236 Auto Heating & Air Conditioning
Note: In this and subsequent chapters,
colored shading used on gauges indicates
various types of readings:
• Orange shading indicates a normal low-side
reading.
• Purple shading indicates a normal high-side
reading.
• Red shading indicates an abnormal reading.
If the pressure readings are within specifi cations,
observe the evaporator outlet line. It should be covered with
condensed moisture, possibly frozen. This is a visual sign the
system is working properly. If the system uses an accumulator,
touch the inlet and outlet tubes, Figure 15-11. If the system is
fully charged and working properly, the temperature should
be roughly equal at both pipes.
Warning: High pressure lines can become
extremely hot. Touching a line for an ex-
tended period of time can result in a burn.
Briefl y touch the low and high pressure lines. All high side
lines should be hot, while the low pressure lines should be cold.
If the system uses a sight glass, check the glass for foaming. After
the system has been running for fi ve minutes on a reasonably
warm day (70°F or 20°C), the glass should be clear. See the
additional sight glass information included in Figure 15-12.
Check the temperature gauge in the outlet vent. Outlet
temperature will vary with outside air temperature and
humidity. As a general rule, the outlet temperature should
be about 30°F (17°C) lower than the outside air tempera-
ture after the system has been operating for 5-10 minutes. If
the gauge is not showing a reasonable drop in temperature,
something is wrong.
Figure 15-11. Touching the lines going to and from the
accumulator is a quick, but good test of system performance. Do
not attempt this on receiver-drier systems.
Touch inlet and
outlet tubes here
Figure 15-12. Many older refrigeration systems have a sight glass. Observing the sight glass after the refrigeration system has been
running for a few minutes will tell the technician approximately how much refrigerant is in the system. (Nissan)
Item to check Adequate Insufficient Almost no refrigerant Too much refrigerant
State in sight glass
Temperature of
high- and low-
pressure lines
Pressure of system Both pressures on high-
and low-pressure sides
are normal.
Both pressures on high-
and low-pressure sides
are slightly normal.
High-pressure side is
abnormally low.
Both pressure on high-
and low-pressure sides
are abnormally high.
Note: The condition of the bubbles in the sight glass, temperatures, and pressure are affected by ambient temperature
and relative humidity.
There is almost no
temperature difference
between high- and low-
pressure side.
High-pressure side is
hot while low-pressure
side is cold. (A big
temperature difference
between high- and
low-pressure side.)
High-pressure side is
warm and low-pressure
side is slightly cold.
(Not so large a
temperature difference
between high- and
low-pressure side.)
High-pressure side is hot
and low-pressure side is
slightly warm. (Slight
temperature difference
between high- and low-
pressure side.)
CLEAR
Vapor bubbles sometimes
appear when engine
speed is increased or
decreased.
FOAMY or BUBBLY
Vapor bubbles always
appear.
FROSTY
Frost appears.
NO FOAM
No vapor bubbles
appear.