42 Auto Heating & Air Conditioning
R-134a and R-12 hose connectors in Figure 3-4. These
hoses can be replaced if damaged or worn, however,
they are not interchangeable. Most hoses are colored
blue for the low side and red for high. The center hose is
usually yellow.
Hose Fittings and Adapters
Many gauge manifolds are equipped with fi tting adapt-
ers, shutoff valves, and other features. These are attached to
the hoses. Typical valves are used to isolate refrigerant pres-
ent in the hoses. This keeps refrigerant loss to a minimum.
R-12 refrigeration systems made since 1986 have dif-
ferent size service fi ttings to prevent the technician from
accidentally crossing the high and low side connections.
Adapters are used to allow the same gauge manifold to
be used on these refrigeration systems. See Figure 3-5.
Adapters are not needed on R-134a systems since they use
standardized fi ttings.
Attaching and Reading Gauges
To attach and read the gauge manifold, fi rst make sure
the hand valves are closed. Then remove the refrigeration
service fi tting caps and attach the hoses. Remember the
blue hose attaches to the low side of the system, and the
red hose is connected to the high side.
R-134a and R-12 hose connectors have different
fi ttings, and therefore, different attachment methods. R-12
fi ttings are threaded. The hose connector is also threaded
and is screwed onto the fi tting. R-134a fi ttings are some-
what similar to those used on air hoses. The hose connec-
tor is a quick disconnect with an isolation valve. Once the
connectors are installed, open the hose isolation valves
as needed. Do not open the hand valves on the manifold
body.
Once the hoses are in place, you can read the static
pressures. Static pressures are the pressures in the refrigera-
tion system when the system is not operating. If the system
has been off for 30 minutes or more, the high and low side
pressures should be almost the same (within a few pounds
or kPa). Other uses for the gauge manifold will be covered
in the chapters where they apply.
Temperature Gauges
Temperature gauges are used to test the temperature
of the air exiting the HVAC system vents and to check the
temperature of the engine coolant. The two main types of
temperature gauges are mechanical and electronic. They
are discussed in the following paragraphs.
Mechanical Temperature Gauge
The mechanical temperature gauge, Figure 3-6, relies
on an internal bimetal spring to register temperature dif-
ferences. The bimetal spring is a coil of wire made of two
kinds of metal. Each metal expands at a different rate as
its temperature changes. Therefore, changes in temperature
cause the coil to tighten and loosen. The coil is attached
to the gauge pointer. Movement of the pointer against the
gauge face indicates the temperature.
Electronic Temperature Gauge
The electronic temperature gauge uses infrared waves
to measure temperature. Infrared waves are waves similar
to light waves, although they cannot be seen. Temperature
A
Figure 3-4. Hose connectors vary by refrigerant type. A—R-12
connectors. B—R-134a connectors. Note the difference between
the two connectors.
B
Figure 3-5. Some R-12 systems made in the 1980s and early
1990s require special adapters. These adapters are usually
needed to access the high side of the refrigeration system. (Ford)
Flexible
45 Degree 90 Degree
Straight