Service manual

4-8T-294-01
4.4.6 Defrost Mode
Refer to section 3. 1. 4 (Code 27) for description of t he
defrost interval selector and automatic defrost
initiation.
The defrost cycle consists of two distinct sub-cycles.
The first sub-cycle is the de-ice cycle, the second is a
probe check cycle.
NOTE
Aresetofthesteppermotorsuctionmodulation
valve (SMV) is performed at the beginning of
the de-ice cycle.
DefrostmaytakeplaceanytimetheDTSallows,andno
shutdown alarms are active. With these conditions
satisfied, defrost is initiated when one of the following
conditions becomes true:
a. The manual defrost switch (MDS) is closed by the
user. Refer to F i gure 2-6 for location. The MDS is
ignored during Pre-Trip.
b. The defrost interval timer reaches or exceeds the
defrost interval selected and set by the use r.
c. During Pre-Trip (auto, not manual) defrost can
occur during the advanced Pre-Trip tests P-8 and
P-10. Defrost is forced during advanced Pre-Trip
test P-9.
d. When the probe diagnostic logic determines that a
probe check is necessary based on the temperature
values currently reported by the supply and return
probes.
e. When bulb mode is active, and the defrost
terminationsensor(DTS)isbetween0_Cand10_C
(32_F and 50_F).
Whenthedefrostmodeis initiated(seeFigure 4-3),the
controller relay contacts (TH) close to supply power to
the heat contactor(HR)and in turn,energize thedefrost
heaters. The defrost light is illuminated.
Relay TC and TD open to de-energize the compressor
contactor and cool light. Also relay TN opens to
de-energize the condenser fan contactor (CF).
Relay TE and TV open to stop the evaporator fan
motors.
The in-range light remains illuminated during defrost.
When the coil tube sheet temperature reaches 25.6_C
(78_F), [4
_ C and 25.6 _ C (39.2 _ F and 78 _ F) if
configuredforandoperatinginbulbmode],thedefrost
termination sensor (DTS) causes the controller t o end
the defrost cycle and the unit returns to its normal
function.Undercertaincircumstances,defrostmayalso
be forced to terminate through special communication
commands.
Upon completion of the de-ice phase of defrost, the
controller will perform a probe check cycle. The
purposeoftheprobecheckcycleistoperformaperiodic
checkofthecontrollersensorstodetectmalfunctionsor
drift in the sensed temperature that is too small to be
detected by the normal sensor out of range tests. The
system will run for eight minutes in this condition. At
the end of the eight minutes, the primary supply,
primary return and DataCORDER sensortemperatures
will be compared. The controller probe alarms will be
set or cleared based on the conditions seen.
The54.5_C(130_F)heatterminationthermostat(HTT)
will open the circuit if the defrost mode does not
terminate at 25.6_C(78_ F). If termination does not
occur within 2.0 hours, the controller will terminate
defrost. An alarm will be given of a possible DTS
failure.
When the return air falls to 7_C(45_F), the controller
checks to ensure t he defrost termination sensor (DTS)
has dropped to 10_C or below. If it has not, a DTS
failure alarm is given and the defrost mode is operated
by the return t emperature sensor (RTS).
Snap Freeze Option:
NOTE
Controller configuration variable 33 must be
set t o SnAP to activate this option, refer to
Table 3-1.
If the probe check portion of defrost is required
(depending on the configuration of probe check), snap
freeze will run after the probe check cycle. Otherwise,
snap freeze will run immediately following the de-ice
portion of defrost.
The snap freeze cycle consists of running the
compressor without the evaporator fans running for a
period of four minutes with the stepper motor suction
modulation valve (SMV) fully open.
Ifcurrent limiting (seesection3.1.4, Cd32) or pressure
limiting activates during snap freeze, the state of the
SMV valve may change. Whenthe snap freeze cycleis
completed, defrost is formally terminated.
4.4.7 Arctic Mode
With arctic mode enabled, if the ambientis colderthan
--10.0_C there is a 30 minute time delay at startup for
any of t he components in the system, except for the
controller andthe compressorcrankcase heater (CCH),