Outdoor Air Thermistor (Oat); Return Gas Thermistor (Rgt); Thermistor Well Assembly; Replacing Thermistors - Carrier 38APS025 Manual

TROUBLESHOOTING — To perform a thermistor-tempera-
ture sensor check, a high quality digital volt-ohmmeter is re-
quired.
1. Connect the digital voltmeter across the appropriate
thermistor terminals at the J8 terminal strip on the main
base board or J6 terminal strip of the AUX Board. See
Fig. 184 and 185.
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 73-77.
3. To check thermistor accuracy, measure temperature at
probe location with an accurate thermocouple-type tem-
perature measuring instrument. Insulate thermocouple to
avoid ambient temperatures from influencing reading.
Temperature measured by thermocouple and temperature
determined from thermistor voltage reading should be
close, ± 5° F (3° C), if care was taken in applying thermo-
couple and taking readings.
If a more accurate check is required, unit must be shut down
and thermistor removed and checked at a known temperature
(freezing point or boiling point of water) using either voltage
drop measured across thermistor at the J8 terminal, by deter-
mining the resistance with unit shut down and thermistor dis-
connected from J8. Compare the values determined with the
value read by the control in the Temperatures mode using the
scrolling marquee display.
REPLACING THERMISTORS — Special instructions for
replacing the return gas thermistor (RGT), Outdoor air thermis-
tor (OAT), and discharge temperature thermistor (DTT) are
listed below. All other thermistors can be replaced simply by
replacing the old with the new.
CAUTION
Be sure to route wiring away from all refrigerant piping.
Heat from the piping can damage wiring.
Discharge Temperature Thermistor (DTT) —This thermis-
tor is only used when the unit is equipped with a digital com-
pressor. Mount the thermistor in the appropriate location. Once
in place and wired, insulate the device with cork insulation tape
to form a watertight seal around the thermistor and minimize
the ambient influence on the sensor. See Fig. 45 on page 39.
Outdoor Air Thermistor (OAT) — This thermistor for
38APD/APS025-060 units is mounted close to the base of the
unit. It must be mounted off the base rail for an accurate out-
door ambient temperature. See Fig. 46 on page 39 for mount-
ing dimensions.
Return Gas Thermistor (RGT) — Return gas thermistors are
mounted in thermistor wells. The well assembly consists of
three parts, the thermistor well body, seal disc and nut. See
Fig. 186. Remove seal disc and nut from the existing thermis-
tor and install on the new thermistor. Before introducing a
thermistor into the well body, add a small amount of thermal
conductive grease to the thermistor well and end of probe. In-
sert the probe into well. Tighten the retaining nut
finger tight using a back-up wrench. Be sure to loop cable and
secure with a wire tie to create a strain relief as shown in
Fig. 48 on page 40. Once in place and wired, insulate the de-
vice with cork insulation tape, similar to Presstite, to form a
watertight seal around the thermistor and minimize the ambient
influence on the sensor.
Fig. 186 — Thermistor Well Assembly
156
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1
4
NUT
SEAL DISC
THERMISTOR
WELL BODY
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