654 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. as the ones on the system being tested. Close the start switch and then the run switch. Open the start switch after a second or two. If the motor operates correctly, the problem is in the external circuit. Check the electri- cal system up to the compressor. If the motor does not operate when tested, further motor checks are needed. These are explained in Chapter 17, Servicing Electric Motors and Controls. Pro Tip Using Test Cords to Check Continuity The test cords Figure 25-11,Figure Figure 25-13,3 and Figure 25-14 can be used for checking continuity and grounding by using extra jumper cords to include a lightbulb in series with the circuit being tested. Ineffective cooling problems can be caused by the evaporator fan motor or the condenser fan motor. A test cord like the one shown in Figure 25-11 can be used to directly test a fan motor. Clip 1 attaches to the fan, clip 2 attaches to the power terminal, and clip 4 attaches to ground. Open the start switch and leave clip 3 disconnected. Fan motors are usually replaced if they are found to be faulty. Pro Tip Fan Motor Replacement Before removing a fan from a motor shaft, put matching marks on the fan hub and the shaft. This ensures that the fan is positioned correctly on the new shaft. Electrical failure in a mullion heater may cause a door gasket to freeze to the cabinet. The heater must be checked for continuity with a test light or an ohmmeter. Locate the circuit in the wiring diagram. Disconnect both ends of the mullion heater leads. Then, test the heater for continuity. If a mullion heater is defective, look for a second (extra) heater in the insulation. Most cabinets have one. Test it also, and connect it if it has continuity. If there is no extra heating unit, install one of the same wattage (volt-ampere) rating. If the problem is a faulty wire, use a stiff steel wire to pull new wiring through the foamed-in-place insula- tion. If necessary, drill a hole (up to 1/2″) in the back of the refrigerator to help feed the wires. Seal the hole with silicone sealant after the wire or wires are pulled through. If the cabinet temperature is not responding properly to the thermostat, the thermostat may be faulty. A set of alligator clips can be used to jump across the terminals of the thermostat, bypassing it. If the compressor starts with the thermostat bypassed, the problem is in the thermostat. Using Test Cords to Check Continuit y Th e t es t cordsshownin s hown in Figure 25-1 1 , 25-1 , and Figure 25-1 4 can be used for checking continuity and grounding by using extra jumper cords to include a lig htbulb in series with the circuit bein g tested . Fan Motor Replacement Before removing a f an f rom a motor sha ft, put matching marks on the f an hub and the sha ft. This ensures that the f an is positioned correctly on the new sha ft. If the compressor fails to start with the thermostat bypassed, the problem is elsewhere in the circuit. If a thermostat will not start the compressor when the cut-in temperature is reached or if it keeps the system running after the cut-out temperature is reached, it should be replaced. If there is time for a return service call on the following day, set up a data logger to record the cabinet temperature over a 24-hour period. The recorded data will show if the appliance is operating properly. 25.4 Diagnosing Internal Troubles Once you have eliminated external problems as the cause of failure, it is time to troubleshoot the refrig- eration system itself. There are many ways to find the cause of trouble inside a small hermetic system. This is done using gauges, thermometers, and electrical instruments, combined with careful observation. A properly trained service technician should be able to locate the cause of any problem in a system. The evaporator may be partially frosted. This could indicate that not enough refrigerant is flowing through the metering device into the evaporator. As a result, the refrigerant that does enter the evaporator is completely vaporized after passing only partway through it. This lowers the system’s cooling capacity and efficiency. This can be caused by a low refrigerant charge or by a partially clogged metering device. A low refrigerant charge results from a leak in the system. Leaks on new refrigeration units can be the result of manufacturing defects or broken or cracked lines from shipping or installation. Older units may develop leaks as a result of vibration of the tubing, which can cause cracks at the tubing joints. If liquid refrigerant makes it all the way through the evaporator and into the suction line before vapor- izing, frost or sweat may form on the suction line. In a capillary tube system, this can be caused by a bro- ken thermostat or an overcharge of refrigerant. Either of these conditions results in more refrigerant entering the evaporator than there is heat available to vaporize it. As a result, some liquid refrigerant enters the suc- tion line. Liquid refrigerant cannot be compressed by the compressor. If liquid refrigerant enters the com- pressor, it can cause severe mechanical damage. Internal electrical troubles, involving the motor and connections, are very rare. Most internal electrical problems come from air and moisture getting into the compressor shell. This causes corrosion in the motor and eventually a burnout. If liquid refrigerant reaches the compressor, it may remove the oil. The liquid evaporates in the crankcase and carries the oil with it into the condenser. Valves