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4-7
WARNING
Do not use a nitrogen cylinder without a
pressure regulator. Cylinder pressure is approx-
imately 2350 psi (165 kg/cm
@
). Do not use oxygen
in or near a refrigeration system as an explosion
may occur.
d. Back-off regulator adjustment completely. Open
the cylinder valve.
e. Slowly open the regulator valve to increase the
pressure to the applicable pressures listed in Table 4-1 to
open or close the switch. If the ohmmeter reading does
not correspond with the pressure listed in Table 4-1, the
switch is defective and should be replaced.
f. Close cylinder valve and release the pressure
through the bleed-off valve. As the pressure drops, the
applicable switch will open or close. If the ohmmeter
reading does not correspond with the pressure listed in
Table 4-1, the switch is defective and should be replaced.
Table 4-1. Pressure Switch Continuity Check
Ohmmeter Reading
Switch
No Continuity-
Switch Opens at:
Continuity-
Switch Closes at:
High Pressure (HPS)
425
10 psig
300
10 psig
Low Pressure (LPS)
10
3 psig
25
3 psig
Cond. Fan Speed
(CFS)
250
15 psig
190
15 psig
Unloader Press. #2
(UPS)
40
5 psig
23
3 psig
4.12 THERMOSTATIC EXPANSION VALVE
The thermal expansion valve is an automatic device
which maintains constant superheat of the refrigerant gas
leaving the evaporator regardless of suction pressure.
The valve functions are: (a) automatic response of
refrigerant flow to match the evaporator load and (b)
prevention of liquid refrigerant entering the compressor.
Unless the valve is defective, it seldom requires any
maintenance.
a. Replacing the Expansion Valve
1. Pump down the unit. (Refer to section 4.4)
2. Remove left return air access panel.
3. Remove insulation (Presstite) from expansion
valve bulb and remove from suction line.
4. Loosen flare nut and disconnect equalizer line
from expansion valve.
5. Remove flange screws and lift off power head and
cage assemblies. Check for foreign material in valve
body.
6. Install new gaskets and assemble new cage and
power head assemblies.
7. Attach the sensor bulb just below center of the
suction line (4 or 7 o’clock position viewing from cross
section to the suction line, see Figure 4-8). This area must
be clean to ensure positive bulb contact. Do not insulate
the bulb until the superheat is measured.
8. Fasten equalizer tube to expansion valve.
9. Evacuate by placing vacuum pump on the
compressor suction service valve port or suction drop
tube port (located in the engine compartment).
10. Open the inlet service valve to the filter-drier.
Check refrigerant level. (Refer to paragraph 4.5.)
11. Check superheat.
1
2
3
4
5
6
7
8
9
11
8
10
1. Power Head
2. Cap Seal
3. Flare Seal
4. Retaining Nut
5. Adjusting Stem
6. Equalizer
Connection
7. Sensor Bulb
8 Gasket
9. Cage Assembly
10. Body Flange
11. Capscrew
Figure 4-7. Thermostatic Expansion Valve
b. To Measure Superheat
NOTE
When conducting this test the suction pressure
must be at least 6 psig (.42 kg/cm
@
) below the
expansion valve maximum operating pressure
(MOP). Refer to section 1.2.d for applicable
valve settings.
1. Remove insulation from sensor bulb and suction
line if installed..
2. Loosen one bulb clamp and make sure area under
clamp is clean.
3. Place the temperature thermocouple above
(parallel) TXV bulb and tighten loosened clamp making
sure both bulbs are firmly secured to suction line as shown
in Figure 4-8. Place insulation around TXV bulb and
thermocouple.
4. Connect suction gauge to the service port located
on the suction line near the valve.
5. Set temperature controller to it’s coolest setting.
Run unit for at least 20 minutes to stabilize the system
6. Using the temperature/pressure chart (Table 4-2
or Table 4-3) for the applicable refrigerant used,
determine the saturation temperature corresponding to
the pressure taken at the suction service valve.