Chevrolet 1977 light duty truck Service Manual page 56

Fig. IB 6- Compressor M ou n tin g - L6 Engine
T he receiver-dehydrator, used prim arily as a liquid
storage tank, also functions to trap minute quantities of
moisture and foreign material which may have rem ained
in the system after installation or service operations. A
re frig e ran t sight glass
d e h y d ra to r to be used as a quick check o f the state and
condition o f charge o f the entire system. The receiver-
d eh y d ra to r is mounted near the condenser.
S ig h t Glass (G and M o to r Hom e Chassis Models)
W hile having no real function to perform in the
system, the sight glass is a valuable aid in determ ining
w heth er or not the refrigerant charge is sufficient and for
elim inating some guess work in diagnosing difficulties.
The sight glass, is built into the receiver-dehydrator
outlet connection and is designed and located so that a
shortage of refrigerant at this point will be indicated by
the appearance o f bubbles beneath the glass. The dust
cap provided should be kept in place when the sight
glass is not in use.
T h erm o static Expansion Valve (Fig. IB -7 )
C-K Overhead, G C60 and O verhead and Motor
Home Chassis systems use a thermostatic expansion
valve in place o f a float system.
The valve consists prim arily o f the power element,
body, actuating pins, seat an d orifice. At the high
pressure liquid inlet, is a fine mesh screen which prevents
dirt, filings or other foreign m atter from entering the
valve orifice.
W hen the valve is connected in the system, high
pressure liquid refrigeran t enters the valve through the
screen from the receiver-dehydrator or condenser and
passes on to the seat an d orifice. Upon passing through
the orifice the high pressure liquid becomes low pressure
liquid. T he low pressure liquid leaves the valve and flows
into the e vaporator core where it absorbs heat from the
e vapo rato r core and changes to a low pressure vapor.
is bult into the receiver-
Fig. lB-7--Expansion Valve
and leaves the evaporator core as such. The power
element bulb is clamped to the low pressure vapor line
just beyond the outlet o f the evaporator (fig. IB-7).
The operation o f the valve is quite simple. It is a
matter o f controlling opposing forces produced by a
spring and the refrigerant pressures. For example: The
pressure in the power element is trying to push the seat
away from the orifice, while the adjusting spring is
trying to force the seat toward
opposing pressures are established in the design of the
valve so that during idle periods the adjusting spring
tension and the referigerant pressure in the cooling coil
are always greater than the opposing pressure in the
power element. Therefore, the valve rem ains closed.
W hen the compressor is started, it will reduce the
pressure and tem perature of the refrigerant in the
cooling coil to a point where the vapor pressure in the
power element becomes the stronger. The seat then
moves off the orifice and liquid starts to flow through the
valve orifice into the cooling coil.
The purpose o f the power element is to help
determ ine the quantity o f liquid that is being metered
into the cooling coil. As the tem perature of the low
pressure line changes at the blub, the pressure o f the
vapor in the power element changes, resulting in a
change o f the position o f the seat. For example if the
cooling coil gets more liquid than is required, the
tem perature o f the low pressure line is reduced and the
resultant lowering o f the bulb tem perature reduces the
pressure o f the vapor in the power element, allowing the
seat to move closer to the orifice. This immediately
reduces the am ount of liquid leaving the valve. Under
norm al operation, the power element provides accurate
control o f the quantity o f refrigerant to the cooling coil.
O U T L E T
the orifice. These