664 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. 4. Open the nitrogen cylinder valve and set the pressure regulator to a limited pressure. Once set, open the gauge manifold valves and the piercing valves. If the filter-drier is open, nitro- gen will come out of its open end. If the capillary tube is open, a small flow (because the tube is small) will come out its opening. If either one is clogged, there will be no flow through it. Clogged filter-driers must be replaced. Sometimes clogged capillary tubes can be opened with a high- pressure hydraulic pump. Repair and replacement pro- cedures are presented in Chapter 26, Service and Repair of Domestic Refrigerators and Freezers. Pro Tip Filter-Drier Replacement The filter-drier should be replaced any time the refrigerant circuit is opened, regardless of whether the filter-drier is the point of restriction or not.. Post-Condenser (PC) Loop Problems Earlier, it was pointed out that some domestic refrigerator-freezers are equipped with post-condenser (Yoder) loops that heat the cabinet exterior to prevent condensation from forming. These loops carry refriger- ant and therefore can be the source of refrigerant leaks. One problem with PC loop service is the inability to replace the loop if it develops a leak. The most com- mon reason for a leak in a PC loop is a screw that has been mistakenly driven into the loop. If the loop were improperly placed during the foaming process, it could be in the direct path of a planned screw. To determine if the loop is indeed a source of a sealed system leak, the loop must be isolated from the rest of the system. An access valve must be attached, a nitrogen charge pressurized into the loop, and a compound gauge con- nected to monitor pressure. With that part of the sys- tem isolated and pressurized, if the pressure reading Filter-Drier Replacement T he filter-drier should be replaced any time the refrigerant circuit is opened, regardless o f whether the f ilter-drier is the p oint o f restriction or not drops, then there is a leak in the loop. If the pressure holds steady, look elsewhere for the refrigerant leak. Electric Resistance Defrost Problems If a defrost system does not work, ice can build up on the evaporator and make it ineffective. If ice builds up on the evaporator, begin by checking the evaporator fan. If it is working, check the defrost system. Inspect and electrically-test the defrost resistance wire. Hot-Gas Defrost Problems Problems in hot-gas defrost systems are often related to the hot-gas bypass valve. The hot-gas bypass valve is usually operated by a solenoid. This solenoid can fail with the valve in the open position, which would cause the defrost system to operate all the time, keeping the evaporator excessively warm. The solenoid could also fail with the valve in the closed position, which would cause a complete failure of the defrost system. If the evaporator is overloaded with ice, the sole- noid’s electric coil may have failed with the valve in the closed position (open circuit), or the timer may not be operating correctly. Both of these problems can be checked electrically. If the electrical system is operating correctly, the problem is probably a stuck valve stem in the solenoid. Sharply rap the solenoid valve body while the defrost timer switch is closed. If the valve stem comes loose, the surge of hot gas can be heard. The line between the solenoid valve and the evaporator will also become warm to the touch. If the evaporator and the line between the hot- gas solenoid valve and evaporator are warm, the valve is probably stuck open, Figure 25-31. Again, rap the valve sharply while the defrost timer circuit is open. If the valve closes, low-side pressure will start to decrease immediately. The evaporator will start to cool and frost. If the solenoid valve still does not operate after trying these solutions, it must be removed and replaced. 4. Open t h e nitrogen cylinder valve an d set t h e p ressure regulator to a limited pressure. Once set, open the gauge manifold valves and the p iercing valves. If the filter-drier is open, nitro- g en will come out of its open end. If the capillar y tube i s open, a smal l flo w (because the tube is small) sm al l) w w ill il l co me out its opening. I f e ither one is cl clogged g g g ge g g d, there t he re w w ill be n o flo flo w w through th r it .