← Back to Contents
3 diagnosis / 3.1 Mechanical Fault Diagnosis | PDF pages 65-74
3.1 Mechanical Fault Diagnosis
3.1 Mechanical Fault Diagnosis
3.1.1 Mechanical Fault Diagnosis Overview
3.1.1.1 Overview
Mechanical diagnosis is to troubleshoot faults in vehicle,
analyze and make judgments in accordance with relevant
technical standards with the use of special tools, instrument,
equipment and software for the purpose of investigating the
cause of the fault and confirming the location of the fault. In
general, the fault diagnosis of vehicles is performed through
intuitive experience and instrument and equipment. With the
development of vehicle technology, especially the application
of electronic technology and computer technology in vehicles,
the fault diagnosis methods of vehicles have been developed
from observation, hearing, smelling, touching, isolation,
testing, comparison and other traditional experiences to the
use of information technology-based complete, modern fault
diagnosis technology systems for vehicles with the digital,
integrated and intelligent diagnostic equipment as the
auxiliary means.
This section aims to analyze and eliminate common faults
found during driving so as to make it easier for maintenance
personnel to quickly find and eliminate the source of the fault.
3.1.1.2 Precautions
Cautions Regarding Mechanical Fault Diagnosis
warning！
1. Park the vehicle on level ground or in a well-ventilated
operation room.
2. Apply the parking brake and turn off the ignition switch to
prevent the vehicle from slipping during maintenance.
3. Do not operate under the vehicle when it is lifted only with a
jack or lift. Unless the vehicle is parked on a sturdy tripod or
bracket suitable for lifting the vehicle. Place wedges at the
wheels to prevent the wheels from rolling when the tires are
not off the ground.
4. During maintenance, ensure safety and do not damage
parts or operate in violation of regulations.
3.1.2 Integrated Vehicle Faults
3.1.2.1 Noises and Abnormal Sounds in Vehicle
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Refer to Diagnosis of Abnormal Sounds in Steering System -
Abnormal noise in steering system
1
Abnormal Sounds in Steering System
Refer to Diagnosis of Abnormal Sounds in Brake - Abnormal
2
Abnormal sound in brake
Sounds in Brake
3.1.2.2 Tire Wear
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Axle steering occurs when the left and right
Check the dimensional error of the frame, air spring, front axle,
wheelbases of the vehicle are not the same, which
rear axle and other parts on regular basis. When the
1
aggravates the tire wear and even leads to tire
dimensional errors reaches a certain level, the left and right
eating.
wheelbase will not be equal.
Tire wear caused by changes in wheel alignment
Increase the tightness of the front hub bearing
2
angle as a result of overloading
In the event of tire eating caused by wheel camber and kingpin
Significant fluctuations in toe-in of the wheels also
inclination that are out of tolerance, the front axle assembly
3
result in wear
should be replaced
Abnormal tire friction caused by steerable wheel
Strictly control the toe-in of steering wheels
4
oscillation
Replace the steel rim with qualified flatness or process the
The unevenness of steel rim end face is out of
5
plane of the steel rim to make it qualified
tolerance
Avoid severe overloading of the vehicle
6
Overloaded tires
Deflate tires with high air pressure to keep the air pressure
The tire pressure is too high, resulting in crown
7
within the specified range
wear
Tire rotation is not performed in time
Replace the tires in a timely manner
8
Overloaded tires lead to deformation or runout of
Reduce wheel radial runout, end face runout, imbalance of
9
steel rim end face, and the runout of steel rim end
mass distribution, etc.
face also causes runout of tire end face
In the event of rigid deformation of front axle, the front axle
Severe overloading of front axle leads to great
10
should be replaced
deformation of front axle
Diagnostic Content
Step
Measure
Frequent and violent starts, braking, high-torque
Develop correct driving habits and avoid frequent and violent
11
drive, and stops
starts, braking, high-torque drive and stops
Insufficient tire inflation pressure or severe
Avoid severe overloading of the vehicle
12
overload
Inferior quality of parts, such as steering knuckle
bushing deformation, serious wear, excessive
clearance between kingpin and steering knuckle
Replace inferior parts
13
bushing, loose front axle hub bearing, loose rear
axle hub bearing, etc.
