Chevrolet 70 1969 Series, Shop Manual

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TRANSMISSIONS AND CLUTCHES 7B-13
One other point that should be noted is whether the
unit walks out of gear under drive (while pulling a
load) or on a coast load. Also, does the gear hop
occur on smooth or only on rough roads. A num­
ber of items that would prevent full engagement of
gears are:
a. Improperly positioned control island shift
mechanism which limits full travel forward and
backward from the remote neutral position.
b. Improper length shift rods or linkage that
limits travel of control island shift mechanism
from neutral position.
c. Loose bell cranks, sloppy ball and socket
joints.
d. Shift rods, cables, etc., too spongy, flexi­
ble, or not secured properly at both ends.
e. Worn or loose engine mounts if forward
unit is mounted to frame.
f. Control island shift mechanism mount too
flimsy, loose on frame, etc. (tilt cab models).
g. Setscrews loose at remote control joints
or on shift forks inside remote or even inside
transmission unit.
h. Shift fork pads or groove in sliding gear
or collar worn excessively.
i. Worn taper on gear clutch teeth.
j. Transmission and engine out of alignment
either vertically or horizontally.
3. A few items which could move the gear or
shaft out of proper position, particularly on rough
roads are:
a. Use a heavy shift lever extensions.
b. Shift rod poppet springs broken.
c. Shift rod poppet notches worn.
N O T E : When gearshift lever can be held in
to prevent jump-out, detent modifications will
often correct it. When a gear has been allowed to
jump out for a long period generally the cause
must be corrected plus replacement of the af­
fected gears.
d. Shift rod bent or sprung out of line.
e. Shift fork pads not square with shift rod
bore.
f. Excessive end-play in drive gear, main­
shaft or countershaft, caused by worn bearings,
retainers, etc.
g. Thrust washers or faces worn excessively,
missing, etc.
HARD SHIFTING
1. Sliding gear tight on shaft splines.
2„ Insufficient chamfer of sliding gear teeth.
3. Burred mainshaft or sliding gear splines.
4. Misaligned mainshaft.
5. Damaged synchronizer (when used).
6. Improper adjustment of shifting linkage or
excessively worn.
7. Worn shift rods.
8. Worn, sprung shifter fork.
9. Wrong lubricant especially if extreme
pressure type lubricants are added.
10. Free-running gears, seized or galled on
either the thrust face or diameters.
STICKING IN GEAR
1. Insufficient chamfer on detent ball notches.
2. Chips wedged between or under splines of
shaft and gear.
3. Misaligned mainshaft and/or countershaft.
CRASH SHIFTING OR RAKING
OF GEARS
Raking of gears during the manual shift is
usually caused by a defective synchronizer or
improper shifting technique for synchronized
transmission.
When the shift lever moves directly into the
manual shift position without resistance, the rak­
ing of teeth will be audible and felt through the
gearshift lever. This condition does not always
mean the synchronizer is worn out. The following
may cause this condition:
1. Quite often, small chips may lodge in the
synchronizer temporarily, which prevents proper
synchronization and causes raking shifts. Contin­
ued operation of the transmission may either e m ­
bed the chip below the surface of the bronze or
reject it and the synchronizer will return to
normal functioning.
2. Use of improper oils often causes raking
of synchronizer. Heavy oil prevents the syn­
chronizer from breaking through the oil film and
doing the job properly. The above condition usual­
ly occurs, with cold, heavy oil, but the synchroniz­
er begins to work properly when the transmission
oil reaches normal operating temperature.
The use of extreme pressure type lubricants
is not recommended. Glazing of the synchronizer
due to breakdown of oil is especially common with
extreme pressure additives found in multi-purpose
or rear axle type lubricants.
Broken synchronizer components sometimes
jam under the poppet preventing proper move­
ment of the synchronizer, resulting in crash
shifts.
Worn synchronizer components with the loss
of clutching action are usually caused by poor
driver technique, or failure to control engine
speed drop-off during upshift, or failure to bring
engine speed nearly up to governor speed when
down-shifting, causes overwork of synchronizer
and failure to shift. Also, drivers who try to shift
without using the clutch will burn or wear out
manual synchronizers at relatively low mileage.
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L