(a) Growl and humming or, more serious, a grinding
noise. These noises are caused by worn, chipped, rough
One of the major causes of bearing and gear failures
or cracked gears. As gears continue to wear, the grinding
in the auxiliary unit is poor driving habits.
noise will be noticeable, particularly in the gear position
Driver should be taught to always use the lo speed or
that throws the greatest load on the worn gear.
reductions available in the auxiliary unit and keep the
front box in the higher ratios not vice versa.
(b) Hissing or, more serious, a thumping or
Worn and pitted gears, as well as worn and pitted
bumping-type noise. Hissing noises can be caused by
bearings are usually caused by excessive use of the
bad bearings. As bearings wear and retainers start to
auxiliary overdrive gears with the mainbox in lower gear
break up, etc., the noise could change to a thumping or-
Broken teeth in the auxiliary unit are usually caused
by drivers trying to start their vehicles with the auxiliary
(c) Metallic rattles within the auxiliary usually result
unit in the high ratio while the big reduction is made in the
from a variety of conditions. Engine torsional vibrations
front box. Frogging or quick release of clutch gives a
are transmitted to the transmission through the clutch,
jump start also noted for breaking teeth.
which may be amplified and transmitted to the auxiliary
through the connecting propeller shaft. In heavy duty
equipment, clutch discs with vibration dampers are not
Noise is usually very elusive and generally not the
used, so a rattle, particularly in neutral, is common with
fault of the auxiliary; therefore, mechanics should road
diesel equipment. In general, engine speeds should be
test to determine if the driver's complaint of noise is
600 RPM or above to eliminate objectionable rattles and
actually in the auxiliary. Remember that auxiliary units act
vibration during the idle. Always leave the main box in
as sounding boxes and in numerous instances, drivers
neutral and the auxiliary unit in gear when idling. A
have insisted that the noise was in the auxiliary; however,
investigations revealed the noise to be caused by one of
defective or faulty injector would cause a rough or lower
the following conditions:
idle speed and a rattle in the auxiliary. Rattle could also
be caused by excessive backlash in P.T.O. unit
Fan out of balance or blades were bent.
Defective vibration dampers.
Crankshafts out of balance.
Flywheels out of balance.
produce noises. Auxiliaries with low oil levels sometimes
Flywheels mounting bolts loose.
run hotter than normal, as there is insufficient lubricant to
Engine rough at idle producing rattle in gear
cool and cover the gears.
Clutch assembly out of balance.
(e) Squealing, particularly when the auxiliary is
Engine mounts loose or broken.
operating at higher speeds, could be caused by one of
P.T.O. gear not fully engaged or housing not
the free running gears seizing on the thrust face or fluted
diameter temporarily and then letting go. In general, a
Universal joints worn out.
mild seizure will clear itself up and the auxiliary will
Propeller shafts out of balance.
continue to operate very satisfactorily without this defect
Universal joint angles out of plane or at
being known. See .(g) below:
Center bearings in drive line dry, not mounted
accompanied with loud squealing noise. This type of
Wheels out of balance.
seizure is readily apparent to the driver, since the truck
Tire treads humming or vibrating at certain
will suddenly slow down as if the brakes were being
applied. If the truck continues to move ahead, even
Air leaks on suction side of induction system
though the gear shift lever is placed in neutral, it would
specially with turbo-chargers.
indicate the floating gear on the mainshaft had seized.
Depressing the clutch should interrupt the driving torque.
Mechanics should try to locate and eliminate noise by
The seized gear could be checked quite readily by
means other than auxiliary removal, or overhaul.
depressing the clutch and checking the action with the
However, if the noise appears to be in the auxiliary try to
gear shift lever progressively in all shift positions. If
break it down into the following classifications. If
releasing the clutch tends to kill the engine, then this gear
possible, determine what position the gear shift lever is in
position has not seized. In other words, the auxiliary
when the noise occurs. If the noise is evident in only one
would be in two gears at the same time. By a process of
gear position, the cause of the noise is generally
elimination, the gear at fault can be readily identified. See
traceable to the gears in operation.
(g) below: (g) Vibration: Gear seizures on thrust faces or
fluted diameters are usually caused by vibrations in the
power train-this could be engine, propeller shafts, joint
angles rear axle, differentials, etc.