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Modern Refrigeration and Air Conditioning
post-condenser (Yoder) loops that heat the cabinet
exterior to prevent condensation from forming. These
loops carry refrigerant and therefore can be the source
of refrigerant leaks.
One problem with a PC loop service is an inability
to replace the loop if it develops a leak. The most
common 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 and
a nitrogen charge placed in the loop. If the charge
degrades, there is a leak in the loop. If the charge holds,
look elsewhere for the source of 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 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.
Rap the valve body while the defrost timer switch is
closed. If the valve breaks 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
stuck open, Figure 19-31. Again, rap the valve sharply
while the defrost timer circuit is open. If the valve
closes, the low-side pressure will start to decrease
immediately. The evaporator will start to cool and
frost.
If the valve still does not operate after trying these
solutions, it must be removed and replaced.
Condenser
Evaporator
Compressor
Hot-gas line
Solenoid valve
Filter-drier
High-pressure vapor
Low-pressure liquid High-pressure liquid
Low-pressure vapor
Goodheart-Willcox Publisher
Figure 19-31. Diagram of a hot-gas defrost system with solenoid valve stuck open.