GMC X-7425 1973 User manual

X-7425
1973 GMC Motorhome
Maintenance Manual
Table of Contents
Section Description
0 Information, Periodic Maintenance and Lubrication
1 Body, Heating and Air Conditioning
2 Frame
3A Front Suspension
3B Drive Axles
3C Final Drive
4 Rear Suspension
5 Brakes
6A Engine
6K Engine Cooling
6M Engine Fuel System
6T Emission Control Systems
6Y Engine Electrical
7 Transmission
8 Fuel Tank and Exhaust
9 Steering System
10 Wheels and Tires
12 Chassis Electrical
13 Radiator and Coolant Recovery System
14 Bumpers
24 Miscellaneous Living Area Facilities
24A Periodic Maintenance and Lubrication
24B Living Area Electrical System
24C Motor Generator
24D Refrigerator
24E Roof Mounted Air Conditioner (Information not available at time of publication)
24F LP Gas System
24G Furnace
24H Range/Oven
24J Living Area Water System
24K Toilet
24L Holding Tank and Drainage System
24M Thermasan System
24N Cabinets and Furniture
24P Exhaust Vents
24Q Other Equipment (Vacuum Cleaner)
Wiring Diagrams

MIMIC
MOTOR
HOME
SERVICE
GENERAL INFORMATION
DESCRIPTION
The Turbo
Hydra-Matic transmission, Figure 1,
is
a
fully
automatic transmission used for front wheel
drive applications
.
The
Turbo Hydra-matic transmission consists
primarily of a three-element
hydraulictorque con-
verter, a dual sprocket and link assembly
and a com
pound planetary gear
set .
Three
multiple-disc
clutches, a sprag unit, a roller
clutch unit, andtwo
bands provide the friction elements
required to ob-
tain the desired functions of the compound planetary
gear set .
The torque converter, the
dual sprocket
and link,
the clutches, the sprag and roller clutch,
couple the
engine to the planetary gears, providing
three for
ward speeds and one reverse .

The torque con-
verter when required will supplement the gears by
multiplying engine torque .
The torque converter is of welded construction and
is serviced as anassembly
. The unit is made up of two
vaned sections, or halves,
that face each other in an
oil filled housing . The pump half of the
converter is
connected to the engine and the turbine half is, in
effect,
connected to the transmission .
The
torque converter couples the engine to the
planetary
gear set through the use of a drive
sprocket, a link
assembly, and a driven sprocket .
Clockwise engine torque turns the drive sprocket
clockwise, which, in turn, drives the driven sprocket
in a clockwise direction . This
in effect is a reverse in
the direction of enginetorque due
to the side mount-
ing of
the gear unit .
SECTION 7
TRANSMISSION
Contents of this section are listed below :
are :
First =
2 .48 :1
gear
ratio
Second = 1 .48 :1 gear
ratio
Third
=
1 .0 :1 gear ratio
Reverse = 2 .08 :1 gear ratio
OCTOBER, 1973
SUBJECT

