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METHODS OF FAULT DIAGNOSTICS OF SHOCK ABSORBERS

Basic information

The shock absorber is a quite complex, from a technical point of view, detail of the car. If the diagnosis of most suspension components can be done  "by means of mounting", testing at special stands is often necessary to determine the faults of shock absorbers, and especially to identify the causes of these failures. The experience of the major vendors of shock absorbers shows that the main cause of failing of shock absorbers is their unprofessional installation or violation of the terms of use.

Practice shows that the factory defects for shock-absorbers of foreign manufacture seldom exceed 0,5 %. However, in the event of a defect of the shock-absorber, even if the installer's proven guilty, the consumer usually gets a negative impression of the store that sold shock absorbers, and the brand of shock absorbers itself.

Therefore, for positive image of your company it is very important to try to avoid any cases of premature failure of shock absorbers.

The figure shows the structure of the shock-absorber. Possible locations of defects in the shock absorbers are identified by numbers 1-6.

Shock absorberThe most common defects of shock absorbers:

  1. Break of rod seal in the shock-absorber.
  2. Internal damages of the shock absorber: destruction, failure or natural wear of the valve assembly or piston.
  3. Mechanical damage of the shock-absorber: crack, dent in a body, bent rod.
  4. Destruction of the shock absorber: breaking off the rod, disengaging the mounting lug, degradation or destruction of silent blocks.
  5. Inconsistency of properties or degradation of the shock absorber fluid.
  6. Absence of gas in the shock absorber.

The causes of the defects may be different. For example, the break of rod seal may be caused by a violation of installation technologies (damage to the chrome rod cover), and by the wear of shock absorber boots (rod corrosion when wet).

There are several ways to evaluate the working capacity of shock absorbers. They vary in complexity and, therefore, require varying degrees of diagnostic accuracy. Generally, the simpler the method is, the less accurate the results it gives. The following sections are the most common methods for diagnosing shock absorbers, ranked by the accuracy of the result, the defects which can be installed with their help, and the causes of these defects.

Diagnosis on changing stability, controllability and stiffness of automobile suspensions

Shock absorber, as well as any part of the car, is subject to wear. Over time, shock absorber characteristics gradually deteriorate, but the driver does not notice it right away, as he adapts his style of driving under the car's features. This method of diagnosis involves a subjective assessment of the wear condition of shock absorbers. The estimate is made by the deterioration in vehicle performance.
Different makes and models of cars have different parameters of stability, controllability, stiffness of the suspension, which are put in them at the stage of design development. Also, each driver has his own driving style and ideas about the necessary stiffness of the suspension. Therefore, these concepts are always relative and, in each case, are individual in nature.

Thus, the offered method of diagnostics, although allowing the evaluation of the main problems associated with shock absorbers, is quite subjective. Most manufacturers of shock absorbers in their recommendations on troubleshooting these parts recommend using this method to compare the 'behavior' of the car with a certain model, that is, with absolutely identical car, equipped with good shock absorbers. Of course, this is not always possible in practice.

The table shows defects, which can be diagnosed using this method. Typically, this diagnostic method is supplemented with visual inspection of shock absorbers.

Feeling when driving Possible causes
Car suspension is too soft (vehicle is unstable when turning, 'floats' on the road, or rocks back and forth)      Shock absorbers, that do not match the vehicle, are installed
Subjective driver's experience
Absence of the shock absorber fluid in the working chamber of the shock absorber
Valve assembly of shock absorber is worn out
Internal damages of the shock absorber
Shock absorber mount is torn off
Car suspension is too stiff (car 'jumps' even on small bumps, bumps in the road pass to the body)    Subjective driver's experience
 Installed inappropriate shocks absorber or springs
Shock absorber is 'jammed'
Shock absorber 'froze'
Knocking noise from the suspension    Free play in mounting elements of shock absorber
Internal defect of the shock-absorber
The defect is associated with other elements of the suspension
Shock absorber mount is torn off


Diagnosis by rocking the car back and forth

The given method consists in rocking a body of the car and an assessment of the condition of shock absorbers by the number of oscillating movements of the body before the full stop. This method allows you to determine only two "extreme" states of a shock-absorber: either a shock absorber is completely worn out (broken mount or coupling rod, worn out valve unit, there is no shock absorber fluid in the working chamber), or a shock absorber is 'jammed' or completely 'stuck'. Attempts to determine the degree of shock absorber wear, in this case, are doomed to failure, because the force exerted by the shock absorber depends on the speed of movement of the rod. In addition, different vehicles, as noted above, have structurally different parameters of suspension stiffness. Some models of cars initially have fairly "soft" suspension. When the vehicle is moving, speed of a piston rod of the shock absorber is much higher than that achieved by rocking the car. Therefore, it is impossible to determine the degree of wear of the shock absorber in this case. Usually, such a way of identifying the causes of malfunctions of shock absorbers is complemented by a visual method of diagnosis.

