116
DIAGNOSTIC ALARM CODES AND POSSIBLE
CAUSES
T048 (Ckt A, Oil Return Not Reliable with Only One Comp
Available)
T049 (Ckt B, Oil Return Not Reliable with Only One Comp
Available) — Alert codes T048 and T049 are for circuits A
and B respectively and are active for 90 and 105 ton units only.
The 90 and 105 ton units have 3 compressors per circuit. If
load conditions are such that only one compressor is running
on a circuit, a second compressor is periodically turned on to
equalize compressor oil levels. If a second compressor is
unavailable for oil return, the circuit will be shut down, and an
alert will be generated. The alert will automatically clear, and
the circuit will restart when a second compressor becomes
available.
T051 (Circuit A, Compressor 1 Failure)
T052 (Circuit A, Compressor 2 Failure)
T053 (Circuit A, Compressor 3 Failure)
T059 (Circuit A, Compressor 4 Failure)
T054 (Circuit B, Compressor 1 Failure)
T055 (Circuit B, Compressor 2 Failure)
T056 (Circuit B, Compressor 3 Failure)
T060 (Circuit B, Compressor 4 Failure — If the current sen-
sor board reads OFF while the compressor relay has been com-
manded ON for a period of 4 continuous seconds, an alert is
generated.
Any time this alert occurs, a strike will be logged on the af-
fected compressor. If three successive strikes occur the com-
pressor will be locked out requiring a manual reset or power re-
set of the unit.
The clearing of strikes during compressor operation is a
combination of 3 complete cycles or 15 continuous minutes of
run time operation. So, if there are one or two strikes on the
compressor and three short cycles (ON-OFF, ON-OFF, ON-
OFF) occur in less than 15 minutes, the strikes will be reset to
zero for the affected compressor. Also, if the compressor turns
on and runs for 15 minutes straight with no compressor failure,
the compressor's strikes are cleared as well.
NOTE: Until the compressor is locked out, for the first two
strikes, the alert will not be broadcast to the network, nor will
the alarm relay be closed.
The possible causes are:
• High-pressure switch open. The high pressure switch
(HPS) for each circuit is wired in series with the com-
pressor contactor coils of each compressor on the circuit
to disable compressor operation immediately upon a high
discharge pressure condition. If the high-pressure switch
opens while the MBB or CXB is commanding the com-
pressor ON, the compressor stops and the CSB no longer
detects current, causing the control to activate the alert.
• Compressor circuit breaker tripped.
• Failed CSB or wiring error.
To check out alerts T051, T052, T053, T054, T055, T056,
T059, T060:
1. Turn on faulty compressor using Service Test mode. If
the compressor does not start, then most likely the prob-
lem is one of the following: HPS is open, compressor cir-
cuit breaker is tripped, incorrect control wiring, or incor-
rect compressor wiring.
2. If the compressor starts, verify that the indoor and out-
door fans are operating properly.
3. If the CSB is always detecting current, then verify that the
compressor is on. If the compressor is ON, check the con-
tactor and the relay on the MBB or CXB. If the compres-
sor is OFF and there is no current, verify the CSB wiring
and replace if necessary.
4. Return to Normal mode and observe compressor opera-
tion to verify that compressor current sensor is working
and condenser fans are energized.
A051 (Circuit A, Compressor 1 Stuck On Failure)
A052 (Circuit A, Compressor 2 Stuck On Failure)
A053 (Circuit A, Compressor 3 Stuck On Failure)
A059 (Circuit A, Compressor 4 Stuck On Failure)
A054 (Circuit B, Compressor 1 Stuck On Failure)
A055 (Circuit B, Compressor 2 Stuck On Failure)
A056 (Circuit B, Compressor 3 Stuck On Failure)
A061 (Circuit B, Compressor 4 Stuck On Failure) — If the
current sensor board reads ON while the compressor relay has
been commanded OFF for a period of 4 continuous seconds, an
alarm is generated and the HVAC Mode will display Compres-
sor Stuck On. These alarms are only monitored for a period of
10 seconds after the compressor relay has been commanded
OFF. This is done to facilitate a service technician forcing a re-
lay to test a compressor.
When the HVAC Mode indicates a compressor stuck on
condition, the following will occur:
1. The outdoor fans will continue to control head pressure.
2. The supply fan will remain on.
3. Heating will be disabled.
A manual reset or power reset of the unit is required for
these alarms.
The possible causes are:
• Compressor contactor has failed closed.
• Relay output on MBB or CXB that drives compressor
contactor has failed closed.
• Failed CSB or wiring error.
To check out alarms A051, A052, A053, A054, A055,
A056, A059, A061:
1. Place the unit in Service Test mode. All compressors
should be OFF.
2. Check for welded compressor contactor.
3. Verify there is not 24 vac across the contactor coil of the
compressor in question. If 24 vac is measured across coil,
check relay on MBB or CXB and associated wiring.
4. Verify CSB wiring.
5. Return to Normal mode and observe compressor opera-
tion to verify that compressor current sensor is working
and condenser fans are energized after compressor starts.
T057 (Circuit A, High Pressure Switch Failure Alert)
T058 (Circuit B, High Pressure Switch Failure Alert)
A057 (Circuit A, High Pressure Switch Failure Alarm)
A058 (Circuit B, High Pressure Switch Failure Alarm)
— The high-pressure switch for each circuit is wired in series
with the compressor contactor coils of each compressor on the
circuit to disable compressor operation immediately upon a
high discharge pressure condition. The normally closed
contacts in the switches are calibrated to open at 650 ± 10 psig
which corresponds to a saturated condensing temperature of
155.6 ± 1.3 F. The pressure switches will automatically reset
when the discharge pressure is reduced to 500 ± 15 psig which
corresponds to a saturated condensing temperature of 134.1 ±
2.4 F.
The output of each high-pressure switch is wired to inputs
on the RXB to provide the control with an indication of a high-
pressure switch trip. This alarm could occur when compressors
are off if the wiring to the switch is broken or the switch has
failed open.
If the high-pressure switch trips on a circuit with compres-
sors commanded on, the discharge pressure is recorded. If the
recorded discharge pressure is between 630 and 660 psig (satu-
rated condensing temperature between 153.0 and 156.9 F), and
is also less than the value recorded on any previous
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