Mercedes C-Class

W203 since 2000 of release

Repair and car operation

Mercedes-Benz
+ Mercedes-Benz Cars of a class C (W-203)
+ Operation manual
+ Routine maintenance
+ Engine
+ cooling and heating Systems
+ Power supply system and release
- Engine electric equipment
   + ignition and engine management System
   + System of preheat of the diesel engine
   - Diagnostics of systems of electronic control and diagnostic equipment
      Diagnostics of electronic control systems by the engine, injection, ignition and auxiliary brake systems
      The controler of interface of the personal computer with onboard system of self-diagnostics of OBD II according to protocols of the SAE standards (PWM and VPW) and ISO 9141-2
      Diagnostics of electronic system of the automatic conditioner of air (KV)
      Oscillograph application for supervision of signals in chains of control systems
   + charge and start Systems
+ Manual transmission
+ Automatic transmission
+ Coupling and power shafts
+ Brake system
+ Suspension bracket and steering
+ Body
+ Onboard electric equipment

Oscillograph application for supervision of signals in chains of control systems

Digital multimeters perfectly are suitable for check of the electric chains which were in a static condition, and also for fixing of slow changes of traced parameters. At carrying out the dynamic checks which are carried out on the working engine, and also at identification of the reasons sporadic failures by absolutely irreplaceable tool there is an oscillograph.

Some oscillographs allow to keep oscillograms in the built-in module of memory with the subsequent conclusion of results to the press or their transfer to the carrier of the personal computer already in stationary conditions.

The oscillograph allows to observe periodic signals and to measure tension, frequency, width (duration) of rectangular impulses, and also levels of slowly changing tension.

The oscillograph can be used for:

· Identifications of failures of unstable character.
· Checks of results of the made corrections.
· Activity control a lambda probe of a control system of the engine equipped with the catalytic converter.
· The analysis of the signals developed a lambda probe, which deviation of parameters from norm is unconditional the certificate of violation of serviceability of functioning of a control system as a whole, - on the other hand, correctness of a form of impulses given out a lambda probe can serve as a reliable guarantee of absence of violations in a control system.

Reliability and simplicity of operation of modern oscillographs do not demand any special special knowledge and experience from the operator. Interpretation of received information can be easily made by a way of elementary visual comparison of the oscillograms removed during check with given below typical for various sensors and actuation mechanisms of automobile control systems of temporary dependences.

Parameters of periodic signals

Each signal removed by means of an oscillograph, can be described by means of the following key parameters:
· Amplitude: A difference of the maximum and minimum tension (In) a signal within the period;
· Period: Duration of a cycle of a signal (мсек)
· Frequency: Quantity of cycles in a second (Hz);
· Width: Duration of a rectangular impulse (ms, microsec);
· Porosity: The relation of the period of repetition to width (In foreign terminology return is applied porosity parameter called by the running cycle, expressed in %);
· Signal form: Sequence of rectangular impulses, individual emissions, sinusoid, sawtooth impulses, etc.

Usually characteristics of the faulty device strongly differ from reference that allows the operator easily and quickly visually to reveal the refused component.

Signals of a direct current - tension of a signal is analyzed only. This sort of signals are developed by the devices presented on illustrations.

Signals of an alternating current - are analyzed amplitude, frequency and a signal form.

The frequency modulated signals - are analyzed amplitude, frequency, a form of a signal and width of periodic impulses. Sources of similar signals are the devices presented on illustrations.

The signals modulated on width of an impulse (ShIM) - are analyzed amplitude, frequency, a form of a signal and porosity of periodic impulses. Sources of similar signals are the devices presented on illustrations.

The form of a signal given out by an oscillograph depends on a set of various factors and can change considerably. In a look told before starting replacement of a suspected component in case of discrepancy of a form of the removed diagnostic signal with the reference oscillogram, it is necessary to analyse the received result carefully:

Tension

Zero level of a reference signal cannot be considered as absolute basic value, - "zero" of a real signal, depending on concrete parameters of a checked chain, it can appear shifted rather reference within a certain admissible range.

The full amplitude of a signal depends on supply voltage of a checked contour and also can vary rather reference value in certain limits.

