GMC 4000 Series Service Manual page 454

mechanism preventing full engagement of the
sliding clutch gear, or -
b. If the gear has been shifted completely
into position some other malfunction which could
move the gear out of its proper location.
2. A number of items which would prevent
full engagement of gears are:
a. Shift fork pads or groove in sliding gear
or collar worn excessively.
b. Worn taper on gear clutch teeth.
c. Transmission and engine out of alignment
either vertically or horizontally.
3. A few item s which could move the gear or
shaft out of proper position, particularly on rough
roads are:
a. Use of heavy shift lever extensions.
Shift
b. rod poppet sp rin g s broken.
c. Shift rod poppet notches worn.
NOTE: 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,
m issing, 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 synchronizing unit.
6. Improper adjustment of shifting linkage
or excessively worn.
7. Worn shift rods.
8. Worn, sprung shifter fork.
9. Wrong lubricant
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
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
transmission.
G M C S E R V I C E M A N U A L
especially if extreme
on d e t e n t ball
for synchronized
TRANSMISSION ON-VEHICLE SERVICE
When the shift lever moves directly into
the manual shift position without resistance, the
raking of teeth will be audible and felt through
the gearshift lever. This condition does not al­
ways 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. Con­
tinued operation of the transmission may either
embed 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.
tion usually occurs with cold, heavy oil, but
the synchronizer begins to work properly when
the transmission oil reaches normal operating
temperature.
The use of extreme pressure type lubri­
cants is not recommended. Glazing of the syn­
chronizer cone due to
especially common with extreme pressure addi­
tives found in multi-purpose or rear axle type
lubricants.
Broken synchronizer components sometimes
jam under poppet preventing proper movement of
synchronizer cone, 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
downshifting, 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.
OIL LEAKS
1. Oil level too high.
2. Wrong lubricant in unit.
3. Non-shielded bearing used at front or rear
bearing cap. (Where applicable.)
4. Seals (if used) defective or omitted from
bearing cap, wrong type seal used, etc.
5. Transmission breather omitted, plugged
internally, etc.
6. Cap screws loose,
from remote control, shifter housing, bearing
caps, power take-off, or covers, etc.
7. Welch " seal" plugs loose or m issing en­
tirely from machine openings in case.
8. Oil drain-back openings in bearing caps
or case plugged with varnish, dirt, covered with
gasket material, etc.
9. Broken g a s k e t s ,
Sec. 7B
Page 443
The above condi­
breakdown of oil is
omitted or missing
gaskets shifted or