Volvo Symptom D12D US02 Service Manual page 70

Group 20
Noise
Tracing the cause of abnormal sounds is often difficult,
as the noise can be "transferred" to various parts of the
vehicle.
It may seem that the noise originates from different
locations, depending on where you stand and listen.
Vibrations
Vibrations are easily noticed, but finding their exact cause
is as difficult as finding the source of abnormal sounds.
The sources of vibrations often vary. Engine vibrations,
vibrations felt at a particular road speed, or vibrations
caused by irregularities in the road surface, can each set
up a different pattern of frequencies.
Engine power is transferred from the cylinders via the
transmission and the final drive to the tire contact with
the road surface. The transfer of power can set other
components in motion creating noise and vibrations, such
as excessive torsional or transverse vibration.
Fault Tracing
Temporary faults can be difficult to trace since the fault
may not occur when the vehicle is brought in to be
checked.
Fault tracing should be performed based on what the
driver has experienced. The information that the driver
has provided in conjunction with the gathering of factual
data should form the basis of how the problem should
be resolved.
Whenever possible, try to recreate the problem in an
environment and situation similar to the one described
by the driver.
Begin by trying to determine what area causes the
noise or vibration. For example, does the engine,
part of the chassis structure, or any body building
adaptations cause the noise?
One way to find the source of abnormal noise or vibration
is to make an educated guess and to temporarily
disconnect or disengage any suspected component.
Problems during idling (in a stationary vehicle)
The operation of an engine generates a large number of
frequencies which can be transferred to other components
which then begin to oscillate.
A typical example of this is the idling vibration which is
experienced in the cab. Sometimes simply adjusting the
engine idling speed can remedy the problem. A small
change in engine speed, up or down, may be sufficient.
Since there are a number of different cab variants, there
is no general solution for the best idling speed for the
engine. Each vehicle is unique in this regard.
66
D
An open mind and the ability to see beyond the obvious is
oftentimes necessary to find the source of the problem.
Torsional vibration
This can be caused by universal joints operating at the
incorrect angle, poor clutch conditions, or abnormal
engine torque spikes. This can twist the drive shaft or
move it away from the straight line, resulting in vibration.
Transverse vibration
This is caused by imbalance in the drive shaft or related
components. If one side is heavier, centrifugal force is
applied to the drive shaft, resulting in vibration. This
varies with engine speed.
Begin by making a list of suspected components. Test
one component at a time. It is important to be systematic
when doing this.
Example:
A screeching noise from the engine appears to follow
changes in the engine speed. All indications are that the
noise comes from the front of the engine.
A educated guess is that the noise is from the alternator
or the fan.
•
Remove the alternator belts. Listen to determine
whether the noise is still there.
•
Remove the fan belt. Listen to determine whether the
noise is still there.
If the problem was found, the guess was right. Otherwise
you will need to continue with new assumptions and new
attempts.
Note that the cylinder balancing values indicate that
adjustment is necessary to obtain an even idle speed. A
high or low value, in which no fault code has been set,
must not result in a unit injector being replaced. When
fault tracing it may be helpful to check whether there is
a difference between the idling and the power take-off
speeds. No cylinder balancing is carried out when the
power take-off is engaged.
As a rule the engine also operates differently.