Chapter 15 Refrigeration System Diagnosis and Leak Detection 237
Next, unplug the blower motor connection. This
is usually easiest to do at the resistor assembly. With the
blower not turning, there is no heat load on the evapo-
rator, and the compressor clutch should cycle off within
30 seconds. If the system uses an evaporator pressure con-
trol valve (STV, POA, VIR, EPR) pressure should drop to
28-30 psi (193-207 kPa).
System Underc harge
A system undercharge is the most frequent problem
found in a refrigeration system. Depending on how much
refrigerant is left in the system, gauge pressures will read much
lower than normal, even when factoring in air temperature.
Low pressure tubing and hoses will feel warmer while high
pressure tubing will feel cooler. The outlet temperature will
be higher than normal. If the system has a sight glass, bubbles
or foam will be present. Figure 15-13 shows typical readings
from an undercharged system.
The cause of a system undercharge is usually leaks,
but could be caused by failing to fi ll the system with the
proper charge. An undercharged system will not only pro-
vide inadequate cooling, but will fail to carry the necessary
lubricants through the system. This can lead to reduced
compressor life and eventual failure.
System Overcharge
System overcharge occurs quite frequently, in fact,
almost as frequently as system undercharges. The fi rst sign
of a system overcharge is much higher than normal system
pressures, Figure 15-14. Cooling will be affected as the
evaporator, accumulator/receiver-drier, and other system
components are fl ooded with refrigerant.
In some cases, a system appearing to be overcharged
contains air (sometimes called noncondensible gas or
NCG). An overcharged system should be checked for leaks
as the extra refrigerant may have been added because the
system was undercharged. If no leaks are found, recover
the refrigerant charge, evacuate, and recharge the system.
Restriction in Lines, Orifi ce Tubes, and
Expansion Valves
System restrictions can easily be found by feeling
the system’s lines, hoses, and components. If the high side
becomes cold at any point before the orifi ce tube or expan-
sion valve, that spot is restricted. A restriction in the high
side is usually located at the orifi ce tube or expansion valve,
depending on the system. However, restrictions in lines and
components, such as evaporators and condensers, can occur.
Figure 15-15 shows typical system pressures and symptoms
when a line, orifi ce tube, or expansion valve is restricted.
Gauge pressures may be affected by the presence
of a restriction. However, as mentioned earlier, variable
displacement compressor systems may show little or no
change. If a restriction is present, gauge pressures will usu-
ally be lower than normal and there will be no cooling.
Orifi ce tube and expansion valve restrictions can be caused
by a defective compressor, a ruptured desiccant bag, or
contaminants such as dirt or corrosion. If the system uses a
thermostatic expansion valve, the sensing bulb should be
tested for proper operation. A defective expansion valve
can give readings similar to a plugged orifi ce.
Figure 15-13. Gauge readings for a system undercharge condition. The red areas indicate the regions
where the gauge needles will typically be. They may cycle (increase and decrease in pressure) if the
compressor clutch is cycling.
Refrigeration System Undercharged
Low Side (Suction)
Symptoms: Poor cooling, A/C outlet warm, rapid compressor cycling (orifice tube systems),
sight glass has bubbles or foam.
Orifice Tube
Exp. Valve
VDOT
(Varies with ambient temperature) (Varies with ambient temperature)
Low to Normal
Low
Normal
Low
Low
Low
High Side (Discharge)
0
10
20
30
4
0
50
60
70
80
90
100
110
120
0
50
100
150
200
250
3
00
350
400
450
500