Precheck Procedure; Checking The Stepper Valve - Carrier 69NT40-541-001 Operation And Service

7.18.1

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.
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 controller and drive mod-
ule 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 cur-
rent setting and control logic will allow. The current level should be high. A lower discharge pressure 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 pressures, and current draw will go significantly 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 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. 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 approximately 6°C
(11°F) less than the current box temperature, so the unit goes into pulldown. Run the unit for approximately
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 approxi-
mately one minute.
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
occurs, 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
allowable 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 procedure to check out the SMV system.
7.18.2

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 resis-
tance. 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
NOTE
NOTE
Section 7.18.3 to check out the controller.
7–27 T-316