In chains of a direct current the amplitude of a signal is limited to supply voltage. As an example it is possible to give a chain of system of stabilization of turns of the idling (IAC) which alarm tension does not change in any way with change of turns of the engine.

In chains of an alternating current the amplitude of a signal already unequivocally depends on frequency of work of a source of a signal, so, the amplitude of the signal which is given out by the sensor of provision of a cranked shaft (CKP) will increase with increase of turns of the engine.

In a look told if the amplitude of a signal removed by means of an oscillograph appears excessively low or high (up to trimming of top levels), it is enough to switch only a working range of the device, having passed to the corresponding scale of measurement.

At check of the equipment of chains with electromagnetic management (for example, IAC system) at shutdown of a food throws of tension which can be ignored quietly in the analysis of results of measurement can be observed.

It is not necessary to worry also at emergence of such deformations of the oscillogram, as a skashivaniye of the lower part of the forward front of rectangular impulses if, of course, the fact of a vypolazhivaniye of the front is not a sign of violation of serviceability of functioning of a checked component.

Frequency

Frequency of repetition of alarm impulses depends on working frequency of a source of signals.

The form of a removed signal can be edited and brought to a look convenient for the analysis by switching on an oscillograph of scale of temporary development of the image.

At supervision of signals in chains of an alternating current temporary development of an oscillograph depends on frequency of a source of the signal, defined by engine turns.

As it was already told above, for reduction of a signal to a legible look it is enough to switch scale of temporary development of an oscillograph.

In certain cases characteristic changes of a signal appear developed specularly rather reference dependences that speaks a reversivnost of polarity of connection of the corresponding element and, in the absence of a ban on change of polarity of connection, can be ignored in the analysis.

Typical signals of components of control systems of the engine

Modern oscillographs are usually equipped only with two alarm wires in a compartment with a set various щупов, allowing to carry out device connection practically to any device.

The red wire is connected to a positive pole of an oscillograph and is usually connected to the plug of the electronic control unit (ECM). The black wire should be connected to reliably earthed point (weight).

Injectors

Management of structure of an air and fuel mix in modern automobile electronic systems of injection of fuel is carried out by timely correction of duration of opening of electromagnetic valves of injectors.

Duration of stay of injectors abroach is defined by duration of the electric impulses developed by the control unit submitted on an entrance of electromagnetic valves. Duration of impulses is measured in milliseconds and usually does not fall outside the limits a range of 1-14 ms.

Often on the oscillogram it is possible to observe also a series of the short pulsations following directly an initiating negative rectangular impulse and supporting the electromagnetic valve of an injector abroach, and also the sharp positive throw of tension accompanying the moment of closing of the valve.

Serviceability of functioning of ECM can be easily checked by means of an oscillograph by a way of visual supervision of changes of a form of an operating signal at a variation of working parameters of the engine. So, duration of impulses at an engine provorachivaniye on single turns should be slightly higher, than at operation of the unit on low turns. Increase of turns of the engine should be accompanied by respective increase in time of stay of injectors abroach. This dependence is especially well shown when opening butterfly valve by short pressing a gas pedal.

By means of thin щупа from a set enclosed to an oscillograph connect a red wire of the device to the injector ECM plug of a control system of the engine. Shchup of the second alarm wire (black) oscillograph reliably earth.

Analyse a form of the engine of a signal read out during a provorachivaniya.

Having started the engine, check a form of an operating signal on single turns.

Having sharply pressed a gas pedal, lift frequency of rotation of the engine to 3000 rpm, - duration of operating impulses at the moment of acceleration should increase considerably, with the subsequent stabilization at level equal, or slightly smaller peculiar to idling turns.

Fast closing of a butterfly valve should lead to the flattening of the oscillogram confirming the fact of a perekryvaniye of injectors (for systems with an otsekaniye of supply of fuel).

At cold start the engine needs some enrichment of an air and fuel mix that is provided with automatic increase in duration of opening of injectors. In process of warming up duration of operating impulses on the oscillogram should be reduced continuously, gradually coming nearer to value typical for single turns.

