Thermistor Troubleshooting - Carrier 48FP Controls Operation And Troubleshooting

is at low speed, the speed will be set to high and the 5 minute
timer will be restarted. The fan will be locked on high speed
until the saturated suction temperature exceeds 65 F.
Alarm code 80 signals a circuit A failure, and alarm code 81
signals a circuit B failure. Reset is manual, and start-up of the
circuit is normal after the alarm has been cleared. Possible
causes of the fault condition are a combination of low entering
outdoor-air temperature, low evaporator-fan cfm, low refriger-
ant charge, plugged filter drier, partially closed liquid line ser-
vice valve, or pressure transducer failure.
NOTE: This alarm is valid only when the unit has refrigerant
pressure transducers.
Alarm Codes 82 and 83 (High Suction Superheat) — This
alarm is valid only when unit is configured for pressure trans-
ducers and suction sensors (thermistors), and mechanical cool-
ing is on.
If the suction superheat is greater than 45 F (7 C) for more
than five minutes, the alarm trips and the circuit shuts down.
Alarm code 82 signals a circuit A failure, and alarm code 83
signals a circuit B failure. Reset of this alarm is manual. The
circuit will start normally after the alarm condition has been
corrected. Possible causes for this alarm are low refrigerant
charge, plugged filter drier, partially closed liquid line service
valve, or a faulty thermistor or transducer.
Alarm Codes 84 and 85 (Low Suction Superheat) — This
alarm is valid only when the unit is configured for pressure
transducers and suction sensors (thermistors), and mechanical
cooling is on.
If the suction superheat is less than 3 F (–16 C) for more
than five minutes, the alarm trips and the affected circuit shuts
down. Alarm code 84 signals a circuit A failure, and alarm
code 85 signals a circuit B failure. Reset of this alarm is man-
ual. Start-up of the circuit is normal after the alarm has been
corrected. Possible causes of the alarm include a faulty thermo-
static expansion valve (TXV), thermistor, or transducer.
Alarm Code 86 (Illegal Configuration) (Sizes 034-078
Only) — This fault indicates a configuration code error, and
the unit is not allowed to start. Refer to Service Function
section and on page 88 for factory configuration val-
ues and on page 88 for user configuration variables.
Check all configuration codes and set points and correct any
errors.
Alarm Code 88 (Hydronic Coil Freeze Stat) — The hydron-
ic coil freeze stat alarm requires a field supplied, normally
open, temperature actuated switch connected to PSIO2 at plug
J7 (bottom). The alarm is activated by a 24-v signal generated
by the switch when it closes. The economizer will be set at
minimum position, heating coil valve will be fully open, and
supply-air fan will be shut off. This may be caused by low tem-
perature outdoor-air used with minimum airflow, during IAQ
purge mode with low temperature outdoor air, or because the
outdoor-air damper is jammed open.
Reset of this alarm is automatic once the problem is
corrected.
Alarm Code 89 through 92 (Pressurization, Evacuation,
Smoke Purge, and Fire Shutdown, respectively) — When
the unit is equipped with an optional smoke control and a fire
system is installed, these 4 modes are provided to control
smoke within areas serviced by the unit. The unit must be
equipped with an economizer, power exhaust or return-air fan
options, and the control option module to support these modes.
The building fire alarm system closes field supplied, normally
open, dry contacts connected to PSIO2 at plug J7 (bottom) to
activate the alarms.
Reset of this alarm is automatic once the problem is
corrected.
Alarm Code 93 (Linkage Failure — DAV System Only) —
A linkage failure alarm is generated when the linkage
has stopped updating the TSM linkage tables within the last
5 minutes.
NOTE: This alarm can only be generated after linkage has
updated the table at least one time since initialization.
The unit controls enter the linkage default mode if the link-
age is enabled, but the communications link has been lost. With
the controls having reverted back to stand-alone operation, the
existing sensors, previously overridden by linkage, will be
used. This may be caused by a loose connection or a broken
wire.
Reset of this alarm is automatic once the problem is
corrected.
Alarm Code 94 (Building Pressure) — If the building
pressure is greater than the building pressure set point plus
0.25 in. wg for 30 seconds, the alarm will be generated. This
may be caused by a power exhaust fan failure or a leak or ob-
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struction in either the /
be controlled or the tubing routed to atmosphere from the
building pressure control. This alarm is valid only when the
unit is configured for modulating power exhaust or return-air
fan.
Reset of this alarm is automatic once the problem is
corrected.
Alarm Code 95 (Duct Static Pressure) — If the duct pressure
is greater than the static pressure set point plus 1.0 in. wg for
30 seconds, or equal to or greater than 5.0 in. wg for 15 sec-
onds, then the alarm will trip, and the supply-air fan will shut
off for 5 minutes. This may be caused by variable frequency
drive (VFD) or IGV actuator motor failure or a leak or obstruc-
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tion in the / -in. plastic tubing routed from the VFD or IGV
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duct pressure control to the ductwork connection, or all the ter-
minals are closed.
Reset of this alarm is automatic once the problem is
corrected.
Alarm Mode 97 (Indoor-Air Quality Failure) — This alarm
is valid only when the unit is configured with the PSIO2 con-
trol option module, the unit is equipped with field-supplied
IAQ sensors, and the VENT option is set at either "1" or "3".
If the IAQ set point (IAQS) is less than the IAQ low refer-
ence value or greater than the high reference value, an alarm
will be generated. An alarm will also occur when the IAQ pri-
ority is configured as low and the IAQ sensor reading exceeds
the IAQ set point.
Reset of this alarm is automatic once the problem is
corrected.
Thermistor Troubleshooting —
system uses thermistors to measure temperatures of the supply
and return air, outdoor air and space temperature, and the satu-
rated condensing and suction temperatures of the refrigerant
circuits. See Table 1 and Fig. 38-40 for thermistor locations.
The resistance versus temperature and electrical characteris-
tics for thermistors in the system (except space temperature)
are identical. To obtain an accurate reading, a high-impedance
meter (such as a digital meter) must be used.
Thermistors in the unit control system have a 5-vdc signal
applied across them any time the unit control circuit is ener-
gized. The voltage drop across the thermistor is directly pro-
portional to the temperature and resistance of the thermistor.
To determine temperatures at the various thermistor
locations:
1. Disconnect the thermistor from the processor board.
2. Measure the resistance across the appropriate ther-
mistor using a high quality digital ohmmeter.
3. Use the resistance reading to determine the thermistor
temperature using Tables 59 and 60.
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-in. plastic tubing routed to the area to
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The unit control