United Technologies Carrier Weathermaker 48A2, Controls, Start-Up, Operation, Service, And Troubleshooting

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T420 (R-W1 Jumper Must be Installed to Run Heat in Service
Test) — This alert occurs when a request for a heat output has
occurred yet the W1 input is not high. A jumper must be
installed between R and W1 when trying to test heat in Service
Test. The alert will clear when Service Test is exited or if
another Service Test mode is selected. Remove jumper when
done using Service Test if the unit is operating with a thermo-
stat. The jumper should only be left in place if the unit is
operating with a space temperature sensor.
T421 (Thermostat Y2 Input On without Y1 On) — This alert
occurs in Thermostat Mode when Y2 is energized and Y1 is
not. Verify thermostat and thermostat wiring. When Y2 turns
on, the software will behave as if Y1 and Y2 are both on. When
Y2 turns off, the software will behave as if Y1 and Y2 are both
Off. This alert resets automatically when Y1 is turned on.
T422 (Thermostat W2 Input On without W1 On) — This alert
occurs in Thermostat Mode when W2 is energized and W1 is
not. Verify thermostat and thermostat wiring. When W2 turns
on, the software will behave as if W1 and W2 are both on. When
W2 turns off, the software will behave as if W1 and W2 are both
off. This alert resets automatically when W1 is turned on.
T423 (Thermostat Y and W Inputs On) — This alert occurs in
Thermostat Mode when Y1 or Y2 is energized simultaneously
with W1 or W2. Verify thermostat and thermostat wiring. The
software will enter either the cooling or heating mode depend-
ing upon which input turned on first. This alert resets automati-
cally when Y1 and Y2 are not on simultaneously with W1 and
W2.
T424 (Thermostat G Input Off On a Cooling Call) — This
alert occurs in Thermostat Mode when the fan is not requested
(G = ON) during cooling (Y1 or Y2 = ON). Verify thermostat
and thermostat wiring.
T500 (Current Sensor Board Failure – A1)
T501 (Current Sensor Board Failure – A2)
T502 (Current Sensor Board Failure – B1)
T503 (Current Sensor Board Failure – B2) — Alert codes
500, 501, 502, and 503 are for compressors A1, A2, B1, and
B2 respectively. These alerts occur when the output of the cur-
rent sensor (CS) is a constant high value. These alerts reset au-
tomatically. If the problem cannot be resolved and the CS
board must be replaced, the CS board can be temporarily dis-
abled while securing a replaced board. A CS board is disabled
by setting Configuration

COOL

CS.A1, CS.A2, CS.B1 or
CS.B2 to Disable.
If the current sensor board malfunctions or is not properly
connected to its assigned digital input, an alert will be generat-
ed. It takes 2 to 4 seconds to log the alert. If the alert is logged,
it stays for a minimum of 15 seconds to provide the application
a reasonable time to catch the failure. Compressors will be not
be inhibited by this failure. Recovery is automatic. Reason for
failure may be a faulty current sensor board, incorrect wiring,
or a damaged input on the MBB control board.
A700 (Supply Air Temperature Sensor Failure) — This alarm
indicates a failure of the sensor supply air temperature sensor
or the leaving air temperature sensor (if using hydronic heat).
This alarm occurs when the temperature sensor ( Tempera-
tures

AIR.T

SAT ) is outside the range –40 to 240 F (–40 to
116 C). This alarm resets automatically. The cause of the alarm
is usually a faulty thermistor, a shorted or open thermistor
caused by a wiring error, or a loose connection.
T701 (Staged Gas 1 Thermistor Failure)
T702 (Staged Gas 2 Thermistor Failure)
T703 (Staged Gas 3 Thermistor Failure) — If any of the staged
gas thermistors ( Temperatures

AIR.T

S.G.L1-3 ) fails, an
alert will be generated and the remaining thermistors will be
averaged together ( Temperatures

AIR.T

S.G.LS ) without
the failed thermistor. Recovery is automatic. Reason for failure
may be incorrect wiring, faulty thermistor, or a damaged input
on the staged gas control board (SCB).
A704 (Staged Gas Leaving Air Temperature Sum Total Fail-
ure) — If all three staged gas thermistors ( Tempera-
tures

AIR.T

S.G.L1-3 ) fail (the sensor is outside the range
of –40 F to 240 F), staged gas will be shut down and this alarm
will be generated. Recovery is automatic. Reason for failure
may be faulty wiring, faulty thermistors, or damaged inputs on
the staged gas control board (SCB).
T705 (Limit Switch Thermistor Failure) — A failure (the sen-
sor is outside the range of –40 F to 240 F) of this thermistor
( Temperatures

AIR.T

S.G.LM ) will cause an alert to occur
and a disabling of the limit switch monitoring function for the
staged gas control board (SCB). Recovery is automatic.
Reason for failure may be due to faulty wiring, a faulty
thermistor, or a damaged input on the staged gas control board
(SCB).
A706 (Hydronic Evap Discharge Thermistor Failure) — If
the unit is configured for Humidi-MiZer, then the unit has a
thermistor ( Temperatures

AIR.T

CCT ) installed between
the evaporator coil and the Humidi-MiZer coils that functions
as the evaporator discharge temperature thermistor for cooling.
If this thermistor fails, an alarm will be generated and the sys-
tem will be shut down. Recovery is automatic. Reason for fail-
ure may be due to faulty wiring, a faulty thermistor, or a dam-
aged input on the EXV control board.
T707 (Digital Scroll Discharge Thermistor Failure) — If the
RXB control board is not receiving a signal from the discharge
temperature thermistor, the alarm is generated. The thermistor
may be missing, disconnected, or a wire may be broken. The
alert will be generated and the digital scroll capacity will be
locked at 50%. Reset is automatic.
MAJOR SYSTEM COMPONENTS
General —
The 48/50A Series package rooftop units with
electric cooling and with gas heating (48A units) or electric
cooling and electric heating (50A units) contain the
Comfort Link electronic control system that monitors all opera-
tions of the rooftop. The control system is composed of several
components as listed below. See Fig. 20-26 for typical control
and power component schematics. Figures 27 and 28 show the
layout of the control box, unit, and thermistor and transducer
locations.
Factory-Installed Components
MAIN BASE BOARD (MBB) — See Fig 29. The MBB is
the center of the Comfort Link control system. The MBB con-
tains the major portion of the operating software and controls
the operation of the unit. The MBB has 22 inputs and 11 out-
puts. See Table 101 for the inputs and output assignments. The
MBB also continuously monitors additional data from the
optional ECB1, ECB2, SCB, SCB2, EXV, and CEM boards
through the LEN communications port. The MBB also inter-
faces with the Carrier Comfort Network ® system through the
CCN communications port. The board is located in the main
control box.
ECONOMIZER BOARD (ECB1) — The ECB1 controls the
economizer actuator and the power exhaust fans. The ECB1
operates the economizer motor using a digital communication
signal that also provides status and diagnostics for the econo-
mizer motor. See Fig. 30. The ECB1 also controls the operation
of the power exhaust motors and provides up to 6 stages of dig-
itally sequenced power exhaust either based on the economizer
motor position or the building pressure. The board has 4 inputs
and 6 outputs. Additionally, ECB1 provides an output that will
send a 4 to 20 mA signal to a field-installed VFD power ex-
haust accessory. Details can be found in Table 102. The ECB1
board is located in an auxiliary box located at the end of the
unit behind the filter access door. The board also contains
a second LEN port than can be used with the accessory
Navigator™ display.