PAGE N0 .
General Information . . ... . ... . . ... . ... . ....
. ... . .... . ....... . .... . ........ . . ... . ... . . ... . ..... . .... . .... . . ..... . ..... . . ... . ..... . . 7-1
Trouble Diagnosis .... . .... . ... . . . . . . ... . .... . ... .
............ . ... . .... . .... . .... . ... . . ... . ..... . .... . .... . . ..... . .... . . ... . . ..... . . 7-28
On Vehicle Servicing . . ... . ... . . . . .. ...
. .... . ........ . ....... . ... . .... . .... . .... . ... . . ... . ..... . .... . .... . . ..... . ..... . . . . . . ..... . . 7-42
Transmission Replacement . . . . . ... . ... . . ... . ... .
... . . . . . . ... . .... . ... . ......... . . . . . . .... . . ... . . .... . . ..... . .... . .... . . ..... . . 7-46
Transmission Overhaul . . ... . . . . . . ... . ... . . ... . ... . .... . . .
. . ... . .... . ... . ......... . . . . . . .... . .... . . .... . . ... ... ... . . .... . . ..... . . 7-49
Transmission
Specifications
. . . .. ...
. ... . . ... . ... . ... . . . . . . ... . ... . . ... . .... . .... . . . . . . .... . .... . . .... . .... ... ... . . .... . . .... . . . 7-86
Special Tools . . . ... . . ... . . ... . . ... . . . . . . ... . ... . . . . . . ... . ... . . . . . . ... . ... . . . . . . ...
. .... . . ... . . ... . ..... . . ... . . .... . ..... . . .... . . ... . . . . 7-86
When the engine makes the converter pump re-
volve, it sends oil against the turbine, making it re-
volve
also . The oil then returns in a circular flow
back to the converter pump, continuing this flow as
long as the engine is running .
The converter also has a smaller vaned section,
called a
stator, that
funnels
the
oil
back to the con-
verter pump
through
smaller
openings, at increased
speed . The speeded up oil directs additional force to
the engine-driven converter pump,
thereby
multiply-
ing engine torque .
A hydraulic system pressurized byan internal-
external type gear pump provides the working pres-
sure required to operate the friction elements and
automatic controls .
External control connections to the transmission
Manual Linkage-To select the desired operating
range .
Engine Vacuum-To
operate a vacuum modula-
tor unit .
12 Volt Electrical Signal-To operate an electrical
detent solenoid .
Gear or Torque ratios of the transmission are as
follows :

7- 2

TRANSMISSION
GOVERNOR ASSEMBLY
OUTPUT CARRIER AND
INTERNAL
GEAR
REAR UNIT INTERNAL
GEAR
SUN GEAR
MAINSHAFT
GOVERNOR DRIVEN
GEAR
GOVERNOR
DRIVE
GEAR
OUTPUT
FLANGE
SUN GEAR SHAFT
REACTION
CARRIER
LO
ROLLER
CLUTCH
GOVERNOR PIPES
CONVERTER
PUMP
TURBINE
STATOR ROLLER
CLUTCH
STATOR
SHAFT
TURBINE SHAFT
INTERMEDIATE CLUTCH
INTERMEDIATE CLUTCH,
PISTON
CENTER SUPPORT
SPEEDOMETER
DRIVEN
GEAR
ASSEMBLY
INTERMEDIATE SPRAG CLUTCH

CENTER
REAR
SERVO
SUPPORT
ASSEMBLY
BOLT
DETENT
SOLENOID
PUMP LINK ASSEMBLY
FRONT SERVO
ASSEMBLY

A-2076
First and reverse gears can be multiplied by as
much as 2 .2 :1, depending upon the slip speed of the
converter pump and turbine .
A vacuum modulator is used to sense engine
torque input to the transmission automatically . The
vacuum
modulator transmits this signal to the pres
sure regulator,
which controls line pressure, so that
all torque requirements
of the transmission are met
Figure 1-Turbo Hydra-matic Transmission
and proper
shift
spacing is
obtained
at
all throttle
openings .
The downshift solenoid is activated byan adjust-
able switch at the accelerator pedal . When the throt-
tle is opened sufficiently to close this switch, the
solenoid in the transmission is activated, causing a
downshift at speeds below approximately 70
miles
per hour . At lower speeds,
downshifts will occur at

