Chevrolet 10 Series 1970 Service Manual page 634

ACCESSORIES 15-4
When the solenoid is d e -e n e rg ize d , the nose re tra cts
and cam s the ends o f the " U " clip outward so that it r e ­
le a s e s the rubber drum.
The solenoid also op erates a vacuum switch sim u l­
taneously with the clutching and declutching of the " U "
clip. The solenoid operated vacuum va lv e s lid es o v e r two
p o rts in the R egu lator w all. One p ort is connected to
m anifold vacuum and the other is connected to a " T e e "
fittin g. When the solenoid is d e-e n e rg ize d , the va lve
c lo se s the m anifold vacuum p ort and opens the " T e e "
p o rt to the inside o f the R egu lator case. When the s o le ­
noid is en ergized , the va lv e connects m anifold vacuum
to the " T e e " fittin g, at which point a ir is blended to the
p ro p er proportion and im p ressed upon the Servo unit
accordin g to the dictates o f the R egu lator.
During system operation the fo llow in g events occur:
1. C ar speed below 30 M P H --n o function of the pu ll-in
circ u it because the rubber drum has not rotated fa r
enough to close the solenoid points. No p u ll-in cu r­
rent can flow through the solenoid co il. The solenoid
co il is re c e iv in g a sm all curren t via the 40 ohm
resista n ce w ire unless the brake pedal is depressed,
engagem ent sw itch fu lly d ep ressed , o r the ignition
switch is " o f f " .
2. C ar speed above 35 M P H — the tang on the rubber
drum has clo sed the solenoid points.
circ u it is now ready fo r engagem ent.
3. D r iv e r p a r tia lly p re s s e s Engagem ent S w itch --fu ll
voltage flow s through the solenoid to pull it into
operation. Solenoid cam tension on the " U " clip is
rele a s e d and the clip g rip s the rubber drum. Simul­
taneously, the vacuum switch applys m anifold vacuum
to the " T e e " fittin g.
with a ir bein g introduced fro m the R egulator. The
balance o f a ir and vacuum is im p ressed upon the
S ervo to p ro vid e fo r in itial throttle positioning.
4. D r iv e r re le a s e s the
flow s to the solenoid through the 40 ohm w ire and
since the solenoid is "p u lle d in " , the reduced cu r­
rent flow is su fficien t to hold it in position.
5. The ca r begins to ascend a h ill- - c a r speed drops
sligh tly (v e r y s ligh tly ) and the m agnetic fo r c e on the
driven disk o f the speed sensor is d ecreased. The
disk rota tes s lig h tly (as would a speedom eter shaft
The pull-in
H ere the vacuum is blended
Engagem ent Sw itch --cu rren t
Fig. 6 —S ervo, R egulator and Hose Routing
because o f h airsprin g tension) turning the rubber
drum. Since the " U " clip is gripping the drum, it
m oves the a ir bleed va lve
co v e rs the a ir bleed ports m ore. W ith le s s a ir b leed ­
ing into the " T e e " fitting a higher vacuum le v e l is
achieved at the S ervo diaphragm, opening the throttle
angle to c o r r e c t fo r the underspeed condition.
6. The ca r begins to descend a h ill- - c a r
crea ses sligh tly and the a ir bleed va lv e m oves in that
d irection which uncovers the a ir bleed p orts. With
m ore a ir bleedin g into the " T e e " fittin g, a lo w er
vacuum le v e l is achieved at the S ervo diaphragm
d ecrea sin g the throttle
o versp eed condition.
7. D r iv e r a c c e le ra te s by p ressin g a c c e le ra to r p e d a l--
ca r speed in crea ses and the system
m oving the diaphragm to d ecrea se th rottle opening.
Since a bead chain is used, the chain m e re ly r e ­
la xes and has no effe c t on th rottle operation. A fte r
the d r iv e r re le a s e s pressu re fro m
th rottle w ill close until car speed d ecrea ses to the
p r e - s e t speed. A t that point the R egu lator bleeds
le s s a ir to the " T e e " fittin g which opens the throttle
enough to maintain the p r e -s e t speed. The system
returns to a stable condition.
8. D r iv e r d e s ire s higher
a c c e le ra to r until new speed is
p re s s e s Engagement Switch fu lly and re le a s e s button
--s p e e d sensing assem bly t r ie s to turn would de­
crea se the th rottle opening until the d r iv e r fu lly
d ep resses the Engagem ent Switch. Then the current
is cut o ff to the solenoid which retra cts; the s o le ­
noid cam expands the " U " clip relea sin g its grip
on the rubber drum. The drum and disk assem bly
then rotates to a new position because o f the higher
car speed. When the
shuts o ff vacuum to the " T e e " fittin g and opens the
vacuum port to atm ospheric p ressu re within the
in that d irection which
speed in­
angle to c o r r e c t fo r the
responds by
the pedal, the
con trolled speed, p re s s e s
reached, and de­
solenoid re tra c ts , it also
CHEVROLET TRUCK SERVICE MANUAL