Refrigeration Maintenance (Rev 1/99)
55
Run On Electric Operation
c. Turn the unit OFF and disconnect the electric
power supply. Start the truck and run the unit in
cool on engine operation for a minimum of 15
minutes.
17. Check the receiver tank sight glass with the unit running
in cool, the box temperature at 0 F (-20 C), and a head
pressure of at least 180 psi (1241 kPa) for R-134a sys-
tems and 275 psi (1896 kPa) for R-404A systems. The
ball should be at the top of the sight glass.
If not, open the valve on the refrigerant drum to deliv-
ery liquid and open the low side valve on the gauge
manifold. Add refrigerant liquid until the ball in the
receiver tank sight glass reaches the top of the sight
glass, then close the valve on the refrigerant drum and
close the low side valve on the gauge manifold.
18. Stop the unit, shut OFF the truck engine and remove the
gauge manifold set.
19. The above conditions MUST be established each time
the refrigerant level is checked or if refrigerant needs to
be added for any reason.
NOTE: To prevent oil migration from one compressor to
another, and for proper oil return when a compressor is
operating, operate a compressor for a minimum of 15 min-
utes. Do not operate the compressor for shorter intervals.
Checking the Refrigerant Charge
If the unit has an insufficient charge of refrigerant, the evap-
orator will be “starved” and the box temperature will rise
even though the unit is operating. Also, an insufficient
charge does not circulate enough oil to properly lubricate
the compressor. The charge may be determined by inspec-
tion of the refrigerant through the receiver tank sight glass
with the following conditions established:
Testing the Refrigerant Charge with an Empty Box
1. Place a test box over the evaporator.
2. Install a gauge manifold set.
3. Run the unit in cool on engine operation until the ther-
mometer reads 0 F (-18 C). By allowing the box to leak
a small amount, you will be able to maintain 0 F (-18 C).
4. The discharge or head pressure gauge should be at least
180 psi (1241 kPa) on the gauge manifold for R-134a
systems and 275 psi (1896 kPa) for R-404A systems.
If the pressure is below this, it can be raised by cover-
ing a portion of the condenser grille with a piece of
cardboard.
5. Under these conditions the ball should be at the top of
the sight glass to indicate a full charge.
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