Carrier 50CR - Manual (Page 28)

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Step 14—System Information
LOSS--OF--CHARGE SWITCH
The loss of charge switch is a protective device wired into control
circuit (low voltage). It shuts off the compressor if abnormally low
pressures are present in the refrigeration circuit.
NOTE: Because these switches are attached to refrigeration system
under pressure, it is not advisable to remove this device for
troubleshooting unless you are reasonably certain that a problem
exists. If switch must be removed, remove and recover all system
charge so that pressure gauges read 0 psi. Never open system
without breaking vacuum with dry nitrogen.
CHECK DEFROST THERMOSTAT
There is a liquid header with a brass distributor and feeder tube
going into outdoor coil. At the end of one of the feeder tubes, there
is a 3/8--in. OD stub tube approximately 3 in. long (See Fig. 27). The
defrost thermostat should be located on stub tube.
NOTE: There is only one stub tube used with liquid header, and on
most units it is the bottom circuit.
The defrost thermostat signals heat pump that conditions are right
for defrost or that conditions have changed to terminate defrost. It
is a thermally--actuated switch clamped to outdoor coil to sense its
temperature. Normal temperature range is closed at 30° ± 3°F and
open at 80° ± 5°F.
TROUBLESHOOTING
Refer to the Cooling and Heating Troubleshooting Chart (Table 7)
for troubleshooting information.
START--UP CHECKLIST
Use the Start--Up Checklist at the back of this manual.
Defrost
Sensor
Unit Top
Detail A
See
Detail A
A06068
Fig. 27 -- Defrost Sensor Location