Carrier 69NT40-541-505, Operation And Service Manual, Page: 110 / 143

T-368 7–18
7.17 SUCTION MODULATION VALVE
On start up of the unit, the SMV will reset to a known
open position. This is accomplished by assuming the
valve was fully open, driving it fully closed, resetting the
percentage open to zero, then opening to a known 21%
staging position.
Figure 7.24 Suction Modulation Valve (SMV)
7.17.1 Pre-check Procedure
a. Check unit for abnormal operation.
b. Check charge. If refrigerant is low repair as
required and again check operation.
c. If sufficient capacity cannot be maintained or
unit is tripping excessively on high pressure
switch (HPS) in high ambients, check coils and
clean if required.
d. If capacity or control cannot be maintained, turn
unit OFF, then back ON. This will reset the SMV
in the event the controller lost communication
with the valve, and may correct the problem.
NOTE
Carefully listen to the valve. During reset, the
valve will make a ratcheting noise that may be
heard or felt as it is attempting to close. If this
can be heard or felt, it indicates that the con-
troller and drive module are attempting to close
the valve, and may serve as a quick indication
that the drive module is in working order.
e. During the first few minutes of unit operation,
compressor reliability enhancement logic
(CREL) may be in effect. This places the valve
at a 21% staging position and is sufficient to
drive the temperature of the supply probe down
several degrees during this interval.
f. After the CREL time-out has been met, the valve
will start responding to the control logic and open or
close relative to the demand. Scrutinize the unit
operation for a few minutes. While in pulldown, the
unit will open the SMV to a maximum discharge
pressure of 325 psig in high ambient conditions, or
as much as the current setting and control logic will
allow. The current level should be high. A lower dis-
charge pressure will be seen in lower ambient tem-
peratures. Once the unit has reached set point, the
SMV will go into control mode. Both the discharge/
suction pressures, and current draw will go signifi-
cantly lower. Once below set point, the suction
pressure should go into a vacuum within several
minutes. Should the operation differ as mentioned,
the SMV, controller, or wiring, may be faulty.
g. Check for correct wire location at the stepper
motor plug and the environmental connector
(EC). Make sure that the wires terminate in
accordance with the wire markings (addresses).
h. Attach a manifold gauge set (refer to Section
7.2 ). If the unit is operating in the perishable
mode, proceed to step i. If the unit is operating
in the frozen mode, proceed to step j.
i. Perishable operation: If the operation of the unit
is in question, place the set point to approxi-
mately 6°C (11°F) less than the current box tem-
perature, so the unit goes into pulldown. Run
the unit for approximately one minute. Record
readings on gauges and current draw. The cur-
rent draw and pressures should go up. Place set
point 0.5°C (0.9°F) above current box tempera-
ture to fully modulate valve, and run for approxi-
mately one minute.
NOTE
The unit may shut off for a short time. Wait
until the unit self starts and sufficient time
has elapsed to cause the valve to fully mod-
ulate. Record new gauge readings and cur-
rent draw. The suction pressure should go
into a vacuum and the current draw should
have gone down. If little or no change to the
suction pressure or current draw occurs, this
is an indication of a malfunctioning SMV.
j. Frozen operation: In frozen mode the valve
tends to stay open as much as possible. Again,
this is dependent upon current limit setting and
control logic. Turn the unit OFF and ON, as in
the perishable mode, and watch the gauges.
The valve will run at 21% open if CREL logic is
active, and will open to maximum allowable after
this. Depending on ambient conditions, there
should be an increase in suction pressure and
current draw as the valve opens. However, at
times, this may be difficult to fully determine.
k. If the unit still does not operate properly, stop
unit and proceed to the following step to check
out the SMV system.
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