Specifications

66
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 — There are several condi-
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 78, 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 — If a single circuit stops
incorrectly, there are several possible causes. The problem
should be investigated using information from the Alarms and
Alerts section on page 78.
Service Analysis — Detailed service analysis can be
found in Tables 75-77 and in Fig. 12.
Restart Procedure — Before attempting to restart the
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.
Thermistor Troubleshooting — The electronic con-
trol uses five 5K-thermistors for the saturated condensing
temperature sensor SCT.A and SCT.B. See Tables 78 and 79
for temperature vs. resistance data.
When replacing thermistors SDT.A and SDT.B, reuse the
original hardware. These thermistors must be clamped tightly
to the hairpins of the condenser.
The EDT, OAT, RAT, LAT, T55, T56, and T58 space tem-
perature sensors use 10K thermistors. Resistances at various
temperatures are listed in Tables 80 and 81.
THERMISTOR/TEMPERATURE SENSOR CHECK — A
high quality digital volt-ohmmeter is required to perform this
check.
1. Connect the digital voltmeter across the appropriate ther-
mistor terminals at the J8 terminal strip on the Main Base
Board.
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 78-81.
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 — The electronic con-
trol uses 2 suction pressure transducers to measure the suction
pressure of circuits A and B. The pressure/voltage characteris-
tics of these transducers are in shown in Table 82. The accuracy
of these transducers can be verified by connecting an accurate
pressure gage to the second refrigerant port in the suction line.