Service manual

6-24T-294-01
be in effect. This places the valve at a 21% staging
positionandissufficienttodrive thetemperatureofthe
supplyprobedownseveraldegreesduring thisinterval.
After the CREL time-out has been met, the valve will
start responding to the control logic and open or close,
relativetot hedemand.Scrutinizetheunit operationfor
afewminutes.Whileinpulldowntheunitwillopenthe
SMV t o a maximum discharge pressure of 350 psig in
high ambient conditions, or as much as t he current
setting and control logic will allow. The current level
shouldbehigh. Alowerdischargepressurewill beseen
in lower ambient temperatures. Once t he 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
suctionpressureshouldgoi ntoavacuumwithinseveral
minutes. Should the operation differ as mentioned, the
SMV or drive m odule may be faulty.
4. Attach a manifold gauge set, refer t o section 6.1.
Perishable operation:
If the operation of the unit is in question, place the set
point to approximately 6°C(11°F) lessthan thecurrent
box temperature, so the unit goes into pulldown. Run
theunitforapproximatelyoneminute.R ecordreadings
on gauges and current draw. The current draw and
pressuresshouldgoup.Placeset pointto 0.5°C(0.9°F)
abovecurrent box temperature to fullymodulatevalve,
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 occurs,
this is an indication of a malfunctioning SMV.
Frozen operation:
Infrozenmodethevalvewilltendto stayopenasmuch
aspossible.Again, this is dependent upon current limit
settingandcontroll ogic.Turn theunit OFFandON,as
intheperishablemode,andwatchthegauges.Thevalve
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.
5. Ifthe unit still does not operate properly, stop unit,
and check out the SMV system, refer to section
6.24.b.
b. Checking the Stepper valve
Disconnect the four pin connector to the stepperSMV.
With a reliable digital ohmmeter, check the winding
resistance.Innormalambient,thevalveshould have72
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, refer to section 6.24.e.
c. SMA-12 portable stepper drive tester
The SMA-12 portable stepper drive tester (P/N
07-00375-00) is a battery operated stepperdrivewhich
will allow opening and closing of the SMV, which
allows a more thorough check of the motor.
To check operation:
1. Stop the unit, disconnect the four pin connector
fromthesteppermodule tot he valvelocatedonthe
suction line near the valve, andattach theSMA-12
stepper drive to the connector going to the valve.
2. SettheSMA-12pulsepersecond(PPS)toonePPS
and either open or close valve. Each LED should
light sequentially until all four are lit. Any LED
failing t o light indicates an open on that leg which
indicates apoor connection or an open coil. Repair
or replace as required to achieve proper operation.
3. Restartunit,setthesteprateto200PPSonSMA-12
for the valve, and close stepper valve while
watchingthe suctiongauge. Within one minutethe
suction pressure will go into a vacuum. This is an
indication t hat the valve is moving.
4. If no change in suction pressure is detected, check
for resistance (refer to section 6.24.b.), and check
connections for proper continuity and retest. If the
valve is functioning and all connections andmotor
resistancearegood, checkthedrivemodule.(Refer
to section 6.24. d.)
5. Ifthevalveisdeterminedasfaultyaftercompleting
the above steps, perform a high side pump down.
Remove valve powerhead assembly, and replace
withaNEWvalvepowerheadassembly, torquenut
to 35 ft-lb, evacuate low side, and open all valves.
WARNING
DO NOT disassemble piston from NEW
powerhead assembly! Doing so may result
in damage to piston.