Electrical Maintenance (Rev 6/98)
45
Defrost Components
Manual Defrost Switch
The manual defrost switch is located on the control panel.
Pressing the manual defrost switch initiates a defrost cycle
when the evaporator coil sensor temperature is below 7.2° C
(45° F).
Evaporator Coil Sensor
The evaporator coil sensor is mounted in the evaporator and
senses the evaporator coil temperature to control defrost. If
the evaporator coil temperature is below 7.2° C (45° F), a
defrost cycle may be initiated by the microprocessor, the air
switch, or the manual defrost switch. The microprocessor
terminates the defrost cycle when the evaporator coil tem-
perature rises to 13.9° C (57° F).
Air Switch
The air switch automatically places the unit on defrost when
ice accumulation on the evaporator coil builds up to a point
where the air flow across the coil is restricted.
Restricted air flow results in a pressure difference between
the evaporator coil inlet and outlet. The air switch senses the
pressure differential across the coil and initiates the defrost
cycle.
The air switch is preset at the factory to 35.6 ± 2mm (1.4 ±
0.08”) H
2
O. Normally readjustment is not necessary unless
the switch has been tampered with or does not function
properly due to factors affecting air circulation, such as
bulkhead construction and duct work.
Microprocessor Temperature Controller
Automatic Defrost Initiation By Defrost Timer
An internal defrost timer in the microprocessor automati-
cally places the unit on defrost. The defrost intervals are
every 4 hours when the trailer temperature is not in-range,
and every 6 hours when the temperature is in-range.
NOTE: If the return air sensor temperature goes out of
range during unit operation, defrost cycles again are initi-
ated on the 4-hour interval. Even if the return air sensor
temperature returns to in-range in just 2-3 minutes, the
next defrost cycle will be initiated based on the 4-hour
interval.
The defrost interval will then return to the 6-hour in-
range defrost interval as long as the return air sensor tem-
perature remains in-range. However, the microprocessor
will not respond to an out-of-range condition for 45 min-
utes after the termination of a defrost cycle.
Automatic Defrost Termination By Evaporator Coil
Sensor
When the evaporator coil sensor temperature rises to 13.9°
C (57° F), the microprocessor terminates defrost.
Automatic Defrost Termination By Defrost Timer
The defrost timer in the microprocessor will terminate
defrost after 30 or 45 minutes if the evaporator coil sensor
temperature has not risen to 13.9° C (57° F). The defrost
timer will terminate a defrost cycle initiated by the manual
defrost switch, the air switch or the microprocessor. The
microprocessor will also record an alarm.
NOTE: If the evaporator coil sensor fails during the
defrost mode, the microprocessor will terminate defrost
after 30 minutes.
Defrost Cycle
The defrost cycle may be initiated by the manual defrost
switch, the air switch or the microprocessor when the evap-
orator coil temperature is below 7.2° C (45° F). Immedi-
ately upon initiating defrost, the three-way valve should
shift to heat, the damper should close, the unit should run on
low speed and the defrost light should come on.
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