6-17
T-296
2-1/8 inch Nut
1
2
1. Suction Modulation Valve (SMV)
2. Stepper Drive (SD)
Figure 6-24 Suction Modulation Valve (SMV)
6.18.1 Precheck 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 valve 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 control-
ler 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, com-
pressor reliability enhancement logic (CREL) may be
in effect. This places the valve at a 21% staging posi-
tion and is sufficient to drive the temperature of the
supply probe down several degrees during this inter-
val.
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 op-
eration 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 discharge pres-
sure will be seen in lower ambient temperatures.
Once the unit has reached set point, the SMV will go
into control mode. Both the discharge/suction pres-
sures, and current draw will go significantly lower.
Once below set point, the suction pressure should go
into a vacuum within several minutes. Should the op-
eration differ as mentioned, the SMV, drive module, or
wiring may be faulty.
g. Check for correct wire location at the drive module
(SD) 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 paragraph 6.3. If
the unit is operating in the perishable mode, proceed
to step i. I f the unit t 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 approximately 6
°
C
(11
°
F) less than the current box temperature, so the
unit goes into pulldown. Run the unit for approximate-
ly one minute. Record readings on gauges and cur-
rent draw. The current draw and pressures should go
up. Place set point to 0.5
°
C (0.9
°
F) above current box
temperature to fully modulate valve, and run for
approximately 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 modulate.
Record new gauge readings and current 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 oc-
curs, this is an indication of a malfunctioning SMV.
j.
Frozen operation:
In frozen mode the valve will tend
to stay open as much as possible. Again, this is de-
pendent 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 al-
lowable after this. Dependent on ambient conditions,
there should be an increase in suction pressure and
current draw as the valve opens, however, this may
be difficult at times 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.
6.18.2 Checking The Stepper valve
a. Checking with ohmmeter
Disconnect the four pin connector to the stepper SMV.
With a reliable digital ohmmeter, check the winding
resistance. In normal ambient, the valve should have 72
to 84 ohms measured on the red/green (a-b terminals)
and on the white/black (c-d terminals) leads. If an infinite
or zero reading occurs, check connections and replace
the motor. If near normal or normal reading occurs,
proceed to step 6.18.4 to check out the controller.
b. Checking with SMA-12 portable stepper drive tester
The SMA-12 portable stepper drive tester (Carrier
Transicold P/N 07-00375-00) is a battery operated
stepper drive which will open and close the SMV, which
allows a more thorough check of the motor.
To check operation:
1. Stop the unit, disconnect the four pin connector from
the stepper module to the valve (see Figure 6-24)
and attach the SMA-12 stepper drive to the connec-
tor going to the valve.
2. Set the SMA-12 pulse per second (PPS) to one PPS
and either open or close valve. Each LED should
light sequentially until all four are lit. Any LED failing
to light indicates an open on that leg which indicates
a poor connection or an open coil. Repair or replace
as required to achieve proper operation.
3. Restart unit, set the step rate to 200 PPS on SMA-12
for the valve, and close stepper valve while watching
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