Functional Check - GMC 1976 ZEO 6083 Maintenance Manual

Figure 9-T.V .S . Location (Typical)
the spark timing slightly and speeds up the engine .
The result is less heat rejected to the coolant together
with higher fan speeds for better cooling action .
When the engine has cooled down, 216°F. (102 .2°C .)
the TVS switch moves the valve back to retard spark
timing.
VACUUM HOSE ROUTING TO T.V .S .
SWITCH (FIGURE 9)
Port "D" Vacuum hose to the
distributor vacuum advance.
Port "C" Vacuum hose to the
carburetor ported spark port .
Port ""MT"" Vacuum hose to intake
manifold elbow.
FUNCTIONAL CHECK
To test the switch function, disconnect the dis-
THROTTLE RETURN CONTROL (TRC)
(CALIFORNIA ONLY)
The Throttle Returq Control system is designed
to reduce hydrocarbon emissions during deceleration
by controlling the rate of throttle closing, causing a
more complete burning. The system consists of:
throttle lever actuator ; vacuum control valve; solen-
oid valve (figure 11).
THROTTLE LEVER ACTUATOR
The Throttle Lever Actuator is mounted as part
of the carburetor assembly . This device controls the
position of the primary throttle plates a preset
EMISSION CONTROL SYSTEMS 6T- 7
Figure 10-Thermostatic Vacuum Switch
tributor vacuum hose at port "D" of the T.V .S .
switch, see Figure 10, connect a vacuum gauge and
check for vacuum with the engine idling at normal
operating temperature. If more than 5 in . Hg. of
vacuum is present and the hoses are connected to the
proper ports, check further with instruments de-
signed to test the switch such as BT-7002.
The switch must be installed with a soft setting
sealant on the threads.
amount in excess of curb idle when engine vacuum
is applied. This actuating vacuum is controlled by a
separate vacuum control valve .
The Throttle Lever Actuator bracket is secured
by two bolts torqued to 20-25 ft . lbs. The actuator is
mounted to the bracket by a single nut. This nut can
be used to adjust the stem to a travel requirement of
.260 inch (See "Throttle Actuator Adjustment") .
VACUUM CONTROL VALVE
The Vacuum Control Valve is crimped to a