lesser throttle
openings without use of the switch .
The
oil
cooler is
located in the right hand tank of
the radiator .
The transmission is cooled by directing
oil from the
converter to the radiator . Oil returning
from the radiator feeds the
transmission lubrication
system
.
The
oil system incorporates an intake pipe
and
filter assembly . The filter assembly shouldbe
re-
placed after each 12,000 miles or 12
months .
The transmission quadrant
has six selector posi-
tions that enable the driver to control the operation
of the transmission under various
driving conditions .
The six selector positions appearon
the quadrant in
the followingsequence, from left to right
; PARK-
park, R-reverse, N-neutral, D-drive,
S-super, L-lo .
PARK-Park position positively
locks theoutput
flange to the transmission case
by means of a locking
pawl and prevents the
vehicle from rolling either
forward or backward
. For this reason, it is recom-
mended that
the engine be started with transmission
selector lever
in Park position . If it is necessary to
re-start the
engine with vehicle rolling, place selector
lever in Neutral
and start the engine .
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 . It is
recommended that Neutral be used to start the en
gine only if it is necessary to re-start
the engine with
the vehicle rolling . At all other times use Park .
D-Drive is used for all normal
driving conditions
and maximum economy . Drive has
three
gear
ratios
from starting to direct drive . Downshifts
are availa
ble for safe passing by depressing
the accelerator
pedal .
S-Super adds performance for
congested traffic
or engine braking in hilly
terrain
.
The Super range
has the same starting gear
ratio as Drive, but pre-
vents the transmission
from shifting above second
speed to
retain acceleration when extra performance
is desired .
L-Lo range permits operation at a lower
gear
ratio, and shouldbe used where
maximum engine
braking is desired, such as in descending a steep
grade
.
When selector lever is moved from Drive to
Lo range at normal highway speeds, the transmission
will shift to second gear and remain in second
gear
until vehicle speed is reduced to
approximately 45
mph . The transmission will then shift to first gear
and remain in first gear regardless of vehicle or en-
gine speed, until selector lever is moved
back into
either Super or Drive position
.
TRANSMISSION

7- 3
HYDRAULIC SYSTEM
PRESSURE CONTROL
The transmission is controlled
automatically by
a hydraulic system, Figure 2 .
Hydraulic pressure is
supplied by the transmission
oil pump, which is en-
gine driven . Main line pressure
is controlled by a
pressure regulator valve train
located in the trans-
mission case and by the
vacuum modulator which is
connected to engine vacuum
. The pressure regulator
controls line pressure automatically,
in response to a
pressure signal from a modulator
valve in such a way
that the torque requirements of the
transmission
clutches are met and proper
shift spacing is obtained
at all throttle openings
and vehicle speeds .
To control line pressure
properly, modulator
pressure is used which
varies in the same manner as
torque input to the
transmission . Since the torque
input to the clutches
is the product of engine torque
and converter
ratio, modulator pressure mustcom-
pensate
for changes in either or both of these .
To meet these requirements,
modulator pressure
is regulated by engine vacuum,
which is an indicator
of engine torque, (Governor pressure
also controls
modulator pressure) . It
will
decrease
with an in-
crease in vehicle speed to compensate for the
chang-
ing converter torque ratio, by virture
of
the
governor
pressure influence .
VACUUM MODULATOR
ASSEMBLY
The engine vacuum
signal is received by the
vacuum modulator,
Figure 3, which consists of an
Figure
2-Pressure Control

7- 4

TRANSMISSION
Figure 3-Vacuum Modulator Assembly
evacuated metal bellows, a diaphragm and a spring .
These are so arranged that the
bellows and spring
apply
a force that acts on the modulator valve so that
it increases modulator pressure .
Engine vacuum and the enclosed spring oppose
the bellows
and spring to control modulator pres-
sure .
To reduce the effect of altitude
on shift points,
the effective area of the diaphragm is different
than
that of the bellows . Atmospheric
pressure acts on the
resulting differential area
to reduce modulator pres-
sure .
GOVERNOR ASSEMBLY
The
vehicle speed signal to the transmission
is
supplied by
the governor, which is driven by the
output flange . The
governor consists of flyweights
and a regulator valve .
Centrifugal force of the fly-
weights is imposed
on the regulator valve, causing it
to regulate a pressure signal
that increases with
speed .
Governor pressure acts
on the modulator valve
to cause modulator pressure to decrease
as vehicle
speed increases .
FUNCTIONS OF VALVES AND
HYDRAULIC CONTROL UNITS
Line Pressure
Regulator
Regulates line pressure
to satisfy engine torque .
Manual Valve
Establishes the range of transmission operation,
PARK,
R, N, D, S, or L, as selected
by the vehicle
operator
through the manual selector lever .
Governor
Assembly
Generates a speed
sensitive oil pressure that in-
creases with output
shaft or vehicle speed . Governor
pressure is used
to control the shift points
and
modulator pressure regulation
.
Vacuum Modulator
Valve
Provides modulator pressure
that senses engine
torque and vehicle speed . The
vacuum modulator is
used to vary the shift
points according to throttle
opening and to raise line
pressure proportional to
input torque to the transmission
.
1-2 Shift Valve
Activates the 1-2 and 2-1
shifts .
1-2
Regulator Valve
Controls
the flow of modulator pressure
to the
1-2 shift valve
to regulate the minimum shift point
.
1-2 DetentValve
Senses regulated modulator
pressure tending to
hold 1-2 shift valve downshifted
and provides an area
for detent pressure for detent
2-1 shifts .
2-3 Shift Valve
Activates the 2-3 and 3-2 shifts .
2-3 Modulator
Valve
Senses modulator
pressure to apply a variable
force
that tends to hold the 2-3 shift valve down-
shifted .
3-2 Valve
Shuts off
modulator pressure from acting on the
shift valves after the
direct clutch has been applied .
This allows fairly heavy
throttle operation in third
speedwithout downshifting .
In third speed, detent
pressure or modulator pressure above
87 psi can be
directed to the shift valves to provide
the downshift
forces .
1-2 Accumulator Valve
Regulates drive oil to a proportional
lesser value

