Thermo King MD-MT, Operator'S Manual

Page: 110 / 192

Refrigeration Maintenance
110
3. Raise the discharge pressure of the
compressor by blocking the condenser coil air
flow by covering the roadside condenser grille
with a piece of cardboard.
NOTE: The discharge pressure should never
be allowed to exceed a pressure of 450 psig
(3103 kPa) on R-404A and R-502 systems.
4. Failure of the HPCO system to stop
compressor operation should be investigated
first by checking the control circuit operation
and secondly by HPCO switch replacement.
Compressor Pump Down and
Checkout
1. Install a gauge manifold set on the
compressor. Open the service valves (crack
the valves off of the back seated position) to
access system pressure.
2. Adjust the thermostat setpoint well below
compartment temperature and operate the unit
in Cool until the temperature is stabilized (at
least 5 minutes).
3. Close (front seat) the suction service valve.
Pump the compressor down into a deep
vacuum (25 in. Hg [-85 kPa] vacuum). If the
suction pressure does not rise above 10 psig
(69 kPa) in 2 minutes, perform check
procedures on the three-way valve and the
bypass check valve. If the pressure does rise,
go to step 4.
4. Front seat discharge service valve and
equalize the compressor to 1 to 2 psig (7 to
14 kPa). Disassemble and inspect the
discharge valve plates.
Evacuating and Charging
Procedures
Set-up Electrical Controls
1. Remove the 20A wire from the engine low oil
pressure switch.
2. Remove the CYCLE-SENTRY module from
the condensing unit control box.
3. Disconnect the condenser inlet solenoid wires
(2 pin connector) from the main wire harness.
4. Disconnect the HGSA and RHGS wires from
the multi-temp relay board.
5. Place jumpers from the HGSA and RHGS
wires to the 2A circuit in the control box
(circuit breaker CB2).
6. Turn the Zone 1 and Zone 2 unit On/Off
switches On.
7. Set both thermostats to Cool.
NOTE: The electrical controls should
remain set-up like this throughout the
procedure. It may be necessary to use a
battery charger. Make sure that the battery
voltage is sufficient to keep the solenoids
energized.
Set-up and Test Evacuation
Equipment
1. Connect a 2-stage vacuum pump equipped
with a vacuum gauge to the suction service
valve, the discharge service valve, and the
receiver outlet valve.
2. Back seat the suction service valve, the
discharge service valve, and the receiver
outlet valve.
3. Open the vacuum valve at the vacuum pump
and start the pump.
4. Evacuate the hoses to 100 microns, or the
lowest achievable level below 500 microns.
WARNING: Any time the discharge valve
is front seated, disconnect the unit battery
or power source to prevent accidental
compressor start-up.