GMC 1976 ZEO 6083 Maintenance Manual page 378

6T- 8 EMISSION CONTROL SYSTEMS
THROTTLE LEVER ACTUATOR
NEUT .,START, BACK-UP
AND SAFETY SWITCH
ENGINE VACUUM
(VALVES OPEN ONLY)
EXTENDED POSITION
(WHEN VACUUM IS APPLIED)
PRIMARY THROTTLE LEVER
VACUUM CONTROL VALVE
VENT TO ATMOSPHERE
A-4134
mounting bracket and mounted separately from the
carburetor by one bolt turqued to 25-30 ft . lbs. This
on/off valve senses engine vacuum when the solenoid
valve is open, and opens above a preset high vacuum
level. The valve, when open, allows a vacuum signal
to be applied to the throttle lever actuator as long as
the preset vacuum level is exceeded .
SOLENOID VALVE
The Solenoid Valve is mounted separately from
the carburetor by one bolt torqued to 95-120 in . lbs.
This on/off valve opens when the transmission selec
tor lever is in a forward drive position, allowing en-
gine vacuum to be applied to the vacuum control
valve.
TRC SYSTEM DIAGNOSIS
CONTROL VALVE - CHECKING
PROCEDURE :
1 . Disconnect valve to carburetor hose at the
Figure 1 1-Throttle Return Control System
carburetor and connect to an external vacuum
source equipped with a vacuum gauge.
2. Disconnect the valve to actuator hose at the
actuator and connect to a vacuum gauge. (figure 12).
3 . Place finger firmly over the end of the bleed
fitting (the foam air filter need not be removed) .
4. Apply 23 in . Hg . vacuum to the control valve
and seal off the vacuum sburce . The gauge on the
actuator side should read the same as the source
gauge. If not, then the valve is sticking or needs
adjustment . A sticking valve must be replaced . If the
vacuum drops off on either gauge (finger still on the
bleed fitting), the valve is leaking and must be re-
placed .
5. With a 23 in . Hg . vacuum level in the valve,
remove finger from bleed fitting. The vacuum read-
ing on the actuator side will drop to zero and the
reading on the source gauge will drop to a value
which will be designated as the valve set point. If this