Chevrolet 10 Series 1968, Service Manual, Page: 679 / 713

ACCESSORIES 15-5
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 speedometer cable from the
transmission which turns a disk shaped ferrite magnet.
Facing the magnetic disk is the driven brass disk
mounted on a shaft with the rubber drum mounted on the
same shaft. A spiral hairspring connects the shaft to the
housing and allows it to rotate through an angle which is
proportional to car speed. If the car 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 gear drive for the magnetic disk is a 90 degree
nylon gear drive with the driven gear rotating 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 assembly
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 car speed. When the car reaches about 20 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 sole­
noid coil so that under 20 mph, no Regulator operation
is possible.
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 com­
prise the speed clutch of the regulator. When the sole­
noid 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 car speed will rotate the drum and move
the “ U” clip 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
car speed changes from the speed at which the solenoid
was energized. The direction of drum rotation is such
that resulting bleed valve operation will cause the Servo
to decrease engine power if the car exceeds the preset
speed and increase engine power if car speed decreases.
The air which passes out the orifice tube enters the
Regulator 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 retracts
and cams the ends of the “ U” clip outward so that it re ­
leases the rubber drum.
SPEED SW ITCH
D RIV E A D A PTO R ASSEM BLY
INPUT CO N N EC TIO N
VACUUM CONNECTOR
SERVO
EN GIN E VA CU UM
CO N N ECTO R
VA CU UM V A LV E
L O C K -IN
C f i
M
SOLENOID PLUNGER
SIDE VIEW SECTION END VIEW SECTION
SO LEN O ID C O IL AIR FILTER
M AGNET
TO SPEEDOM ETER
C O N TR O L V A LV E
CLUTCH SPRING
SPEED DISC
FIELD PLATE
H AIRSPRIN G
SO LEN O ID & CLUTCH
H O U SIN G ASSEM BLY
O R IFICE TUBE &
CO N N ECTO R A SSEM BLY
F ig . 8 —Regulator - Cross Section
CHEVROLET TRUCK SERVICE MANUAL