GMC 1976 ZEO 6083 Maintenance Manual page 644

The transducer is electrically engaged and disen-
gaged through operation of the engagement switch
and the electric brake release switch . It has two
subassemblies which make up the unit : one being the
magnetic speed sensing assembly and the other being
the solenoid actuated vacuum switch, air bleed and
filter, and low limit speed switch assembly .
Magnetic Speed Sensing Assembly
The speed sensing assembly operates in the same
manner as a speedometer unit except that instead of
rotating a needle through an angle proportional to
the vehicle speed, it rotates a rubber drum which is
clutched to the air bleed valve when the system is in
operation. The assembly is driven by the speedome-
ter cable from the transmission which turns a disk
shaped ferrite magnet .
Facing the magnetic disk is the driven copper
disk mounted on a shaft with the rubber drum
mounted on the same shaft. A spiral hairspring con
nects the shaft to the housing and allows it to rotate
through an angle which is proportional to vehicle
speed. If the vehicle doubled its speed, the shaft
would rotate to twice its previous angle as may be
seen by noting the operation of a speedometer. The
driven disk is sandwiched between the magnetic disk
and a field plate. The field plate forms a returning
path for the magnetic field from the magnetic disk .
The input shaft drives both the magnetic disk and
the speedometer drive cable.
Vacuum Switch, Air Bleed and Filter, and Low
Limit Speed Switch
The end of the shaft from the speed sensing as-
sembly with the rubber drum extends into the air
bleed metering assembly . This rubber drum has a
tang extending from its surface which allows a set of
points to close at a specific speed. When the vehicle
reaches about 31 mph, the rubber drum has rotated
far enough (moved by the brass driven disk in the
magnetic field) so that its tang has allowed a spring
loaded electrical point to contact another point.
These points are in series with the solenoid coil so
that under 24 mph, no transducer operation is possi-
ble.
Surrounding the rubber drum is a "U" shaped
spring clip which is held spread away from the drum
by the nose or cam of the solenoid when the solenoid
is in the relaxed position . The rubber drum and this
clip comprise the speed clutch of the transducer.
When the solenoid is energized, the solenoid nose
moves toward the drum and releases the ends of the
clip . The clip springs inward and attaches itself by
friction to the drum . Now, any change in vehicle
speed will rotate the drum and move the "U" clip
CHASSIS ELECTRICAL
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just as a speedometer moves its needle . The top of the
"U" clip is attached to the air bleed valve. The clip
moves a sleeve which slides on the orifice tube
thereby covering and uncovering air ports in the wall
of the tube (the tube inner end is plugged) whenever
vehicle speed changes from the speed at which the
solenoid was energized . The direction of drum rota-
tion is such that resulting bleed valve operation will
cause the servo to decrease engine power if the vehi-
cle exceeds the preset speed and increase engine
power if vehicle speed decreases . The air which
passes out the orifice tube enters the transducer
through the openings in the solenoid housing, passes
through the oil wetted polyurethane filter, and then
enters the orifice tube ports.
When the solenoid is de-energized, the nose re-
tracts and cams the ends of the "U" clip outward so
that it releases the rubber drum .
The solenoid also operates a vacuum switch
simultaneously with the clutching and declutching of
the "U" clip . The solenoid operated vacuum valve
slides over two ports in the transducer wall . One port
is connected to manifold vacuum and the other is
connected to a "Tee" fitting. When the solenoid is
de-energized, the valve closes the manifold vacuum
port and opens the "Tee" port to the inside of the
transducer case . When the solenoid is energized, the
valve connects manifold vacuum to the "Tee" fitting,
at which point air is blended to the proper proportion
and impressed upon the servo unit according to the
dictates of the transducer .
During system operation the following events oc-
curs :
1 . Vehicle speed below 24 mph - no function of
the pull-in circuit because the rubber drum has not
rotated far enough to close the solenoid points . No
pull-in current can flow through the solenoid coil .
The solenoid coil is receiving a small current via the
40 ohm resistance wire unless the brake pedal is
depressed, engagement switch fully depressed, or the
ignition switch is "off'.
2. Vehicle speed above 31 mph - the tang on the
rubber drum has closed the solenoid points . The
pull-in cirucit is now ready for engagement .
3. Driver partially presses engagement switch -
full voltage flows through the solenoid to pull it into
operation. Solenoid cam tension on the "U" clip is
released and the clip grips the rubber drum . Simul-
taneously, the vacuum switch applys manifold
vacuum to the "Tee" fitting. Here the vacuum is
blended with air being introduced from the trans-
ducer. The balance of air and vacuum is impressed
upon the servo to provide for initial throttle position-
ing.