Thermo King MD-MT, Operator'S Manual

Page: 64 / 192

Electrical Maintenance
64
Defrost System
Each evaporator should be run through a Defrost
cycle during After Start Inspections and during
scheduled maintenance inspections to test the
defrost system components. To check the Defrost
cycle, run the evaporator on Cool and set the
thermostat to drop the evaporator coil temperature
below 42 F (6 C). Press the
M
ANUAL
D
EFROST
key.
The evaporator should shift from the Cool cycle
to the Defrost cycle.
If the evaporator continues to Cool, double check
the evaporator coil temperature and refer to the
Defrost Cycle Checkout Procedure.
The air switch setting should be checked annually.
Refer to “Air Switch Testing and Adjustment” on
page 70.
Zone 1 Evaporator Defrost Cycle
Diesel Operation
The Defrost cycle can be initiated with the
M
ANUAL
D
EFROST
key, by the air switch, or by the defrost
timer in the TG-V Controller when the defrost
termination switch is closed.
Initiating the Defrost cycle energizes the defrost
relay, which energizes the HGS circuit and the 29
circuit, and opens the LLS-FER circuit. The HGS
circuit energizes the HGS, LLS, and CIS (and the
RTPS and PV when the RTPC is closed). The 29
circuit causes the TG-V to energize the 2K relay,
which shifts the condensing unit into high speed.
Opening the LLS-FER circuit de-energizes the
evaporator fans.
The CIS and HGS divert heated refrigerant vapor
to the evaporator coil, melting frost.
De-energizing the evaporator fans keeps the heat
in the evaporator and prevents warm air from
passing over the load.
NOTE: The unit will not Defrost during normal
unit operation unless the Defrost cycle is
initiated while the evaporator temperature is
below 42 F (6 C).
The defrost termination switch will open and
de-energize the defrost relay when the frost has
melted and the evaporator temperature rises above
52 F (11 C). The TG-V Controller then determines
the mode of operation.
Zone 1 Evaporator Defrost Cycle
Electric Operation
The Defrost cycle on electric standby operation is
similar to the Defrost cycle on diesel operation.
CAUTION: This unit is equipped with
Auto Start. It will start automatically if the
O
N
key is pressed while the On/Off
switches are in the On position.
CAUTION: CYCLE-SENTRY Equipped
Units—Place the Auto Start/Stop -
Continuous Run switch in the Cont Run
position before performing any diagnosis
and service operations on the unit. The
unit may start at any time without prior
warning if the
O
N
key has been pressed
while the On/Off switches are in the On
position and the Auto Start/Stop -
Continuous Run switch is in the
CYCLE-SENTRY position.