12
2. If theexpected result is not achieved,remove the solenoid
plugfrom the compressorand with the unit running and the
User Interface or thermostat calling for hig h stage, test the
voltageoutput at the plug with a DC voltmeter. The read-
ingshould be 24 volts DC.
3. If the correct DC voltage is at the control circuit molded
plug,measure the compressorunloader coil resistance. The
resistanceshould be 32 to 60 ohms depending on com-
pressor temperature. If the coil resistance is infinity,mu ch
lower than32 ohms, or isgrounded, the compressor must
be replaced.

TemperatureThermistors

Thermistors are electronic devices which sense tem perature. Asthe
temperature increases, the resistance decreases. Thermistors are
used to sense outdoor air (OAT) and coil temperature(OCT).
Refer to Fig. 7 for resistance values versus temp erature.
0
10
20
30
40
50
60
70
80
90
0
(-17.77)
20
(-6.67)
40
(4.44)
60
(15.56)
80
(26.67)
100
(37.78)
120
(48.89)
TEMPERATURE °F (°C)
RESISTANCE (KOHMS)
THERMISTOR CURVE
A08054
Fig. 7 --R esistance Values VersusTemperatu re
If the outdoor air or coilthermistor should fail, the controlwill
flashthe appropriatefault code. (See Table4.)
IMPORTANT: The outdoor airthermistor and coil thermistor
shouldbe factory mounted inthe final locations. Ch eck to ensure
thermistors are mounted properly per Fig . 8 and Fig. 9.

ThermistorSensor Comparison

The control continuouslymonitors and compares the outdoor air
temperaturesensor and outdoor coil temperaturesensor to ensure
properoperating conditions. The comparison is:
SIncooling if the outdoor air sensorindicates ≥10_F
(≥5.6_C)warmer than the coilsensor (or) the outdoor airsensor
indicates ≥20_F(≥11_C)cooler than the coil sensor, the
sensorsare out of range.
SInh eating if theoutdoor air sensor indicates≥35_F(≥19.4_C)
warmerthan the coilsensor (or) the outdoor airsensor indicates
≥10_F(≥5.6_C)co oler than the coil sensor, the senso rs are out
ofrange.
Ifthe sensors are out of range, the control willflash the appropriate
faultcode as shown in Table4.
The thermistorcomparison is not performedduring low ambient
coolingor defrost operation.

FailedThermistor Default Operation

Factory defaults havebeen provided in the event of failure of
outdoor air thermistor (OAT) and/or outdoor coil thermistor
(OCT).
If the OATsensor should fail, low ambient cooling will not be
allowedand the one--minuteoutdoor fan off delay will not occur.
Defrost will be initiated based on coil temperature and time.
If the OCT sensor shouldfail, low ambient cooling will not be
allowed. Defrost will o ccur at each time interval during heating
operation,but will terminateafter 5 minutes.
Ifth ere is a thermistor out of range error,defrost will occu r at each
time interval during heating operation , but will terminate after 5
minutes.
Count the number of short and long flashes to determinethe
appropriateflash code. Table4 givespossible causes and actions
related to each error.
OATThermistormust be
locked in place with spherical
nibend facing towardsthe
frontof the control box
Fig.8 --Outdoor Air Thermistor (OAT)Attachment
OCTT hermistor
mustbe se cured
tighton stub tu be.
Fig.9 --Outdoor Coil Thermistor (OCT) Attachment
25HPA6