W203 since 2000 of release
Repair and operation of the car
+ Mercedes-Benz cars of class C (W-203)
+ Operation manual
+ Routine maintenance
+ Cooling system and heating
+ Power supply system and release
- Engine electric equipment
+ System of ignition and engine management
+ System of preheat of the diesel engine
- Diagnostics of systems of electronic control and diagnostic equipment
Diagnostics of electronic control systems of the engine, injection, ignition and auxiliary brake systems
The controller of interface of the personal computer to onboard system of self-diagnostics of OBD II under protocols of the SAE standards (PWM and VPW) and ISO 9141-2
Diagnostics of electronic system of the automatic air conditioner (AC)
Application of an oscillograph for observation of signals in chains of control systems
+ Systems of a charge and start
+ Manual transmission
+ Automatic transmission
+ Coupling and power shafts
+ Brake system
+ Suspension bracket and steering
+ Onboard electric equipment
Application of an oscillograph for observation of signals in chains of control systemsDigital multimeters perfectly are suitable for check of the electric chains which are in a static state, and also for fixing of slow changes of the traced parameters. When conducting the dynamic checks which are carried out on the working engine and also at identification of the reasons sporadic failures the oscillograph becomes absolutely irreplaceable tool.
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 evidence of violation of serviceability of functioning of a control system in general - on the other hand, correctness of a form of the impulses given a lambda probe can serve as a reliable guarantee of lack 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 the obtained information can be easily made by 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
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 - only signal tension is analyzed. Signals of this sort are developed by the devices presented on illustrations.
Signals of alternating current - are analyzed amplitude, frequency and a form of a signal.
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 form of the signal given by an oscillograph depends on a set of various factors and can change considerably. In a look told before starting replacement of the suspected component in case of discrepancy of a form of the removed diagnostic signal with the reference oscillogram, it is necessary to analyse carefully received result:
Zero level of a reference signal cannot be considered as absolute basic value, - "zero" real signal, depending on concrete parameters of the checked chain, it can be shifted rather reference within a certain admissible range.
Full amplitude of a signal depends on the supply voltage of the checked contour and also can vary rather reference value in certain limits.
In chains of a direct current amplitude of a signal is limited to supply voltage. It is possible to give a chain of system of stabilization of turns of idling (IAC) which alarm tension does not change with change of turns of the engine in any way as an example.
In chains of alternating current signal amplitude already unambiguously depends on the frequency of work of a source of a signal, so, amplitude of the signal given by the sensor of provision of a bent shaft (CKP) will increase with increase in turns of the engine.
In a look told if amplitude of the signal removed by means of an oscillograph is excessively low or high (up to trimming of top levels), it is only enough to switch the working range of the device, having passed to the corresponding scale of measurement.
When checking the equipment of chains with electromagnetic management (for example, the IAC system) at power off tension throws 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 bevelling 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 the checked component.
Frequency of repetition of alarm impulses depends on the working frequency of a source of signals.
The form of the 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 observation of signals in chains of alternating current temporary development of an oscillograph depends on the frequency of a source of a signal determined by engine turns.
As it was already told above, for reduction of a signal to a legible look it is enough to switch the scale of temporary development of an oscillograph.
In certain cases characteristic changes of a signal are developed specularly rather reference dependences that is explained by reversibility of polarity of connection of the corresponding element and, in the absence of the 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 with only two alarm wires in a compartment with a set of the various probes allowing to carry out connection of the device practically to any device.
The red wire is connected to a positive pole of an oscillograph and is usually connected to the plug of an electronic control unit (ECM). The black wire should be connected to reliably grounded point (weight).
Control of composition of air-fuel mix in modern automobile electronic systems of injection of fuel is exercised by timely adjustment 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 given on an entrance of electromagnetic valves. Duration of impulses is measured in milliseconds and usually does not exceed the limit of the range of 1 - 14 ms.
Often on the oscillogram it is possible to observe also a series of the short pulsations following directly the 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 visual observation of changes of a form of the operating signal at a variation of working parameters of the engine. So, duration of impulses at an engine provorachivaniye on single turns has to be slightly higher, than during the operation of the unit on low turns. Increase in turns of the engine has to be followed by corresponding increase in time of stay of injectors abroach. This dependence is especially well shown when opening a butterfly valve by short pressing the accelerator pedal.
By means of the thin probe from the set attached to an oscillograph connect a red wire of the device to the injector ECM plug of a control system of the engine. Reliably ground the probe of the second alarm wire (black) oscillograph.
Analyse a form of the signal engine which is read out during a provorachivaniya.
Having started the engine, check a form of the operating signal on single turns.
Having sharply pressed the accelerator pedal, lift the frequency of rotation of the engine to 3000 rpm, - duration of the operating impulses at the time of acceleration has to increase considerably, with the subsequent stabilization at the level equal, or slightly smaller peculiar to idling turns.
Bystry closing of a butterfly valve has to lead to the oscillogram flattening confirming the fact of an overshoot of injectors (for systems with fuel supply trim).
At cold start the engine needs some enrichment of air-fuel mix that is provided with automatic increase in duration of opening of injectors. In process of warming up duration of the operating impulses on the oscillogram has to be reduced continuously, gradually approaching value, typical for single turns.
In systems of injection in which the injector of cold start is not applied at cold start of the engine the additional operating impulses which are shown on the oscillogram in the form of pulsations of variable length are used.
Typical dependence of duration of the operating impulses of opening of injectors on the working condition of the engine is presented in the table given below.
Start the engine and compare the oscillogram removed from an exit of the inductive sensor with given on an illustration reference. Increase in turns of the engine has to be followed by increase in amplitude of the pulse signal developed by the sensor.
Lambda probe (oxygen sensor)
Connect an oscillograph between the plug a lambda probe on ECM and weight.
If the removed signal is not wavy, and represents linear dependence, then, depending on tension level, it demonstrates excessive reimpoverishment (0 - 0.15 In), or reenrichment (0.6 - 1 In) air-fuel mix.
At insufficient image sharpness slightly knock on the block of cylinders around placement of the sensor of a detonation.
At increase in turns of the engine the frequency of a signal has to increase in direct ratio.
Unevenness of throws can be caused by the excessive resistance of a secondary winding, and also malfunction of a condition of VV of a wire of the coil or a candle wire.