Chevrolet 10 Series 1968, Service Manual

Page: 506 / 713

CLUTCHES AND TRANSMISSIONS 7-43
TURBO HYDRA-MATIC TRANSMISSION
INDEX
Page
General D e s c r ip t io n .......................................................... 7-43
Maintenance and A d ju s tm e n ts .......................................... 7-44
Transm ission F l u i d ....................................................... 7-44
Fluid Checking P r o c e d u r e ............................................. 7-44
Fluid L e v el Indicator....................................................... 7-45
Shift Control Linkage A d ju stm en t................................ 7-45
Detent Switch A d ju s tm e n t............................................. 7-46
Neutral Safety Switch A d ju stm en t................................ 7-46
Draining and R efillin g T ra n s m is s io n .......................... 7-46
Pressu re Regulator V a l v e ............................................. 7-46
Control Valve B o d y ....................................................... 7-46
G o v e r n o r .......................................................................... 7-47
Modulator and Modulator V a l v e ................................... 7-47
Page
Parking L i n k a g e ............................................................. 7-47
Rear S e a l.......................................................................... 7-48
Other Service O p e r a t io n s ................................................ 7-48
Transm ission Replacement ............................................. 7-48
Turbo Hydra-M atic Diagnosis P r o c e d u r e ....................... 7-48
S e q u e n c e .......................................................................... 7-48
Oil L evel and Condition C h e c k ................................... 7-48
Manual L in k ag e ................................................................ 7-49
O il L e a k s .......................................................................... 7-49
Case P oros ity —R e p a ir ................................................... 7-49
Oil P ressu re C h e c k ....................................................... 7-49
Vacuum Modulator A s s e m b l y ....................................... 7-50
Transm ission Shift Points .......................................... 7-51
GENERAL DESCRIPTION
The Turbo Hydra-Matic transmission is a fully auto­
matic unit consisting primarily of a 3-element hydraulic
torque converter and a compound planetary gear set.
Three multiple-disc clutches, one sprag unit, one roller
clutch, and two bands provide the friction elements re­
quired to obtain the desired function of the compound
planetary gear set.
The torque converter couples the engine to the plane­
tary gears through oil and provides hydraulic torque
multiplication when required. The compound planetary
gear set produces three forward speeds and reverse.
The 3-element torque converter consists of a pump or
driving member, a turbine or driven member, and a sta­
tor assembly. The stator is mounted on a one-way roller
clutch which will allow the stator to turn clockwise but
not counter-clockwise.
The torque converter housing is filled with oil and is
attached to the engine crankshaft by a flex plate and al­
ways rotates at engine speed. The converter pump is
an integral part of the converter housing, therefore the
pump blades, rotating at engine speed, set the oil within
the converter into motion and direct it to the turbine,
causing the turbine to rotate.
As the oil passes through the turbine it is traveling in
such a direction that if it were not re-directed by the
stator it would hit the rear of the converter pump blades
and impede its pumping action. So at low turbine speeds,
the oil is re-directed by the stator to the converter pump
in such a manner that it actually assists the converter
pump to deliver power or multiply engine torque.
As turbine speed increases, the direction of the oil
leaving the turbine changes and flows against the rear
side of the stator vanes in a clockwise direction. Since
the stator is now impeding the smooth flow of oil, its
roller clutch releases and it revolves freely on its shaft.
Once the stator becomes inactive, there is no further
multiplication of engine torque within the converter. At
this point, the converter is merely acting as a fluid
coupling as both the converter pump and turbine are being
driven at approximately the same speed - or at a one-to-
one ratio.
A hydraulic system pressurized by a gear type pump
provides the working pressure required to operate the
friction elements and automatic controls.
External control connections to transmission are:
Manual Linkage - To select the desired op­
erating range.
Engine Vacuum - To operate a vacuum mod­
ulator unit.
12 Volt Electrical - To operate an electrical
Signal detent solenoid.
A vacuum modulator is used to automatically sense any
change in the torque input to the transmission. The vacu­
um modulator transmits this signal to the pressure
regulator for line pressure control, to the 1-2 accumu­
lator valve, and to the shift valves so that all torque
requirements of the transmission are met and smooth
shifts are obtained at all throttle openings.
The detent solenoid is activated by an electric switch
on the carburetor. When the throttle is fully opened, the
switch on the carburetor is closed, activating the detent
solenoid and causing the transmission to downshift for
passing speeds.
The selector quadrant has six selector positions: P,
R, N, D, L2, L I.
P. - Park position positively locks the output shaft to
the transmission case by means of a locking pawl
to prevent the vehicle from rolling in either
direction. The engine may be started in Park
position.
NOTE: Park position is not provided for 30
Series Truck.
R. - Reverse enables the vehicle to be operated in a
reverse direction.
N. - Neutral position enables the engine to be started
and run without driving the vehicle.
D. - Drive Range is used for all normal driving con­
ditions and maximum economy. Drive Range has
three gear ratios, from the starting ratio to
direct drive. Detent downshifts are available by
depressing the accelerator to the floor.
L2. - L2 Range has the same starting ratio as Drive
Range, but prevents the transmission from shift­
ing above second speed to retain second speed
acceleration when extra performance is desired.
L2 Range can also be used for engine braking.
L2 Range can be selected at any vehicle speed,
and the transmission will shift to second gear and
CHEVROLET TRUCK SERVICE MANUAL