that increases as modulator pressure increases, to
control engagement of the intermediate clutch .
Detent Valve
Shifts when line
oil
is
exhausted at the end of the
valve when the detent solenoid is energized . This
directs detent oil to the 1-2 and 2-3 modulator valves
and allows
the detent regulator valve to regulate .
Detent Regulator Valve
When the detent valve shifts, the detent regulator
is freed to allow drive oil to enter the detent passage
and thus becomes regulated at 70 psi . Dctcnt pres
sures will also flow into the modulator passage
which
flows to the shift valves . Detent pressure acting on
the modulator valve limits the
low limit of modulator
pressure to the value of detent
pressure .
Rear Servo and
Accumulator Assembly
The
rear
servo piston applies the rear band for
engine braking in Lo range 1st gear . It applies the
band in Reverse
to hold the reaction carrier to pro-
vide the reverse gear
ratio .
On the 1-2
shift in Drive or Super, the rear ac-
cumulator piston serves as an accumulator for the
intermediate clutch
to provide a smooth shift .
Front
Servo
TRANSMISSION

7- 5
The front
servo piston applies the
front
band to
provide engine braking in 2nd gear in Super and Lo
ranges . The front
accumulator
piston
cushions the
apply of the direct clutch, and in conjunction with a
series of check balls controlling orifices, is a part of
the timing for the release of the direct clutch .
To prevent the
apply
of the front band in Neu-
tral, Drive and Reverse ranges, oil is directed from
the manual valve to the release side of the servo
piston .
In Drive the servo release oil from the manual
valve is used to charge the servo in preparation for
the apply of the direct clutch .
Direct clutch oil is directed to the front servo
accumulator piston where spring force plus direct
clutch pressure stroke the piston up against the force
of servo release oil . This lowers the clutch apply
pressure for a smooth engagement .
The release of the direct clutch and the exhaust-
ing of the front servo accumulator is
slowed down by
the three check balls and three orifices which permits
a soft return of the drive load to the intermediate
roller clutch and also
allows engine rpm to increase
during a detent 3-2
downshift in preparation for the
lower gear
ratio, which results in a smooth shift and
better acceleration
.

