Thermo King MD-MT, Operator'S Manual

Page: 117 / 192

117
Refrigeration Service Operations
NOTE: It is generally good practice to replace
the filter-drier whenever the high side is opened
or when the low side is opened for an extended
period of time.
Commissioning Unit
The following procedure should be followed after
any of these conditions:
• The compressor has been replaced
• There has been a large oil loss in the system
• There has been a complete clean up or flush of
the refrigeration system.
1. Set both zone thermostats to their highest
setpoint.
If the ambient temperature is above set point
range, jump power from the 2 circuit to HGS
and RHGS to allow the valves to open.
2. Run the unit for ten minutes to allow for
wetting of the system.
3. Set both zones for Cool (remove jumpers if
installed in the previous step).
Low Side Pump Down
NOTE: Operate both units in Cool for 2 to 5
minutes with the service valves in their normal
operating positions before performing the low
side pump down.
1. Install a gauge manifold at the compressor.
2. Operate the Zone 1 and Zone 2 units in Cool,
front seat the receiver tank outlet valve, and
pump down the low side to 20 to 25 in. Hg
vacuum (-68 to -85 kPa). Turn the Zone 1
On/Off switch Off.
If the unit pumps down acceptably and then
holds at least a 15 in. Hg vacuum (-51 kPa) for
2 minutes or more, it can be assumed that the
receiver tank outlet valve, the compressor
discharge valve plates, the HGS solenoid, the
RHGS solenoid, and the purge valve solenoid
are sealing properly.
3. Prepare to perform service on the unit’s low
side by equalizing the high side and low side
pressures.
a. Disconnect the purge valve solenoid wires
from the main wire harness.
b. Keep the Zone 1 thermostat set for Cool,
turn the Zone 1 On/Off switch On. Set the
Zone 2 thermostat for Heat and leave the
Zone 2 On/Off switch On for 1/2 second.
The gauge manifold’s high pressure gauge
will indicate a pressure decrease and the
low pressure gauge will indicate a pressure
increase when the RHGS solenoid opens.
c. Turn the Zone 2 On/Off switch Off. Set
the Zone 1 thermostat for Heat and leave
the Zone 1 On/Off switch On for 1/2
second. The gauge manifold’s high
pressure gauge will indicate a pressure
decrease and the low pressure gauge will
indicate a pressure increase when the HGS
solenoid opens.
d. Set both the thermostats for Heat and turn
both of the On/Off switches On. The
manifold’s high and low pressure gauges
will equalize.
NOTE: If the box temperatures are high, the
HGS and RHGS solenoids must be energized
with a test lead.
NOTE: Repeat the pump down procedure if
the pressures equalize above 20 psig
(138 kPa). If suitably low pressures cannot
be achieved after the third pump down, the
refrigerant must be recovered to perform
service on the low side.
e. If the reading on the gauge manifold’s
high pressure gauge increases after the
high and low side pressures have been
equalized, the condenser inlet check valve
is leaking.
f. Remember to reconnect the purge valve
solenoid wires to the main wire harness
when the service procedures have been
completed.