GMC 4000 Series Service Manual page 452

NOTE:
Subject
Manual Transmission S erv icin g ..............................................
Manual Transmission Trouble D ia g n o s is .....................
Transmission Replacem ent..................................................
Rear Oil Seal Replacement..................................................
M A N U A L T R A N S M IS S IO N
TROUBLE DIAGNOSIS
The following trouble diagnosis information
will a ssist in locating transm ission troubles, but
in addition will serve as a guide to find the
"cause" to prevent reoccurence.
Whenever possible,
prior to overhaul. Mechanics usually get second
or third hand reports of trouble experienced with
the unit and these reports do not always accurate­
ly describe the actual conditions.
symptoms seem to indicate trouble in the trans­
mission; while actually the trouble may be caused
by the axle, propeller shaft, universal joint, en­
gine or clutch. Therefore, before removing trans­
m ission or related components to locate trouble,
always road test to check possibility that trouble
may exist in other closely associated units. If the
mechanic can drive, road testing will be more ef­
fective; however, just riding with the driver can
be very informative.
Many times the answer to the trouble is ap­
parent when the unit is inspected prior to disas­
sembly, but this evidence is often lost when the
parts are separated. If possible, check the unit
prior to disassembly. Bear in mind that a careful
inspection of the unit should be made as each dis­
assem bly step is performed.
It is poor practice to disassemble a unit or
complete transmission as quickly as possible
without bothering to examine the parts as they
come down. It happens many times that a me­
chanic has completely disassembled a unit and
failed to find the cause of the trouble because he
did not bother to examine the parts as they came
apart. After the transmission is disassembled,
check the lubricant for foreign particles which
often reveal sources of trouble that are over­
looked during the disassembly.
G M C S E R V I C E M A N U A L
SECTION 7B
On Vehicle SeAuice Opesvationl
-
Allison transmission overhaul information is contained in
separate publication Form Number SA 1126E.
MANUAL TRANSMISSION SERVICING
road test the vehicle
Sometimes
Page No.
NOISY OPERATION
Noise is usually very elusive and generally
not the fault of the transmission; therefore, me­
chanics should road test to determine if the
driver's complaint of noise is actually in the
transmission.
Noise Arising Outside Transmission
In numerous instances, drivers have insisted
that the noise was in the transmission, however,
investigations revealed the noise to be caused by
one of the following conditions:
1. Fan out-of-balance or blades were bent.
2. Defective vibration damper.
3. Crankshaft out-of-balance.
4. Flywheel out-of-balance.
5. Flywheel mounting bolts loose.
6. Engine rough at idle producing rattle in
gear train.
7. Clutch assembly out-of-balance.
8. Engine mounts loose or broken.
9. Power take-off engaged.
10. Universal joints worn out.
11. Propeller shafts out-of-balance.
12. Universal joint angles out of plane or at
excessive angle.
13. Center bearings in drive line dry - not
mounted properly, etc.
14. Wheels out-of-balance.
15. Tires treads humming or vibrating at
certain speeds.
16. Air leaks on suction side of induction
system - especially with turbo-chargers (if used).
Noise Arising In Transmission
Mechanics should try to locate and eliminate
noise by means other than transmission removal,
or overhaul. However, if the noise appears to be
in the transmission try to break it down into the
following classifications. If possible, determine
what position the gearshift lever is in when the
noise occurs. If the noise is evident in only one
gear position, the cause of the noise is generally
traceable to the gears in operation.
Sec. 7B
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