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Sec. 3
Page 42B
GM COACH MAINTENANCE MANUAL
BODY
attaching cover to heat outlet. Cover, with ther-
mostat installed, can be set on end without dis-
connecting wires from thermostat to permit access
to Grad-U-Stat.
2. Exhaust air pressure from air system.
Disconnect air line from port marked "B." This
is the line leading to the water modulation valve.
Connect an air pressure test gauge to this port.
Build up air pressure in air system.
3. Check the air temperature at the bellows
with an accurate thermometer.
CAUTION: Do not touch the thermo-
meter bulb or the bellows with hand
while accomplishing the following opera-
tions, as body heat will affect both units
and erroneous readings and adjustments
will be obtained.
4. Loosen locking latch screw, disengage latch
from adjusting screw, then turn the adjusting screw
to set the indicator at the temperature shown on
the thermometer.
5. Observe pressure reading on air pressure
test gauge.
If
pressure reading is 7-1/2 pounds,
no adjustment is required.
If
pressure is above
7-1/2 pounds, turn the calibration nut and main
spring to shorten the spring until the correct
reading (7 -1/2 pounds) is obtained.
If
pressure
is below 7-1/2 pounds, turn calibration nut and
main spring to lengthen the spring.
6. After correct adjustment is obtained, re-
move gauge and reconnect air line to "B" port.
Turn adjusting screw to set the indicator at de-
sired operating temperature (75
°
F is factory
setting), place locking latch over adjusting screw,
and tighten latch screw.
7. Position heat outlet cover on outlet and
attach with four screws and special nuts.
Grad-U-Stat Removal (Fig. 12)
1. Remove four screws and special nuts at-
taching cover to heat outlet. Cover, with overrule
thermostat installed, may then be set on end with-
out disconnecting wires from thermostat to per-
mit access to Grad- U -Stat.
2. Exhaust air pressure from air system.
3. Remove center screw at each end of unit
attaching mounting plate to Grad- U -Stat box. Re-
move two screws attaching air line connector head
to Grad-U-Stat; one screw is located on either side
and at opposite ends of bellows, accessible from
the top.
Lift Grad-U-Stat assembly out of box.
4.
If
necessary to remove Grad-U-Stat box,
which includes air lines and connector head, dis-
connect air lines from elbows at side of box, then
remove two screws attaching box to floor.
Grad-U-Stat Installation (Fig. 12)
1. If
new Grad-U-Stat box is being installed,
install elbows in air ports and tighten firmly; leave
both elbows pointing up after final tightening.
2. Position box on floor and attach with two
screws.
If
box is new, remove screw and spacer
from each connector head post, then remove con-
nector head protector.
Make
sure gasket on con-
nector head is in good condition.
3. Position Grad-U-Stat in box, with the two
small holes in bottom of air control valve regis-
tered with holes in air line connector head gasket.
Insert two screws, with toothed lock washers, down
through frame and thread into connector head
posts. Tighten screws firmly. Insert screw, with
toothed lock washer, down through each end of
mounting plate and thread into posts in box.
4. Connect main air line to elbow in port
marked "M." Connect air pressure test gauge to
port marked "B."
Build up air pressure in air
system.
5. Test Grad-U-Stat operation, and adjust
if
necessary, and complete the installation as previ-
ously directed in steps 3 through 7 under "Test
and Adjustment."
WATER MODULATION VAL VE
Water modulation valve (fig 13) is an air-
operated water control valve installed in heater
line leading from left-hand under floor heater core
to right-hand heater core (fig 10).
Modulation
valve controls the flow of hot water through the
heater cores in accordance with the air pressure
delivered to
it
by the Grad-U-Stat. Operation of
Grad-U -Stat is described previously. Modulation
valve is mounted on under side of aisle floor in
forward baggage compartment, and is secured to
mounting bracket with two U-bolts and a mounting
clamp.
Modulation Valve Removal
1.
Exhaust air pressure from air system.
Disconnect air line from modulation valve.
2. Drain heating system as previously directed
under "Draining."
3. Disconnect water line from each side of
valve.
4. Remove nuts and lock washers from two
U-bolts securing valve to mounting bracket, re-
move U-bolts and mounting clamp, then remove
modulation valve assembly.
Modulation Valve Disassembly (Fig. 13)
1. Loosen set screw securing bellows spider
to valve bonnet.
Separate spider from bonnet,
then lift spider and bellows assembly off over
valve stem and spring. Remove screws attaching
cover to spider and remove cover and bellows
assembly.
2. Unscrew bonnet from valve body, then re-
move bonnet, valve, and stem assembly from valve
body.
I
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