Thermo King SB-190 30, Manual

Page: 68 / 119

Refrigeration Maintenance (Rev. 12/03)
58
Testing for an Overcharge
Use the following procedure to identify a Thermo King unit
with an excessive refrigerant charge:
1. Install a calibrated gauge manifold on the compressor.
2. Use the Service Test Mode to run the unit in high speed
cool. Refer to the appropriate Operation and Diagnosis
Manual for specific information about the Service Test
Mode.
3. Operate the unit in high speed cool long enough to sta-
bilize system pressures and reduce the box temperature
to approximately 60 F (16 C) or colder.
4. Observe discharge pressure and cover the condenser to
increase the discharge pressure approximately 75 to
100 psi (500 to 690 kPa) above observed pressure.
NOTE: If the liquid level in the receiver sight glass
drops during step #4, the unit is not overcharged and
it is not necessary to complete the procedure.
5. Remove the condenser cover to rapidly reduce dis-
charge pressure.
6. Observe the receiver tank sight glass and the unit dis-
charge pressure.
7. By the time the discharge pressure drops approximately
50 psi (345 kPa), the liquid level in the receiver tank
should drop.
a. When the discharge pressure stabilizes, the liquid
level will rise.
b. If the liquid level will not drop, the unit most likely
has an overcharge of refrigerant. The refrigerant
level should be adjusted.
To adjust the refrigerant level:
1. Stop the unit and remove some refrigerant with an
approved refrigerant recovery device.
2. Perform a refrigerant level check and repeat the over-
charge test.
3. If the liquid level is low, add refrigerant as follows:
a. Connect a refrigerant tank to the gauge manifold
service line and purge the line.
b. Mid seat the compressor suction service valve.
c. Set the refrigerant tank for liquid removal and open
the hand valve.
d. Operate the unit in high speed cool.
e. Observe the suction pressure and slowly open the
gauge manifold low pressure hand valve to allow
liquid refrigerant to flow into the compressor suc-
tion service valve.
f. Control the liquid flow so the suction pressure
increases approximately 20 psi (138 kPa).
g. Maintain a discharge pressure of at least 275 psi
(1896 kPa) while adding refrigerant.
h. Close the hand valve on the refrigerant tank when
the liquid level approaches the top of the receiver
sight glass.
4. Repeat the overcharge test.
Moisture Indicating Sight Glass
The receiver tank is equipped with a moisture indicating
sight glass. The outer edge of the sight glass has a colored
ring approximately 0.1 in. (2.5 mm) thick. The color of the
ring indicates the moisture content of the refrigerant, but it
is not completely reliable.
Green = Dry
Chartreuse = Caution
Yellow = Wet