Lennox Merit Series 14HPX Installation Instructions Manual page 23

DEFROST SYSTEM SENSORS
Sensors connect to the defrost board through a
field-replaceable harness assembly that plugs into the
board. Through the sensors, the board detects outdoor
ambient, coil, and discharge temperature fault conditions.
As the detected temperature changes, the resistance
across the sensor changes. Figure 14 shows how the
resistance varies as the temperature changes for both
type of sensors. Sensor resistance values can be checked
by ohming across pins shown in table 12.
NOTE − When checking the ohms across a sensor, be
aware that a sensor showing a resistance value that is not
within the range shown in table 12, may be performing as
designed. However, if a shorted or open circuit is detected,
then the sensor may be faulty and the sensor harness will
needs to be replaced.
AMBIENT
SENSOR
Figure 13. Sensor Locations
Sensor
Outdoor
(Ambient)
Coil
Discharge (if
applicable)
Note: Sensor resistance decreases as sensed temperature increases
(see figure 14).
Coil Sensor The coil temperature sensor (shown in
figure 13) considers outdoor temperatures below −35°F
(−37°C) or above 120°F (48°C) as a fault. If the coil
temperature sensor is detected as being open, shorted or
Page 23
Table 12. Sensor Temp. / Resistance Range
Temperature
Range °F (°C)
−35 (−37) to 120
(48)
−35 (−37) to 120
(48)
24 (−4) to 350
(176)
Resistance values Pins/Wire
range (ohms) Color
280,000 to 3750 3 & 4
(Black)
280,000 to 3750 5 & 6
(Brown)
41,000 to 103 1 & 2
(Yellow)
14HPX SERIES