GMC 1976 ZEO 6083 Maintenance Manual page 65

1- 48 AIR CONDITIONING SYSTEM
The vacuum tank is simply a reservoir of vacuum
to be utilized when engine vacuum drops too low to
effectively actuate the vacuum components .
The modes are diaphragm switches which open
and close the various doors in the air movement
system allowing "Heat" "Air Conditioning", "De
frost" and "Vent" as shown on the control panel
(figures 23 and 24).
The control panel consists of a temperature valve
and a select valve (See figure 22). This is the control
center for directing vacuum through vacuum lines to
the different modes to achieve the temperatures
desired for passenger and driver comfort. More in-
formation concerning the vacuum system and the
routing of the lines and operation of the modes will
be covered later in this section.
PRIMARY CAUSES OF SYSTEM
FAILURE
Leaks
A shortage of refrigerant causes oil to be trapped
in the evaporator . Oil may be lost with the refriger-
ant at point of leakage. Both of these can cause com-
pressor seizure.
Oil circulates in the globules with the vapor. It
leaves the compressor by the action of the pistons
and mixes with the refrigerant liquid in the con
denser . The oil then enters the evaporator with the
liquid and, with the evaporator properly flooded, is
returned to the compressor through the low pressure
line. Some of the oil returns as globules in the vapor
Figure 22-Control Panel Components
but more importantly, it is swept as a liquid along the
walls of the tubing by the velocity of the vapor. If the
evaporator is starved, the oil cannot return in suffi-
cient quantities to keep the compressor properly lu-
bricated .
High Temperature and Pressure
An increase in temperature causes an increase in
pressure. This accelerates chemical instability in
clean systems. Other results are brittle hoses, "O"
ring gaskets, and by-pass valve diaphragms with
possible decomposition, broken compressor dis-
charge reeds, and seized compressor bearings .
A fundamental law of nature accounts for the
fact that when a substance, such as a refrigerant, is
increased in temperature, its pressure is also in-
creased.
Any chemical reactions caused by contaminants
already in the system are greatly accelerated as the
temperature increases. A 15°F . (9 .4°C.) rise in tem-
perature doubles the chemical action .
While temperature alone can cause the synthetic
rubber parts to become brittle and possibly to decom-
pose, the increased pressure can cause them to rup-
ture or blow .
As the temperature and pressure increases, the
stress and strain on the discharge reeds also in-
creases. This can result in broken reeds. Due to the
effect of the contaminants caused by high tempera-
ture and pressure, compressor bearings can be
caused to seize.
High temperature and pressure is also caused by
air in the system .
Air in the System
Air results from a discharged system or careless
servicing procedures . This reduces system capacity
and efficiency and causes oxidation of oil into gum
and varnish.
When a leak causes the system to become dis-
charged, the resulting vacuum within the system will
cause air to be drawn in . Air in a system is a non
condensable gas and will build up in the condenser
as it would in an air compressor tank. The resultant
heat produced will contribute to the conditions dis-
cussed previously .
Many systems are contaminated and also re-
duced in capacity and efficiency by careless servicing
procedures .
Too frequently, systems which have been open to
the atmosphere during service operations have not