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TROUBLESHOOTING
The scrolling marquee display shows the actual operating
conditions of the unit while it is running. If there are alarms or
there have been alarms, they will be displayed in either the cur-
rent alarm list or the history alarm list. The Service Test mode
allows proper operation of the compressors, fans, and other
components to be checked while the unit is not operating.
Complete Unit Stoppage —
tions that can cause the unit not to provide heating or cooling.
If an alarm is active which causes the unit to shut down,
diagnose the problem using the information provided in
the Alarms and Alerts section on page 95, but also check for
the following:
• Cooling and heating loads are satisfied.
• Programmed schedule.
• General power failure.
• Tripped control circuit transformers circuit breakers.
• Tripped compressor circuit breakers.
• Unit is turned off through the CCN network.
Single Circuit Stoppage —
incorrectly, there are several possible causes. The problem
should be investigated using information from the Alarms and
Alerts section on page 95.
Detailed service analysis can be
Service Analysis —
found in Tables 79-82 and in Fig. 19.
Restart Procedure —
machine, check the alarm list to determine the cause of the
shutdown. If the shutdown alarm for a particular circuit has
occurred, determine and correct the cause before allowing the
unit to run under its own control again. When there is problem,
the unit should be diagnosed in Service Test mode. The alarms
must be reset before the circuit can operate in either Normal
mode or Service Test mode.
Humidi-MiZer
®
Troubleshooting —
scrolling marquee or a CCN device to view the status display
and the diagnostic display for information concerning cooling
operation with the Humidi-MiZer system. Check the Current
Alarms and Alarm History for for any unresolved alarm codes
and correct. Verify Humidi-MiZer configuration settings are
correct for the site requirements. If alarm conditions are cor-
rected and cleared, then operation of the compressors, fans, and
Humidi-MiZer valves may be verified by using the Service
Test mode. By attaching temperature probes across the 3-way
valve, verify the temperature profiles satisfy the corresponding
mode setting. See page 23 for Service Test information. In
addition to the Cooling Service Analysis (Table 79), see the
There are several condi-
If a single circuit stops
Before attempting to restart the
Use the unit
Humidi-MiZer Service Analysis (Table 80) for more informa-
tion.
Thermistor Troubleshooting —
for temperature vs. resistance data.
When replacing thermistors SCT.A and SCT.B, reuse the
original hardware. These thermistors must be clamped tightly
to the hairpins of the condenser.
The EDT, OAT, RAT, LAT, SAT, T55, T56, and T58 space
temperature sensors use 10K thermistors. Resistances at vari-
ous temperatures are listed in Tables 86 and 87.
The 48/50A2,A3,A4,A5 units with the optional variable ca-
pacity digital compressor are equipped with a digital scroll dis-
charge thermistor (DTT). The DTT is an 86K thermistor con-
nected to RXB at plug J6, terminals 3 and 4. The resistance
values are listed in Table 88.
THERMISTOR/TEMPERATURE SENSOR CHECK — A
high quality digital volt-ohmmeter is required to perform this
check.
1. Connect the digital voltmeter across the appropriate
thermistor terminals at the J8 terminal strip on the main
base board.
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 83-88.
3. To check thermistor accuracy, measure temperature at
probe location with an accurate thermocouple-type
temperature-measuring instrument. Insulate thermocou-
ple 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
thermocouple 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, or 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.
Transducer Troubleshooting —
A4,A5 units, the electronic control uses 4 pressure transducers
to measure the suction and discharge pressure of circuits A and
B. The pressure/voltage characteristics of these transducers are
shown in Tables 89 and 90. The accuracy of these transducers
can be verified by connecting an accurate pressure gage to the
second refrigerant port in the suction line.
79
See Tables 83-85
On 48/50A2,A3,
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