Table of Contents
Advertisement
ACCESSORIES 15-3
CONTROLLED AIR SUPPLY
FROM REGULATOR
DIAPHRAGM
SPRING
METERING ROD
VARIABLE ORF1CE
ENGINE AAANIFOLD VACUUM
FROM REGULATOR VACUUM VALVE
BEAD CHAIN
VACUUM CHAMBER —*
MOUNTING SCREW
Fig. 4 - Servo Unit
BRAKE RELEASE SWITCHES
Two brake release switches are employed in the Cruise
Master System. When the brake pedal is depressed, an
Electric Release Switch cuts off the voltage supplied to
the engagement switch, hence cuts off power to the Regu-
lator unit. The Regulator is then disengaged and requires
Engagement Switch operation to return it to operation.
A Vacuum Release Switch operates simultaneously with
the electric release switch whenever the brake pedal is
depressed. This switch opens a port to atmospheric
pressure which rapidly bleeds down the vacuum in the
Servo unit thereby returning the throttle to the idle
position.
SERVO UNIT
The Servo unit is a vacuum actuated, variable position
diaphragm assembly which operates the carburetor throt-
tle when the system is in operation (fig. 4). It is powered
by engine intake manifold vacuum and operates the throt-
tle linkage via a bead chain. The Servo has two ports on
the sealed side of the diaphragm housing: one is supplied
manifold vacuum, and the other is connected to a variable
air bleed in the Regulator Unit. The vacuum port is
located at the center of the unit and the air bleed port
is near the outer wall. When vacuum is applied to the
center port, atmospheric pressure moves the diaphragm
which pulls on the bead chain opening the carburetor
throttle. As the diaphragm moves, it positions a tapered
needle within an orifice in the vacuum port so that as the
diaphragm moves toward the port, the orifice becomes
smaller; and as it moves away from the port, the orifice
becomes larger.
The air bleed port serves to supply a variable quantity
of air to the diaphragm chamber which causes the dia-
phragm to attain a balanced state (between the force of
the atmospheric pressure to chamber pressure differ-
ential and the force of the diaphragm and throttle return
springs). In operation then, the following events occur:
Vacuum is applied to the center port and the diaphragm
moves toward the port. The tapered needle restricts
the vacuum port more and more as it moves into the
orifice. If no air was allowed to bleed into the chamber,
the diaphragm would move until it contacted the housing,
however, the Regulator meters bleed air into the chamber
and the diaphragm reaches a point at which the air is
bled out of the chamber through the vacuum port orifice
as fast as it enters the air bleed port. If the Regulator
begins to supply less bleed air, (vehicle speed decreas-
ing) the vacuum in the chamber increases and the dia-
phragm moves toward the vacuum port. In so doing, the
tapered needle closes the vacuum port orifice even more
CHEVROLET CHASSIS SERVICE MANUAL
Advertisement
Table of Contents