7- 6

TRANSMISSION
Forward Clutch - Released
Roller Clutch - Ineffective
Direct Clutch - Released
Front Band - Released
Rear Band - Released
Whenever the engine is running at idle with the
selector lever in "PARK" or "N", oil from the pump
is directed to the :
1 . Converter
a .
Oil Cooler
b . Lubrication System
2 . Vacuum
Modulator Valve
a . Pressure
Regulator BoostValve
b . 1-2
Accumulator and Primary Valves
c . Detent Valve
d . 3-2 Valve
e . 2-3 Shift
Valve Train
f . 1-2 Shift ValveTrain
3 . Manual Valve
4 . Detent Solenoid
5 . Detent Valve
6 . Front Servo
Piston (Neutral only)
PARK OR
NEUTRAL-ENGINE
RUNNING
POWER FLOW
OIL FLOW (FIGURE 4)
Intermediate Clutch -
Released
Intermediate Sprag - Ineffective
In Neutral or Park no bands or clutches areap-
plied, therefore nopower is transmitted .
Cooling and Lubrication
Oil flows from the pump to the pressure regulator
valve which regulates pump pressure . When the
pump output exceeds the demand of line pressure, oil
from the pressure regulator is directed to the con-
verter feed passage to fill theconverter . Oil from the
converter is directed to the transmission cooler . Oil
from the cooler is directed to the transmission lu-
brication system .
Line pressure acts on the :
1 . Manual Valve
2 . Detent Valve
3 . Detent Solenoid
4 . Modulator Valve
5 . Front Servo Piston
Line pressure at the modulator valve is regulated
to a pressure called modulator oil, which acts on the
pressure boost valve, 1-2 accumulator and primary
valves, and passes through the detent valve and 3-2
valve to the 1-2 and 2-3 valve trains .
Summary
The converter is filled .
All
clutches and bands are
released .
The transmission is in Neutral .

MAINLINE
PRESSURE
INTAKE
CONVERTER PRESSURE
GOVERNOR PRESSURE
MODULATOR PRESSURE
DETINT PRESSURE
1
-7 ACCUMULATOR PRESSURE

7- 8

TRANSMISSION
Forward Clutch - Applied
Roller Clutch -
Effective
Direct Clutch -
Released
Front Band -
Released
Rear Band - Released
Intermediate Clutch -
Released
Intermediate Sprag - Ineffective
With the selector lever in
either
Drive
or Super
Range, the forward
clutch is applied . This delivers
turbine torque to the drive sprocket, through the link
assembly to the driven sprocket and mainshaft and
turns the rear internal gear in a counterclockwise
direction . (Converter torque ratio = approximately
2 .2 :1 at stall .)
Counterclockwise motion of the rear internal
gear causes
the rear pinions to turn counterclockwise
to drive the
sun gear clockwise . In turn, the sun gear
drives the front pinions counterclockwise,
thrus
turning
the front internal gear, output
carrier, and
output flange counterclockwise in a
reduction ratio
of approximately 2.48 :1 . Reaction of the
front pin-
ions
against the front internal gear is taken
by reac-
tion carrier
and roller clutch assembly to the
transmission case . (Approximate stall ratio - 5 :1) .
When
the selector lever is moved to either Drive
or Super
position,
the manual valve is repositioned to
allow line pressure to enter the drive circuit . Drive
oil then flows to the :
1 . Forward
Clutch
2 . 1-2 Shift Valve
3 . Governor Assembly
4 . 1-2 Accumulator Valve
5 . Detent Regulator Valve
Basic Control
Drive oil is directed to the forward clutch where
it acts
on two
areas
of the clutch piston to apply the
forward clutch . The first, or inner
area, is fed
DRIVE AND SUPER-FIRST
SPEED
POWER FLOW
(FIGURE 5)
OIL
FLOW (FIGURE 6)
Figure 5-Drive Range-First
Gear (Power)
through
an unrestricted passage . The outer area is
fed
through an orifice to insure a smooth shift into
Drive .
Drive
oil at the governorassembly is regulated to
a variable
pressure . This pressure increases with
vehicle
speed and acts against the ends of the 1-2 and
2-3 shift
valves and an area on the modulator valve .
Drive oil is also regulated to another variable
pressure at the 1-2 accumulator valve . This pressure
is controlled by modulator oil and is directed to the
rear accumulator piston . In addition, to maintain the
lower pressure in the 1-2 accumulator passage, the
1-2 accumulator valve intermittently uncovers the to
oil passage and oil is exhausted at the manual valve .
Summary
The converter is filled . The forward
clutch is
applied . The transmission is in first
gear
.

MAINLINE PRESSURE
INTAKE PRESSURE
CONVERTER PRESSURE
GOVERNOR PRESSURE
MODULATOR PRESSURE
DETENT PRESSURE
1-2 ACCUMULATOR
PRESSURE