The steel rim is out of round or eccentric, and the
Replace the reinforced rim
14
outer tire is repaired due to damage
3.1.2.3 Insufficient Air Pressure Throughout the Vehicle
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
The inflation pump connecting pipe is broken
Weld or replace the inflation pump connecting pipe
1
The inflation pump is faulty
Repair or replace the inflation pump
2
3.1.2.4 Steering Wheel Shaking at High Speeds
Fault Description:
The vehicle is unstable or swings, or even the steering wheel shakes at high speeds or at a certain high speed.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Check whether the front wheel alignment angle and toe-in
The front wheel alignment angle is inaccurate and
1
meet the requirements; If not, adjust them
the toe-in is excessive
Jack the front axle to test the wheels and check the dynamic
Deformed front spokes, unequal number of tire
balance of the wheels and whether the tires are deformed to a
2
bolts, too low front tire pressure or tire repair, etc.
large extent. If necessary, replace the wheel with a one with
result in unbalance
good quality for comparative test
The drive system parts are loosely installed
Tighten drive system components
3
Diagnostic Content
Step
Measure
Check whether the front axle and frame are deformed and
The steering drive shaft is bent, dynamic unbalance
whether the steering drive shaft is bent, and perform dynamic
4
occurs and the front axle is deformed
balance on the drive shaft when possible
Replace the shock absorber
5
Shock absorber fault
3.1.2.5 Driving Deviation
Fault Description:
Check for driving deviation as follows: Generally during driving, center the steering wheel, and then take your hands off the
steering wheel to see if the vehicle is driving in a straight line. If not, it can be determined that the vehicle is deviating from the
road.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Diagnostic Content
Step
Measure
Inconsistent left and right tire pressures
1
Inflate under-inflated tires
Both small toe-in or negative toe-in can cause driving
Inaccurate alignment of the front wheels. The front
deviation. When this occurs, go to a professional maintenance
wheel camber, kingpin angles or kingpin inclination
2
station for testing
are not equal
3.1.3 Frame and Suspension Faults
3.1.3.1 Diagnosis of Faults in Frame and Suspension
Fault Description 1:
The front bottom contacts the ground or the riding height is low.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Suspension components are damaged
Check the suspension components for damage
1
The air spring is faulty
Check the dynamic suspension
2
Fault Description 2:
The vehicle leans to one side.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Front and rear suspension components
Check the front and rear suspensions
1
The air spring is faulty
Check the air spring
2
3.1.4 Wheel and Tire Faults
3.1.4.1 Diagnosis of Driving Deviation
Fault Description:
When the vehicle is deviating to one side, the driver should hold the steering wheel with both hands or apply force on other
side of the steering wheel to ensure normal driving. Otherwise it is easy for the vehicle to deviate from the driving direction.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Diagnostic Content
Step
Measure
Unqualified or worn tires are used, or the tires on
both sides are of different sizes; The air pressure in
Rotate tires to ensure consistent tire pressure
1
tires on both sides is not equal, or the tire one side
is severely worn
The front hub bearing is not properly adjusted,
Adjust the front hub bearing; Correct the front axle to restore to
causing it to be too tight or too loose. The front
2
correct front wheel alignment angle; Adjust the toe-in to correct
wheel alignment angle on both sides is different or
value
changes; The front axle is bent and deformed
One side of the frame is broken; The frame is
Repair the frame and correct the deformation
3
deformed
The rear axle housing is bent and deformed or
Correct or replace the rear axle housing
4
broken
Check and adjust the position of the rear axle relative to the
The rear axle is misaligned with the frame
5
frame
Extreme eccentricity
Note eccentric load and load deflection during driving
6
3.1.4.2 Diagnosis of Front Wheel Vibrations
Fault Description:
The front wheels vibrate to left and right or bump vertically during driving. This will have a great impact on the vehicle speed,
making it uncomfortable to sit in the vehicle.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Tire pressure is inconsistent; Tires are not of the
Check and adjust the tire pressure; Replace the tires with new
1
same size; Severe wear
ones
Carry out dynamic balance test on the wheel assembly to
Wheel dynamic unbalance
2
control the unbalance within the allowable range
Diagnostic Content
Step
Measure
Check and adjust the tightness of the front wheel hub bearing
The front wheel hub bearing is damaged or loose
3
of the vehicle, and replace the damaged hub bearing
Incorrect front wheel alignment
Adjust the front wheel alignment and toe-in
4
3.1.5 Steering System Faults
3.1.5.1 Diagnosis of Steering Wheel Shaking
Fault Description:
The shaking steering wheel and vibrating front wheels can cause the car front end to shake while driving, and the driver has to
slow down in serious condition.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Diagnostic Content
Step
Measure
There is air in steering hydraulic system
Discharge air from the hydraulic system
1
Dirt is clogged in the hydraulic pipeline
Remove dirt from the hydraulic pipeline system
2
Check and eliminate mechanical faults in power steering
Mechanical system is faulty
3
system
3.1.5.2 Diagnosis of Abnormal Sounds in Steering System
Fault Description:
The abnormal sounds heard in the steering system while driving or high speed driving mainly come from the steering gear,
electric steering pump, etc.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
There is air in steering hydraulic system
Discharge air from the steering hydraulic system
1
The electric steering pump is damaged
Check and repair the electric steering pump
2
The steering control mechanism is damaged
Clean or repair the steering gear
3
Mechanical collision sounds in steering system
Adjust or repair the steering transmission mechanism
4
3.1.6 Brake System Faults
3.1.6.1 Diagnosis of Braking Deviation
Fault Description:
Braking deviation is a phenomenon that the vehicle deviates to one side while driving as the left and right braking effects on
the same axle are not uniform. Braking deviation occurs when applying brake or emergency brake; When the car brakes, it
may drift. Minor deviation is allowed, but serious deviation may cause accidents.
