Chevrolet Light Duty Truck 1973 Service Manual page 61

The compressor discharge pressure switch also performs
the function of the ambient switch as the pressure at the
switch varies directly with ambient temperatures. The
compressor should run below 25°F. am bient or 37 psi
not
at the switch. The compressor should run in A /C modes
above 45°F. ambient or 42 psi at the switch.
The switch interacts with other switches so that in an
A /C system where the compressor will
45° ambient the following components should be checked
for continuity:
1. Compressor discharge pressure switch.
2. Master switch (on control head).
If both switches show proper continuity, check the
harness for shorts or improper ground conditions.
G Models
This low refrigerant charge protector system consists of a
superheat shutoff switch located in the rear head of the
compressor, connected in series by an electrical lead to a
thermal fuse.
During normal air conditioning system operating
conditions, current flows through the air conditioner
switch, the ambient switch, and through the thermal fuse
link to the clutch coil to actuate the compressor clutch.
Should a partial or total loss of refrigerant in the system
cause the superheat switch to sense low system pressure
and a high suction gas temperature, the superheat switch
contacts will close. When the contacts close, current flows
to energize the resistance type heater in the thermal fuse.
The resultant heat warms the fuse link to its specific melt
temperature, thus opening the circuit to the compressor
clutch coil. Compressor operation ceases and compressor
damage due to a loss of refrigerant charge is prevented.
The cause of the refrigerant loss must be corrected
the system charged prior to replacing the thermal fuse.
The superheat switch does not have to be replaced when
it cycles and is reusable unless it is determined that the
switch is faulty.
CHEMICAL INGREDIENTS OF AN
AIR CONDITIONING SYSTEM
All systems involve metals, refrigerant, and oil which are
basic and essential. The desiccant, or dehydrating agent,
and another chemical ingredient, synthetic rubber,
makes it even more complex.
All of these ingredients have chemical properties which
are entirely different from each of the others. By proper
selection of the ingredients and controlled processes in
manufacture, plus careful servicing procedures, they can
be combined so that they provide many years of
satisfactory and trouble-free operation.
Only one undesirable element added or allowed to enter
the system can start a chain of chemical reactions which
upsets stability and interferes with the operation of the
unit.
Chemical Instability and Refrigerant
System Failures
A sealed refrigerating system is a complex physical-
chemical combination which is designed for stability
within certain operating limits. If these limits are
exceeded, many physical and chemical reactions occur to
the system. Since the results of these reactions within the
operate above system cannot be easily removed, they build up into a
not
constantly accelerating vicious circle to eventually fail
the system.
Metals
In most cases, metals contribute to the decomposition of
R-12 and oil in varying amounts. All metals are attacked
by acids.
Each of the metals in common use in a system has been
selected for a specific reason; heat conductivity,
durability, strength, and chemical composition.
Under favorable conditions, the amount of decomposi­
tion of Refrigerant-12 and oil produced by these metals
is negligible. If undersirable substances are added and
the temperature is increased, the rate of decomposition
and the production of harm ful acids increases
proportionally.
Refrigerant
The chemical properties of refrigerants are very
important factors in the stability of a system since the
refrigerant penetrates to every part of the unit.
Among the many desirable properties of R-12, is its
stability under operating conditions. While more stable
than the other refrigerants under the same conditions, it
can be caused to form harmful acids which will
eventually fail the system.
and
Oil
Oil is the most complex of all organic chemicals. Its
stability in a refrigerating system is dependent upon the
source of crude oil and its method of refining. A good
refrigeration oil must be free of sludge, gum-forming
substances and impurities such as sulphur. It must be
stabilized to resist oxidation and must have a high
degree of resistance to carbonization.
The chemical properties of the lubricating oil form
another very important consideration in the chemical
stability within the system. Like the refrigerant, it travels
to every part of the system.
The factory obtains the finest oils which have been
refined from the most desirable crudes. It is reprocessed
at the factory before it is charged into a system or
poured into a container for resale. Its viscosity and flash
point are checked and it is forced through many sheets
of filtering paper.
Even the containers in which it is poured for resale are
processed. It is the cleanest, dryest, and purest oil that is
humanly possible to make. Leaving the container
HEATER AND AIR C O N D IT IO N IN G
LIGHT DUTY TRUCK SERVICE MANUAL
1A-35