Chevrolet Light Duty Truck 1973, Service Manual

Page: 497 / 979

E M I S S I O N C O N T R O L S Y S T E M S 6 T - 5
AIR CLEANER ASSY.
VACUUM
DIAPHRAGM
TEMP. SENSING SPRING
FROM BASE OF CARBURETOR
(SOURCE OF INTAKE
MANIFOLD VACUUM)
DAMPER
DOOR
Fig. 5b—Thermostatically Controlled Air Cleaner
signal. A bi-metal strip in combination with a needle
valve type operation accomplishes the temperature
sensitive controlled vacuum leak.
The thermostatically controlled air cleaner functions as
follows (Fig.
6
b):
When the engine is “off” (View A), no vacuum is present
at the sensor unit nor at the vacuum diaphragm. The
force of the vacuum diaphragm spring closes off the
“heated air” passage (snorkel passage open).
When the engine is initially started (View B), the sensor
unit’s bimetal strip senses cool air temperature (below
85°F). The thermostat control valve of the sensor unit
closes its air bleed passage allowing maximum vacuum to
the vacuum diaphragm. Maximum vacuum at the
vacuum diaphragm completely opens the damper door,
closing off the ambient air passage through the snorkel
and opening the air passage feed by the air heat stove.
Should the engine be heavily accelerated while in this
mode, the vacuum level in the system will drop to a low
enough level so that the diaphragm spring will overcome
the vacuum and push the damper door closed permitting
ambient air passage through the snorkel.
As the engine heats up and air passage past the sensor
reaches approximately 128° F (View C), the sensor’s
thermostatic control valve bleed passage is completely
opened by the bimetal spring acting upon the needle. Air
bleeding into the vacuum acting on the diaphragm
assembly lowers the vacuum level enough so that the
diaphragm spring closes the damper door, thus opening
the cold air passage (abmient air through snorkel) and
closing the hot air passage (heat stove passage).
At temperatures between 85° and 128°F (View D),
varying amounts of air is bled into the system,
depending on the exact temperature at the sensor unit.
This results in a vacuum level and damper position
required to maintain carburetor air temperature at from
85° to 128°F when underhood temperatures are below
this range.
VIEW A —ENGINE OFF
SNORKEL TUBE
CONTROL DAMPER ASSM.
HOT AIR PIPE
AIR BLEED VALVE
VIEW B -U N D E R H O O D TEMPERATURE
, CLOSED
BELOW 85°F
DIAPHRAGM SPRING
VACUUM CHAMBER
SNORKEL TUBE
"•n
TEMP. SENSING
SPRING
DIAPHRAGM
CONTROL DAMPER ASSM. 1 |
\
1 i i F -H O " HOT AIR PIPE
VIEW C - UNDERHOOD TEMPERATURE
ABOVE 128°F
AIR BLEED VALVE OPEN
DIAPHRAGM SPRING
VIEW D - UNDERHOOD TEMPERATURE
BETWEEN 8 5 °F A N D 128°F
AIR BLEED VALVE
SNORKEL TUBE
.VACUUM CHAMBER
AIR INLET
CONTROL DAMPER ASSM.
DIAPHRAGM SPRING
VACUUM CHAMBER
SNORKEL TUBE
AIR INLET
TEMP. SENSING
SPRING
DIAPHRAGM
HOT AIR PIPE
CONTROL DAMPER ASSMH f f S
h o t a ir pipe
Fig. 6b—Thermostatically Controlled Air Cleaner Operation
LIGHT DUTY TRUCK SERVICE MANUAL