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
Uneven braking force of wheel brakes
Adjust the wheel brakes uniformly as far as possible
1
Adjust the brake chamber push rod to ensure consistency
Unequal exposure of left and right brake chamber
2
between the left and right brake chamber push rods
push rods
Inconsistent left and right tire pressures
Inflate the tires to ensure consistent tire pressure
3
A wheel friction plate is contaminated by oil or the
Replace the inconsistent friction plates and remove the oil
4
friction plates are of inconsistent model
The frame is deformed or the vehicle is heavily
Correct the frame to keep the vehicle load balanced
5
eccentric
3.1.6.2 Diagnosis of Brake Failure
Fault Description:
Obvious insufficient deceleration is found when depressing the brake pedal to slow down or stop. The vehicle cannot stop
quickly when applying emergency brake, and the braking time and braking distance are extended. No tire drag traces are
found or the tire drag traces are extremely short during inspection on a parked vehicle
Fault Diagnosis and Troubleshooting
Check for possible causes of the fault one by one according to the following steps. Diagnose in a simple-to-complex
progressive manner to effectively save time. Directly skip specific steps concerning the causes that are obviously impossible
through analysis. This diagnostic step is used for reference only.
Step
Diagnostic Content
Measure
The brake pedal free travel is too large
Adjust the free stroke of the brake pedal to the specified value
1
Adjust the air pressure of the brake pipeline to the specified
The air pressure of the air reservoir is insufficient
2
value
Check whether the pipeline between the brake valve and the
The brake system is leaking or the pipeline is
brake air pump is subject to air leakage, and replace the air
3
clogged
pressure pipe if necessary
The brake valve is improperly adjusted or working
Adjust or replace the brake valve
4
poorly
The wheel brake is improperly adjusted or poorly
Adjust or replace the wheel brake
5
working
3.1.7 Vehicle Body Faults
3.1.7.1 Note
In the process of body repair, professional and technical
personnel may use sheet metal repair machines, welding
machines and various grinding and cutting tools to ensure that
the vehicle can be restored to the original level in terms of
geometric dimensions and usability. However, sometimes, you
may not be able to find the faults in the driving system and
assembly mounts that can lead to serious consequences
when repairing an accident vehicle. Therefore, in addition to
the necessary inspections of geometrical dimensions, special
attention must be paid to the following components.
3.1.7.2 Fault Inspection
1. Check that the steering mechanism and steering rod
system can be operated correctly within the number of
turns of the steering wheel, and visually check for bent or
cracked parts.
2. Check the wheels and tires for damage, concentric rotation
and imbalance. Check the tire pattern and sidewall for cuts/
check the tire pressure.
3. After body repair, carry out a road test to ensure the driving
ability of the vehicle, and finally hand over the vehicle to
users.
3.1.7.3 Body Fault Assessment
Slight pits can be repaired without damaging the paint finish. It
can be removed from inside the pit using a special lever tool
(pressure tool). When performing this kind of repair, it requires
that the technician has extensive experience in using special
tools and sufficient knowledge about the materials, and is
capable to identify faults and use correct repair methods. The
pit should meet the following requirements:
The pits should be limited to body positions that are easily
accessible from both sides, as this repair method is rarely
used for double-skinned parts or enclosed sections.
Satisfactory repair results can be achieved for slight shallow
pits with small deformation radius. This method is particularly
applicable to damages caused by hail, or during parking or
transportation.
In case the diameter of the pit is less than 50mm, and the
extent of damage can be assessed according to the size of
the pit.
caution
The deformation of material in the center of the pit must
not exceed the deformation limit.