7.3.4. Application of an oscillograph for observation of signals in chains of control systems
Digital 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 brought below typical for various sensors and actuation mechanisms of automobile control systems of temporary dependences.
Parameters of periodic signals
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Everyone, removed by means of an oscillograph signal, can be described by means of the following key parameters: |
- Amplitude: A difference of the maximum and minimum tension (V) of a signal within the period;
- Period: Duration of a cycle of a signal (ms)
- Frequency: Number of cycles per second (Hz);
- Width: Duration of a rectangular impulse (ms, microsec);
- Porosity: The repetition period relation to width (In foreign terminology the return is applied porosity the parameter called by a running cycle, expressed in %);
- Signal form: Sequence of rectangular impulses, single 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 - only signal tension is analyzed.
Signals of this sort are developed by the devices given on illustrations below.
Signals of alternating current - are analyzed amplitude, frequency and a form of a signal.
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Sensor of a detonation (KS) |
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Inductive RPM meter of the engine |
Frequency-modulated signals - are analyzed amplitude, frequency, a form of a signal and width of periodic impulses. Sources of similar signals are the devices given on illustrations below.
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 given on illustrations below.
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.
Tension
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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 (see. A digital signal [1]) within a certain admissible range (see. Digital signal [2] and Analog signal [1]). Full amplitude of a signal depends on the supply voltage of the checked contour and also can vary rather reference value in certain limits (see. Digital signal [3] and Analog signal [2]). |
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.
Frequency
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.
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At observation of signals in chains of alternating current temporary development of an oscillograph depends on the frequency of a source of a signal [3] 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).
Injectors
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.
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Impulse of management of opening of an injector of fuel |
The typical oscillogram of the managing director of operation of an injector of an impulse is given on an illustration above. 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 provided below.
Inductive sensors
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Condition of the engine |
Duration of the operating impulse, ms |
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Single courses |
1.5 ÷ 5 |
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2000 ÷ 3000 rpm. |
1.1 ÷ 3.5 |
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Full gas |
8.2 ÷ 3.5 |
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Start the engine and compare the oscillogram removed from an exit of the inductive sensor from the given reference. |
Increase in turns of the engine has to be followed by increase in amplitude of the 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 various types giving signals of also various form are used.
The common distinctive feature of all valves is the fact that porosity of a signal has to decrease with increase of the load of the engine connected with inclusion of the additional consumers of power causing decrease in 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 the given ECM of a command signal.
Usually in chains of stabilization of turns of idling the 4-polar step electric motor which description is given below is used. Check of 2-contact and 3-contact IAC valves is made in a similar manner, however oscillograms of the alarm tension given by them are absolutely unlike.
Step electric motor, reacting to the given ECM the pulsing operating signal, makes step correction of turns of idling of the engine according to the working temperature of cooling liquid and the current load of the engine.
Levels of the operating signals can be checked by means of an oscillograph which measuring probe is connected serially to each of four plugs of the step motor.
Warm up the engine up to the normal working temperature and leave it working at single turns.
For increase in load of the engine include headlights, the air conditioner, or, - on models with the power steering, - turn a steering wheel. Turns of idling have to fall for a short time, however right there again be stabilized due to operation of the IAC valve.
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Compare the removed oscillogram to the given reference. |
Lambda probe (oxygen sensor)
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The oscillograms typical for the oxygen sensors of zirconium type which are most often used on cars in which basic tension 0.5B is not used are provided in the Section. Recently the increasing popularity is gained by titanic sensors, рабочийдиапазон a signal of which makes 0 - 5 V, and the high level of tension is given 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 up to the normal working temperature.
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Compare the oscillogram displayed the measuring instrument to the given reference dependence. |
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.
If on single turns of the engine the normal wavy signal takes place, try to squeeze out several times sharply gas loops, - fluctuations of a signal should not exceed the limit of the range of 0 - 1 Century.
Increase in turns of the engine has to be followed by increase in 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 up to the normal working temperature.
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Sharply squeeze out the accelerator pedal and compare a form of the removed signal of alternating current to the provided reference oscillogram. |
At insufficient image sharpness slightly knock on the block of cylinders around placement of the sensor of a detonation.
If it is not possible to achieve unambiguity of a form of a signal, replace the sensor, or check a condition of an electrical wiring of its chain.
Ignition signal at the exit of the amplifier
Connect an oscillograph between the plug of the amplifier of ignition of ECM and weight.
Warm up the engine up to the normal working temperature and leave it working at single turns.
At increase in turns of the engine the frequency of a signal has to 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 up to the normal working temperature and ostavtey working at single turns.
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Compare a form of the accepted signal to the provided reference oscillogram, - positive throws of tension have to have constant amplitude. |
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.