660 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. Moisture in the Refrigerant Circuit Moisture in the refrigerant system will cause a unit to malfunction. The moisture forms ice in the metering device at the point where liquid refrigerant is expanding into the evaporator. Ice closes the opening, blocking flow into the evaporator. This condition can be recognized by the following symptoms: • If the system is completely defrosted, the ice that caused the blockage disappears. The unit will then work properly again. However, the unit will only work until ice again forms at the metering device. • Pressure decreases in the suction line. The compound gauge shows a steady decrease over several hours (even to a vacuum). Then, pressure suddenly becomes normal again. This odd cycle will keep repeating. • Warm the metering device with a safe resistance heater (hot pad) or radiant heat bulb during system shutdown to melt any ice buildup. If the system then begins to work properly, there is definitely moisture in the refrigerant. Moisture in the refrigerant circuit also creates cor- rosion problems within the system. This occurs when refrigerants react with water molecules to form acids. The acids increase the amount of corrosion in the sys- tem. Excessive moisture in a system indicates that the filter-drier is clogged. Refer to Chapter 26, Service and Repair of Domestic Refrigerators and Freezers, for informa- tion about replacing filter-driers. There are certain substances that can be placed in the refrigerant circuit to keep moisture from forming ice. However, a new filter-drier is the best solution. It prevents circulation of the moisture through the sys- tem. It also reduces the chance of oil breakdown, which results in sludge and acid. Wax Manufacturers have removed as much wax as pos- sible from refrigeration oil, but some wax still remains. Some oil circulates with the refrigerant. Sudden expan- sion at the refrigerant control and its low temperature and pressure cause some wax to separate from the oil. The wax collects in the metering device. In time, it may build up sufficiently to restrict flow or completely clog the metering device. If there is a restriction in the metering device, it should be determined whether the restriction is caused by wax or ice. If the metering device is clogged with ice, it may be fixed by installing a new filter-drier, as explained in the previous section. If the metering device is clogged with wax, both the metering device and the filter-drier must be replaced. Procedures for replacing metering devices are presented in Chapter 26, Service and Repair of Domestic Refrigerators and Freezers. Shortage of Refrigerant A shortage of refrigerant is another common cause of poor refrigeration. If a shortage of refrigerant is found, it is often the result of a leak. Small systems have only one or two pounds of refrigerant. Therefore, even the smallest leak will soon cause poor refrigera- tion. A leak with a loss rate as low as one ounce per year can be located and must be repaired. The follow- ing conditions indicate a lack of refrigerant: • Low-side pressure is below normal. • The outlet end of the evaporator is warm. • High-side pressure is below normal. 25.4.3 Diagnosing Specific Component Problems Before removing any system component, be cer- tain that it is the cause of the problem. After diagnos- ing the system by visual inspection and by analyzing system performance, perform pinpoint tests of the sus- pected components before replacing them. The follow- ing are parts that frequently cause trouble: • Compressor. • Filter-drier. • Metering device (capillary tube or AEV). • Hot-gas defrosting valve. Locating Compressor Faults The most expensive service item for a domestic refrigerator is the replacement of the compressor. The technician should carefully check for all other electri- cal and mechanical failures before determining that the compressor needs replacement. When a compres- sor is in good condition, the most common reason for replacement is an electrical fault. Other electrical problems often mislead a service technician into thinking the compressor is at fault. To check a compressor, first disconnect power to the unit and then clean the outside of the compressor dome. Then, remove the cover over the electrical connections. Disconnect the system wiring from the compressor: relay, capacitors, overload cutout, and all wiring. Use an ohmmeter to check motor windings for continu- ity, shorts, and grounds. Refer to Chapter 17, Servicing Electric Motors and Controls. If the unit checks out cor- rectly, connect a test cord to the compressor. Use the correct size of capacitors and fuse. Connect the starting circuit as shown in Figure 25-26. If the system starts and operates correctly with these manual-start electrical connections, the problem is in the external system. It may be in the wiring, ther- mostat, relay, or overload cutout. If the internal electri- cal motor is faulty, the compressor must be replaced. If