DeVilbiss DeVO/MC44-90 - Four-Way Valve

SECTION 7
SERVICE INSTRUCTIONS
3. Remove the thumb screws (2 Fig. 11) and
brackets (1 Fig. 11) that secure sieve beds
to unit vertical divider.
4. Remove the hose clamps and tubing from
fittings at top of sieve beds.
5. Remove hose clamps and reinforced tub-
ing from fittings at bottom of sieve beds.
6. Install new sieve beds in reverse order.
Make sure the end of bed with nine digit
serial number and label is at the top of
unit.
NOTE: Ensure that the sealing caps are
removed from the new sieve beds just
prior to connecting hoses.
7. Leak test hose connections with a certi-
fied leak detection compound such as
“Snoop” or “Search”.
8. Close cabinet covers and secure with cabi-
net fasteners.
Four-Way Valve
The pneumatically operated four-way valve
(14 Fig. 11) alternately distributes pressure
supplied by the compressor to the two mole-
cular sieve beds approximately every 11 sec-
onds. While one bed is being pressurized the
other bed is being exhausted through the
four-way valve. Exhaust gases passing
through the four-way are directed through
the manifold to the purge valve so they can
be released from the unit.
The four-way valve, while operating
pneumatically, uses an external pilot valve
that allows pressure to shift the spool inside
the four-way. This pilot valve receives a signal
(approximately 12-15 VDC) from the MC
board causing it to open and allow pilot
pressure to activate the four-way. When the
four-way is activated the right sieve bed will
pressurize. Once the bed fills the oxygen
accumulator tank, the cycle is changed by
removing the signal to the pilot valve. This
will cause the four-way to deactivate and
pressurize the left sieve bed.
NOTE: The four-way valve will not shift until
the oxygen accumulator tank is filled. This
will take approximately 11 seconds de-
pending on altitude and liter flow.
If the four-way does not shift, the alarm
system may be activated and the short and no
cycle lights on the MC board will illuminate.
See Section 5, Alarm System. If the alarm
10/87 16
system is not activated, short cycle times and
low oxygen concentrations will result because
the same bed will continue to pressurize.
There are several reasons for the four-way to
malfunction. The cause of failure must be
determined before the proper corrective
action can be taken.
Some reasons for four-way failure are:
(1) Continuous or no voltage to pilot valve on
four-way.
(2) Little or no pilot pressure.
(3) Foreign matter inside valve preventing
spool from shifting.
(4) Defective or damaged four-way.
(5) O-rings on sleeve may be worn pre-
venting the valve from shifting properly.
To test four-way valve:
1. Connect pressure gauges furnished in the
DeVO Service Kit to the test points (Fig.
22) located on the manifold.
2. Turn unit on and observe the green
indicator light (8 Fig. 12) labeled four-way
on the MC board. When this light is
illuminated the four-way should be acti-
vated and the right bed should be pres-
surizing. If the left bed pressurizes
instead, this indicates that the four-way
did not properly shift.
If the right bed does pressurize first,
observe whether pressure is shifted to the
left bed once the shift point pressure is
reached or if the right bed proceeds to
pressurize again. In either case, if either
bed continues to pressurize without
shifting, further testing is needed. Refer
to the normal cycling sequence on page
24.
3. Place the test leads of a volt meter across
the terminals (See test position C, Figure
16) on top of the pilot valve. Approxi-
mately 12-15 VDC should be measured
while the green four-way indicator light is
illuminated (voltage will be supplied for
approximately 11 seconds or until the
oxygen accumulator tank is filled). If this
reading is not obtained, check for loose or
broken wires from the four-way valve to
the MC board. If no loose or broken wires
are found, replace the MC board.