Volvo Symptom D12D US02 Service Manual page 71

Group 20
Problems while driving
It is more difficult to locate the source of the problem
while driving, it is impossible to walk around the vehicle
to listen and feel.
It is important that it is established whether
the problem is engine speed dependent (the
engine is the source), or if the problem is speed
dependent (the source is a chassis component).
Problems which only occur at high engine loads can also
provide important information.
The easiest way to find the source is to drive at the
"problem speed" and to compare different gears. If no
changes occur when driving in another gear at the same
speed, the fault is in a chassis component. This can be
particularly effective in determining torsional or transverse
vibration problems.
The other question is whether the fault is load dependent
or not. Does the problem occur at full acceleration and
when coasting?
Another example of the cause of faults may be a body
building adaptation. Does the problem only occur with
a particular trailer, or does the problem only occur with
certain loads on the trailer?
Cab vibrations can sometimes be interpreted as being
caused by the engine. The easiest way to determine if
and how the engine is involved is to drive using various
engine controls.
Example:
Noise when engine braking
Braking torque is obtained by the pumping of air and
exhaust gases.
Normal vibration can occur in the exhaust manifold. This
can produce noise but does not affect the braking ability
of the engine brake. It is therefore important to determine
clearly that the braking performance has deteriorated
before starting any further fault tracing.
The following checks should be carried out in order until
the fault is found. These checks also can help eliminate
components with no faults or malfunctions. The number of
tests and the order depends on the following: results from
tests and fault codes, vehicle conditions, vehicle service
and repair history, other symptoms, and information from
the driver.
•
"Step 1: Fault Code Readings" page 68
D
•
Does the problem only occur when driving using
the accelerator pedal? (Is the problem a type of
"accelerator pedal feedback"? )
•
Does the problem only occur when driving with the
cruise control engaged or the speed limiter set? (Is the
problem a control problem?)
If the symptoms only exist when driving using the
accelerator pedal at a particular speed, the problem
could be due to a misaligned trailer chassis, possibly in
combination with a tire out of balance.
The trailer begins to vibrate naturally. The vibrations
are transferred to the cab, and onwards to the driver,
accelerator pedal, the engine, the transmission, the driven
wheels and back to the chassis and the trailer again.
This transfers new energy to the trailer and the vibrations
continue. A "chain reaction" occurs which is very difficult
to remove or discern.
If the problem only exists when driving with the cruise
control engaged or speed limiter set, i.e. as soon as the
speed setting is used for engine control, the fault can
probably be found in the incoming speed signal.
Information about engine load can be useful.
An example is vibrations at a particular speed. If the
vibration is stopped by depressing the clutch pedal, the
drive shaft angles and universal joints should be checked
rather than checking the wheels for imbalance. The
problem will be more noticeable in the lower gears if the
drive shaft angles are incorrect.
Note: The exhaust brake can be mistakenly determined
as being faulty or the source of abnormal noise when
compared to vehicles of different model years. This is a
result of the exhaust brake function to a certain extent
differing between the model years. See Exhaust Brake,
Fault Tracing (Checklist O), Group 253.
•
"Step 2: Visual Inspection (System Checks)" page 68
•
"Step 3: VCADS Pro Tests" page 69
•
"Step 4: Checks" page 70
"Noise and Vibrations, Fault Tracing" page 65
67