Service manual

6-- 17
T--318
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 0.5 degrees C (0.9 degrees 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. W ait 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 tends to
stay open as much as possible. Again, this is depen-
dent upon current limit setting and control logic. T urn
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 allow-
able 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.
6.18.2 Checking the Stepper Valve
a. Checking with ohmmeter
Disconnect the 4-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 6.18.3 to check out the controller.
b. Checking with SMA-12 portable stepper drive tester
The SMA-12 portable stepper drive tester (Carrier
T ransicold P/N 07-00375-00) is a battery--operated
stepper drive that will open and close the SMV, allowing
a more thorough check of the motor.
To check operation:
1. Stop the unit, disconnect the 4-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
the suction gauge. Within one minute the suction
pressure will go into a vacuum. This is an indication
that the valve is moving.
4. If no change in suction pressure is detected, check
for resistance (refer to step 6.18.2), and check con-
nections for proper continuity and retest. If the valve
is functioning, and all connections and motor resist-
ance are good, check the drive module (refer to step
6.18.3).
5. If the valve is determined to be faulty after complet-
ing the above steps, perform a low side pump down.
Remove valve powerhead assembly, and replace
with a NEW valve powerhead assembly, torque nut
to 35 ft-lb, evacuate low side, and open all service
valves.
CAUTION
DO NOT disassemble piston from NEW suc-
tion modulating valve powerhead assem-
bly. Doing so may result in damage to pis-
ton.
6.18.3 Checking the Controller
a. Turn unit OFF.
b. With voltmeter set to read 20 volts DC, attach the pos-
itive lead to MC1 of the 4-pin connector and the nega-
tive lead to the TP9. Turn ON unit and watch the volt
meter. After a short delay, the reading should remain
unchanged at 0 volts. If 5 VDC, check jumper wire
form MC1 to MC8 is in place; if not, install and retest.
6.18.4 Emergency Repair Procedures
In the event that the SMV system has a failure and
replacement components are not readily available, the
system can be bypassed by removing the valve piston.
To remove the piston, do the following:
a. Perform a low side pump down (refer to paragraph
6.4).
b. Remove SMV powerhead by loosening the 2-1/8
inch diameter nut (see Figure 6-24) to relieve any
pressure and then sliding the powerhead out.
c. Remove the piston by loosening the Allen screw and
removing the piston and screw .
d. Install the powerhead assembly (without the piston)
and torque to 35 to 40 foot-lbs.
e. Open all valves.
f. Start the unit.
g. Adjust the suction service valve so that the approxi-
mate temperature OR current limit is maintained.
For perishable loads, it is recommended that the ad-
justment be made so that the available capacity is
slightly larger than the load. The unit will cycle OFF
and ON.
h. Once repair parts become available, repair as re-
quired.
6.19 DPRV CHECK PROCEDURE
The discharge pressure regulating valve (DPRV) is
designed to maintain a minimal level of pressure within
the compressor. Refer to paragraph 2.2 for valve
setting.
The following procedure can be used to perform a
functional check:
a. Install a high pressure gauge at the discharge service
valve.