High Pressure Relief Valve; Low Pressure Cycling Clutch Switch; Refrigerant; Refrigerant Line - Jeep Wrangler 1999 Electronic Service Manual

24 - 8 HEATING AND AIR CONDITIONING
DESCRIPTION AND OPERATION (Continued)
close causing the PCM to turn the compressor clutch
on and off. This prevents compressor operation when
the discharge line pressure approaches high levels.
The high pressure cut-off switch contacts are open
when the discharge line pressure rises above 3100 to
3375 kPa (450 to 490 psi). The switch contacts will
close when the discharge line pressure drops to 1860
to 2275 kPa (270 to 330 psi).
The high pressure cut-off switch is a factory-cali-
brated unit. The switch cannot be adjusted or
repaired and, if faulty or damaged, it must be
replaced.

HIGH PRESSURE RELIEF VALVE

A high pressure relief valve is located on the com-
pressor cylinder head, which is at the rear of the
compressor. This mechanical valve is designed to
vent refrigerant from the system to protect against
damage to the compressor and other system compo-
nents, caused by condenser air flow restriction or an
overcharge of refrigerant.
The high pressure relief valve vents the system
when a discharge pressure of 3445 to 4135 kPa (500
to 600 psi) or above is reached. The valve closes
when a minimum discharge pressure of 2756 kPa
(400 psi) is reached.
The high pressure relief valve vents only enough
refrigerant to reduce the system pressure, and then
re-seats itself. The majority of the refrigerant is con-
served in the system. If the valve vents refrigerant, it
does not mean that the valve is faulty.
The high pressure relief valve is a factory-cali-
brated unit. The valve cannot be adjusted or
repaired, and must not be removed or otherwise dis-
turbed. The valve is only serviced as a part of the
compressor assembly.

LOW PRESSURE CYCLING CLUTCH SWITCH

The low pressure cycling clutch switch is located
on the top of the accumulator. The switch is screwed
onto an accumulator fitting that contains a Schrader-
type valve, which allows the switch to be serviced
without discharging the refrigerant system. The
accumulator fitting is equipped with an O-ring to
seal the switch connection.
The low pressure cycling clutch switch is connected
in series electrically with the high pressure cut-off
switch, between ground and the Powertrain Control
Module (PCM). The switch contacts open and close
causing the PCM to turn the compressor clutch on
and off. This regulates the refrigerant system pres-
sure and controls evaporator temperature. Control-
ling the evaporator temperature prevents condensate
water on the evaporator fins from freezing and
obstructing air conditioning system air flow.
The low pressure cycling clutch switch contacts are
open when the suction pressure is approximately 141
kPa (20.5 psi) or lower. The switch contacts will close
when the suction pressure rises to approximately 234
to 262 kPa (34 to 38 psi) or above. Lower ambient
temperatures, below approximately -1° C (30° F), will
also cause the switch contacts to open. This is due to
the pressure/temperature relationship of the refriger-
ant in the system.
The low pressure cycling clutch switch is a factory-
calibrated unit. It cannot be adjusted or repaired
and, if faulty or damaged, it must be replaced.

REFRIGERANT

The refrigerant used in this air conditioning sys-
tem is a HydroFluoroCarbon (HFC), type R-134a.
Unlike R-12, which is a ChloroFluoroCarbon (CFC),
R-134a refrigerant does not contain ozone-depleting
chlorine. R-134a refrigerant is a non-toxic, non-flam-
mable, clear, and colorless liquefied gas.
Even though R-134a does not contain chlorine, it
must be reclaimed and recycled just like CFC-type
refrigerants. This is because R-134a is a greenhouse
gas and can contribute to global warming.
R-134a refrigerant is not compatible with R-12
refrigerant in an air conditioning system. Even a
small amount of R-12 added to an R-134a refrigerant
system will cause compressor failure, refrigerant oil
sludge or poor air conditioning system performance.
In addition, the PolyAlkylene Glycol (PAG) synthetic
refrigerant oils used in an R-134a refrigerant system
are not compatible with the mineral-based refriger-
ant oils used in an R-12 refrigerant system.
R-134a refrigerant system service ports, service
tool couplers and refrigerant dispensing bottles have
all been designed with unique fittings to ensure that
an R-134a system is not accidentally contaminated
with the wrong refrigerant (R-12). There are also
labels posted in the engine compartment of the vehi-
cle and on the compressor identifying to service tech-
nicians that the air conditioning system is equipped
with R-134a.

REFRIGERANT LINE

The refrigerant lines and hoses are used to carry
the refrigerant between the various air conditioning
system components. A barrier hose design with a
nylon tube inner hose liner is used for the R-134a air
conditioning system on this vehicle. This nylon liner
helps to further contain the R-134a refrigerant,
which has a smaller molecular structure than R-12
refrigerant. The ends of the refrigerant hoses are
made from lightweight aluminum or steel, and use
braze-less fittings.
Any kinks or sharp bends in the refrigerant plumb-
ing will reduce the capacity of the entire air condi-
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