In injection systems in which the injector of cold start is not applied, at cold start of the engine the additional operating impulses which are showing on the oscillogram in the form of pulsations of variable length are used.

In the table provided below typical dependence of duration of operating impulses of opening of injectors on a working condition of the engine is presented.

Inductive sensors

Start the engine and compare the oscillogram removed from an exit of the inductive sensor with given on illustration reference. The increase in turns of the engine should be accompanied by increase in amplitude of a pulse signal developed by the sensor.

Electromagnetic valve of stabilization of turns of idling (IAC)

In automotive industry electromagnetic IAC valves of a set of the various types which are giving out signals of also various form are used.

The common distinctive feature of all valves is that fact that porosity of a signal should decrease with increase of load of the engine, connected with inclusion of additional consumers of the capacity causing fall of turns of idling.

If porosity of the oscillogram changes with increase in loading, however at inclusion of consumers violation of stability of turns of idling takes place, check a condition of a chain of the electromagnetic valve, and also correctness of given-out ECM of a command signal.

Usually in chains of stabilization of turns of idling the 4-polar step-by-step electric motor which description is provided below is used. Check of 2-contact and 3-contact IAC valves is made in a similar manner, however oscillograms of alarm tension given out by them are absolutely unlike.

The step-by-step electromotor, reacting to given-out ECM a pulsing operating signal, makes step correction of turns of idling of the engine according to working temperature of cooling liquid and the current load of the engine.

Levels of operating signals can be checked by means of an oscillograph, measuring щуп which is connected serially to each of four plugs of the step-by-step motor.

Warm up the engine to normal working temperature and leave it working at single turns.

For increase in loading at the engine include headlights, the air conditioner, or, - on models with the power steering, - turn a steering wheel. Turns of idling should fall to short time, however right there again to be stabilized at the expense of IAC valve operation.

Lambda probe (the oxygen sensor)

The oscillograms typical for oxygen sensors most often used on cars of zirconic type in which basic tension 0.5В is not used are provided in the Section. Recently the increasing popularity is gained by titanic sensors, the working which range of a signal makes 0-5 In, and high level of tension stands out at combustion of the impoverished mix, low, - enriched.

Connect an oscillograph between the plug a lambda probe on ECM and weight.

Make sure that the engine is heated-up to normal working temperature.

If the removed signal is not wavy, and represents linear dependence, that, depending on tension level, it testifies to excessive repauperization (0 - 0.15 In), or reenrichment (0.6-1 In) an air and fuel mix.

If on single turns of the engine the normal wavy signal takes place, try to squeeze out sharply several times gas loops, - fluctuations of a signal should not fall outside the limits a range of 0-1 Century.

The increase in turns of the engine should be accompanied by increase of amplitude of a signal, reduction - decrease.

Sensor of a detonation (KS)

Connect an oscillograph between the plug of the sensor of a detonation of ECM and weight.

Make sure that the engine is heated-up to normal working temperature. Sharply squeeze out a pedal of gas and compare a form of a removed signal of an alternating current to the reference oscillogram given on an illustration.

At insufficient image sharpness slightly knock on the block of cylinders around placement of the sensor of a detonation.

If to achieve unambiguity of a form of a signal it is not possible, replace the sensor, or check a condition of electroconducting of its chain.

Ignition signal on an exit of the amplifier

Connect an oscillograph between the plug of the amplifier of ignition of ECM and weight.

Warm up the engine to normal working temperature and leave it working at single turns.

On the screen of an oscillograph the sequence of rectangular impulses of a direct current should stand out. Compare a form of an accepted signal to the reference oscillogram given on an illustration, paying close attention to coincidence of such parameters, as amplitude, frequency and a form of impulses.

At increase in turns of the engine frequency of a signal should increase in direct ratio.

Primary winding of the coil of ignition

Connect an oscillograph between the plug of the coil of ignition of ECM and weight.

Warm up the engine to normal working temperature and leave it working at single turns.

Compare a form of an accepted signal to the reference oscillogram given on an illustration, - positive throws of tension should have constant amplitude.

Unevenness of throws can be caused by excessive resistance of a secondary winding, and also malfunction of a condition of VV of a wire of the coil or a candle wire.