T -304
5--4
Change 07/09
The presence of moisture in a refrigeration system can
have many undesirable effects. The most common are
copper plating, acid sludge formation, “freezing-up” of
metering devices by free water, and formation of acids,
resulting in metal corrosion. An evacuation should take
place after a system repair (replacement of filter drier.
expansion valve, solenoid valve, etc).
5.7.2 Preparation
NOTE
Using a compound gauge (manifold gauge) for
determination of vacuum level is not recom-
mended because of its inherent inaccuracy.
a. Evacuateanddehydrateonlyafterpressureleaktest.
(Refer to paragraph 5.6)
b. Essential tools to properly evacuate and dehydrate
anysystem includeagoodvacuumpumpwithamini-
mumof5cfm(8.5m
3
/hr)volumedisplacement,(CTD
P/N 07-00176-11), and a good vacuum indicator
(CTD P/N 07--00414--00).
c. Keep the ambient temperature above 60°F (15.6°C)
to speed evaporation of moisture. If ambient temper-
ature is lower than 60°F (15.6°C), ice may form be-
fore moisture removal is complete.
5.7.3 Procedure for Evacuation and Dehydrating
System
a. Remove refrigerant using a refrigerant recovery sys-
tem. Refer to paragraph 5.5.2
b. Therecommendedmethodisconnecting3/8”OD re-
frigerant hoses designed for vacuum service as
shown in Figure 5--3.
c. Make sure vacuum pump valve is open.
d. Start vacuum pump. Slowly open valves halfway and
then open vacuum gauge valve.
e. Evacuate unit until vacuum gauge indicates 500 mi-
crons Hg vacuum. Close gauge valve, vacuumpump
valve, and stop vacuum pump.
f. Close off pump valve, and stop pump. W ait five min-
utes to see if vacuum holds.
g. Charge system. Refer to paragraph 5.8.2
5.8 ADDING REFRIGERANT TO SYSTEM
5.8.1 Checking Refrigerant Charge
The following conditions must be met to accurately
check the refrigerant charge.
a. Bus engine operating at high idle.
b. Unit operating in cool mode for 15 minutes.
c. Compressor discharge pressure at least 150 psig
(10.21 bar). (It may be necessary to block condenser
air flow to raise discharge pressure.)
NOTE
Ideal charging conditions are with ambient
above 86°F(30°C) and interior vehicle temper-
ature above 77°F(25°C). Charging to a full
sight glass at lower temperatures may lead to
system overcharge.
d. Under the above conditions, the system is properly
chargedwhen theliquidline sight glaseshowsfull(no
bubbles present).
5.8.2 Adding Full Charge
a. Install manifold gauge set at the in--line suction and
discharge service ports.
b. Evacuateanddehydratesystem.(Refertoparagraph
5.7)
c. Place appropriate refrigerant cylinder on scales. Pre-
pare to charge liquid refrigerantby connectingcharg-
inghosefrom containertocenterconnection ongage
manifold. Purge air from hoses.
d. Note weight of refrigerant and cylinder.
e. Open cylinder valve, backseat discharge valve on
gaugemanifoldandallowliquidrefrigeranttoflowinto
the high side of the system
f. When correct charge has been added (refer to para-
graph 1.3, refrigerant specifications), close cylinder
valve and frontseat manifold discharge valve.
g. Preparethecylinder asrequiredtoallowvaporcharg-
ing. Backseat the manifold suction valve and charge
vapor until the correct charge hasbeen added. Close
cylinder valve and frontseat suction manifold set.
h. Check charge level in accordance with the proce-
dures of paragraph 5.8.1.
5.9 CHECKING FOR NONCONDENSIBLES
To check for noncondensibles, proceed as follows:
a. Stabilize system to equalize pressure between the
suction and discharge side of the system.
b. Check temperature at the condenser and receiver.
c. Check pressure at the discharge (in--line) service
port.
d. Check saturation pressure as it corresponds to the
condenser/receiver temperature. See temperature--
Pressure chart Table Table 5--1. for R134a.
e. If gauge reading is 3 psig or more than the calculated
P/T pressure in step d., noncondensables are pres-
ent.
f. Remove refrigerant using a refrigerant recovery sys-
tem.
g. Evacuate and dehydrate the system. (Refer to para-
graph 5.7.)
h. Charge the unit. (Refer to paragraph 5.8.2.)
5.10 CHECKING AND REPLACING HIGH OR LOW-
PRESSURE CUTOUT SWITCH
5.10.1 Replacing High Or Low Pressure Switches
a. The high and low pressure switches are equipped
withschrader valves to allowremoval andinstallation
without recovering the refrigerant charge.
b. Disconnect wiring from defective switch.
c. Install new cutout switch after verifying switch set-
tings.
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