GMC PD-4103 Maintenance Manual page 41

Sec. 3
Page 42
GM COACH MAINTENANCE MANUAL
BODY
Figure 11-High Temperature Overrule
Thermostat With Cover Removed
has run low and water is replenished, or whenever
sufficient air has accumulated in system to retard
normal flow, of water, heating system should be
bled to expell air.
1. Make sure all drain and vent cocks are
closed and that gate valves, located
in
battery
compartment, are open.
2. Fill coo 1 in g system as directed in
"COOLING SYSTEM - GENERAL" (SEC. 6). With
engine running at half throttle, bleed air from
system as follows:
a. Open vent cock in left side of forward bag-
gage compartment; close vent cock when all air
is expelled.
b. Turn driver's heater control valve knob
clockwise to extreme open position. -Press down
on bleed valve button, accessible through dash
safety compartment; release button when all air
is expelled.
c. Open vent cock in right side of forward
baggage compartment; close vent cock when all
air is expelled.
3. Slow engine down to normal idle and re-
plenish engine cooling system if required,
HIGH TEMPERATURE OVERRULE THERMOSTAT
High temperature overrule thermostat (fig. 11)
is mounted inside of cover which is attached to
hot air outlet under fourth seat from front on
right-hand side. Thermostat contacts are normally
closed, completing circuit through both heater
motor magnetic switch windings; when temperature
in coach rises to temperature at which the ther-
mostat dial is set, contacts open and break the
circuit through the heater motor magnetic switches.
Thermostat has an adjustable dial which may
be set for any temperature at which the contacts
open. While dial is set at the factory at 85°F,
this setting may be raised or lowered as desired.
Dial is locked in position by a lock and lock screw
(fig. 11). Cover retaining screw is secured with
a wire and lead seal to prevent unauthorized per-
sonnel tampering with the dial adjustment. Wire
and seal should be replaced after adjustment is
made.
Thermostat is not reparable in the field and,
except for the manual dial, is non-adjustable.
Sealed silver contacts require no cleaning and
should operate indefinitely.
CAUTION: Any attempt to adjust (other
than by dial} or repair thermostat will
undoubtedly destroy the calibration of
the unit. Shou,ld this occur, thermostat
must be replaced with a new or factory
recalibrated unit.
Thermostat Removal
1. Break lead seal and remove lock wire
from knurled nut which retains thermostat cover.
Remove knurled nut and cover.
2.
Remove four screws, nuts, and lock washers
which attach thermostat and spacers to heat out-
let cover, then remove thermostat and spacers.
3. Disconnect wires from thermostat term i -
nals to complete removal.
Thermostat Installation
1. Connect wires to thermostat terminal num-
bers 1 and 3.
2. Position thermostat on inside of heat out-
let cover, using spacers between thermostat base
and cover, and attach with four screws, lock
washers, and nuts.
3. Loosen dial lock screw, set dial at desired
temperature (85°F recommended), and tighten dial
lock screw. Install thermostat cover and secure
with knurled nut, then install lock wire and lead
seal in knurled nut.
GRAD-U-STAT
Operation (Fig. 12)
Grad-U-Stat is installed on top of coach floor
adjacent to heat outlet under fourth seat from
front on right-hand side. Two air lines connect to
Grad-U-Stat. Main air line, connected to port marked
"M,"
is the feed line, earring air pressure to the
Grad-U-Stat from the air pressure regulatingvalv~
which limits the air pressure to 17 pounds; branch
air line, connected to port marked "B," carries
air pressure from the Grad-U -Stat to the water
modulation valve.
Vapor filled bellows in Grad-U-Stat is sensitive
to inside coach temperature. Expansion and con-
traction of bellows, caused by increasing andde-
creasing coach temperatures, is transmitted to air
control valves in lower portion of unit through
levers. As temperature in coach rises, bellows
expands and exerts downward force on secondary