GMC 1976 ZEO 6083 Maintenance Manual page 642

PARTIALLY DEPRESSED
Full voltage is applied to the solenoid (vehicle
speed over 31 mph) which sets the transducer to
maintain the vehicle speed at the time of transducer
engagement .
FULLY DEPRESSED
No electricity flows to the solenoid and the trans-
ducer is inactive . This position is used by the driver
when he desires to raise or lower his controlled
speed. He may accelerate to his new speed, press the
button fully (transducer releases previously set
speed) and release the button . Upon releasing the
button, it passes through the partially depressed po-
sition and the solenoid is "pulled in", then into
released position which provides "hold in" current.
The driver may also press the button fully with no
pressure on the accelerator pedal. In this case the
transducer releases control of the throttle which re-
turns to idle and the vehicle slows. When the button
is released the solenoid is pulled in and held in re-
spectively and the transducer resumes speed control
at the speed of the vehicle during the moment of
button release (at vehicle speeds over 31 mph) .
BRAKE RELEASE SWITCHES
Two brake release switches are employed in the
Cruise Control System . When the brake pedal is de-
pressed; an electric release switch cuts offthe voltage
supplied to the engagement switch, hence cuts off
power to the transducer unit . The transducer is then
disengaged and requires engagement switch operat-
SERVO UNIT
TRANSDUCER
Figure 53-Servo Unit Installation
CHASSIS ELECTRICAL
12- 37
ion to return it to operation. A vacuum release
switch operates simultaneously with the electric re-
lease 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 posi-
tion .
The servo unit is a vacuum actuated, variable
position diaphragm assembly, which operates the
carburetor throttle when the system is in operation .
It is powered by controlled vacuum from the trans-
ducer and operates the throttle chainage via the ad-
justable link . (See figures 53 and 55). The servo has
a port on the sealed side of the diaphragm housing.
When controlled vacuum is applied to this port, at-
mospheric pressure moves the diaphragm which
pulls on the adjustable chain opening the carburetor
throttle .
The transducer, Figures 54 and 55 is a device
which has two primary functions. First, it is a
vacuum switch which, when engaged by the driver,
supplies vacuum to a "Tee" fitting. Second, it meters
a small variable quantity of air to the "Tee" fitting
where it blends with vacuum, thus providing the
power unit with controlled vacuum which will main-
tain the selected speed. If the transducer begins to
supply less bleed air (vehicle speed decreasing) the
vacuum in the chamber increases and the diaphragm
moves toward the vacuum port . If the transducer