Supplement:

It should be borne in mind that there are shock absorbers with regressive and progressive vibration damping features. Regressive features well extinguish lateral (at passage of turns) and longitudinal (at braking) rolls, and poorly absorb small road bumps. Progressive features well extinguish small bumps, but badly work on turns and at braking. Replacement of shock absorbers with a regressive feature for shock absorbers with a progressive feature may result in damage to the vehicle suspension components. Check by rocking of a body is ineffective because the suspension joints after prolonged use can move with greater resistance, which will be sufficient for rapid damping of rocking of rigid body. Conversely, shock absorbers with a progressive feature, because of the small resistance at low speed movement of the body will slowly extinguish fluctuations even in good condition.

Visual method for diagnosis of shock absorbers

This is the most common method, which, in conjunction with the first two methods of diagnosis, allows, in most cases, to find out the true reasons for failures of shock absorbers. Using this method it is impossible to accurately determine the causes of damage and destruction inside of the shock absorber. It is important to know that one of the most common defects in the internal parts of the shock absorber is their natural wear. When using a visual method of diagnosis, it is often necessary to remove the shock absorber installed on the car, which usually entails significant work and, consequently, costs. It should be noted that when a shock absorber is at work, oil 'mist' on its body and rod is considered to be a norm. But there should be no drops and leaks of oil on a body or a rod.

Diagnosis of shock absorbers at the 'shock-tester'

Shock-tester is a stand for testing shock absorbers, working principle of which consists in one axle of the vehicle rocking with some frequency and amplitude, which determines the speed of damping of fluctuations. This method allows determining the degree of wear of shock absorbers relative to the standard. Damping coefficient values incorporated in the computer of the diagnostic stand serve as a standard, appropriating similar values of the new shock absorber, installed in the car on the assembly line. "The downside" of this method is that the stand diagnoses not so much a condition of shock absorbers, but overall condition of the car's suspension. Therefore, some manufacturers of shock absorbers do not accept results of such testing as diagnostics of shock absorbers.

Checking the shock absorber at the diagnostic stand

This is the most accurate and the most expensive method to diagnose shock absorbers. It is used mainly for the examination of the shock absorber to determine the causes of its failure when the damage is related to the internal structure. The maximum diagnostic accuracy in this method is achieved by testing the shock absorber in particular, not the entire suspension, as in the diagnosis of "shock-tester". This method consists in removing the shock absorber from the vehicle and placing it on the special diagnostic stand to determine its characteristics and compare them with the characteristics specified in the technical documentation for this model of shock absorbers. Discrepancy of characteristics defines the reasons for shock absorber failure. This service is provided by almost all Russian representative manufacturers of shock absorbers. But the timing of this procedure to diagnose damper on the stand can be up to three months. This is because such tests are carried out in the manufacturer's laboratory or in research centers, which are mostly located abroad. For that reason, the majority of representations in disputable cases usually make a decision in favor of the client to avoid time-consuming procedure of delivery of shock absorbers to the factory for diagnosis.

Diagnosis of defects in new and newly installed shock absorbers

Practice shows that the vast majority of defects of shock absorbers already appear at their installation or during the first days of the operation. Therefore, it is necessary to have a full understanding of the specific defects arising from unprofessional installation and possible factory defects of shock absorbers.

The table shows the major defects, which can arise at installation of new shock absorbers, as well as the types of factory defects.

Observed defect Cause Actions
Visible oil drops or leaks on a body and a rod of the new shock absorber If after wiping drops do not resume, then this is the shock absorber lubrication Shock absorber is repaired
Visible oil drops or leaks on a body and a rod of mounted shock absorber Visible mechanical damage on the chrome plated rod of the shock absorber - traces of non-compliance with installation technology, resulting in rupture of the rod seals Replacement of the shock absorber
Visible abrasion on the chrome plated rod of the shock absorber - skew at installation of the shock absorber, resulting in rupture of the rod seals Replacement of the shock absorber
Factory defect Replacement of the shock absorber
At installation of new shock absorbers there is a knocking noise in a suspension bracket Due to the increased stiffness of the suspension bracket, the load on all of its elements increases too Diagnostics of a suspension bracket and replacement of defective components
Insufficient tightening torques for  mounting of shock absorber assemblies Checking the tightening torques. Replacement of shock absorber mounting assemblies, in case of damage
The cartridge is not rigidly fixed inside the damper strut Disassemble and assemble the strut mount in compliance with the installation technology
Mudguard is not fixed Remove the shock absorber and mount it in compliance with the installation technology
Factory defect Replacement of the shock absorber
The 'pumping' of a new shock absorber is the failure Air in the working cylinder of the shock-absorber. The shock absorber was stored in horizontal position Shock absorber is repaired. The problem itself is no longer observed after several cycles of compression/rebound
Factory defect Replacement of the shock absorber
The shock absorber is too stiff, soft or has too short of a stroke. Mounted shock absorber is not appropriate for this car model, a sport shock absorber is mounted. Use the services of professionals when choosing a shock absorber
Rod break at installation Failure to comply with the tightening torque recommended in the repair manual Replacement of the shock absorber
Rod break at operation Skew of the shock absorber at installation Replacement of the shock absorber