure state converts to a liquid form and must be cooled by the condenser before reaching the expansion valve, where it is converted to a vapor/gas. The vaporized, cold refrigerant is drawn back into the compressor through a reed valve and compressed again. The compressor is driven by a drivebelt attached to the crankshaft pulley and energized by an electro-magnetic clutch.


RELATIVE TEMPERATURE OF HIGH AND 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.