7.2.2. 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 of periodic 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 copying to the digital carrier already in stationary conditions.
The oscillograph allows to observe periodic signals and to measure characteristics 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 monitoring lambda probe;
- The analysis of the signals developed a lambda probe which deviation of parameters from norm is the unconditional 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 from the operator of special special knowledge and experience. Interpretation of the obtained information can be easily made by elementary visual comparison of the oscillograms removed during check with the temporary dependences typical for various sensors and actuation mechanisms of automobile control systems given below.
Parameters of periodic signals
Each signal removed by means of an oscillograph can be described by means of the following key parameters (see an illustration of the Characteristic of any signal):
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amplitude – a difference of the maximum and minimum tension (V) of a signal within the period;
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the period – duration of a cycle of a signal (ms);
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frequency – the number of cycles per second (Hz);
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width – duration of a rectangular impulse (ms, microsec);
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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 – the sequence of rectangular impulses, single emissions, a sinusoid, sawtooth impulses, etc.
Characteristics of any signal
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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 (see the illustrations ECT Sensor Signal, the TPS Sensor, the Lambda probe and the MAF Sensor).
ECT sensor signal|
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TPS sensor
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Lambda probe
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MAF sensor
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Signals of alternating current - are analyzed amplitude, frequency and a form of a signal.
Detonation sensor
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Frequency-modulated signals - are analyzed amplitude, frequency, a form of a signal and width of periodic impulses (see illustrations the Inductive CKP sensor, the Inductive CMP sensor, the Inductive VSS sensor, RPM meters and provisions of shaft working at Hall's effect, Optical RPM meters and provisions of shaft and Digital MAF and MAP sensors).
Inductive CKP sensor
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Inductive CMP sensor
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Inductive VSS sensor
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RPM meters and provisions of shaft working at Hall's effect
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Optical RPM meters and provisions of shaft
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Digital MAF and MAP sensors
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The signals modulated on width of an impulse (ShIM) - are analyzed amplitude, frequency, a form of a signal and porosity of periodic impulses (see the illustrations Fuel Injector, the Device of stabilization of turns of X/X (IAC), Primary winding of the coil of ignition, Em the EVAP system adsorber purge valve, EVAP system Valves.
Fuel injector
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Device of stabilization of turns of X/X (IAC)
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Primary winding of the coil of ignition
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Ø/m EVAP system adsorber purge valve
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EVAP system valves
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The form of the signal given by an oscillograph depends on a set of various factors and can change considerably.
In view of 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 (illustration cm the Digital signal and the Analog signal).
Digital signal
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Analog signal
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Tension
Zero level of a reference signal cannot be considered as absolute basic value, - "zero" real signal depending on concrete parameters of the checked chain can be shifted rather reference (see range 1 on an illustration the Digital signal) within a certain admissible range (see range 1 on an illustration the Analog signal).
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 range 2 on an illustration the Digital signal and 2 on an illustration the Analog signal).
In chains of a direct current the level 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 chains with Ý/m management (for example, a control system of idling turns) at power off tension throws (see 4 on an illustration the Digital signal) can be observed which can be ignored quietly in the analysis of results of measurement.
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 (see values 5 on an illustration the Digital signal) 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
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 signal source frequency (on an illustration the Analog 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 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 ECM plug. 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 developed ECM of the electric impulses given on an entrance Ý/m valves. Duration of impulses usually does not exceed the limit of the range of 1 - 14 ms.
The typical oscillogram of the impulse operating operation of an injector is submitted on the illustration Fuel Injector. 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 Ý/m the 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 connect a red wire of an oscillograph to the injector ECM plug. Reliably ground the probe of the second alarm wire (black) oscillograph.
Analyse a form of the signal engine which is read out during a provorachivaniye.
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 a fuel supply cut-off).
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
Start the engine and compare the oscillogram removed from an exit of the inductive sensor with reference, given on an illustration.
Signal of the inductive sensor
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Increase in turns of the engine has to be followed by increase in amplitude of the pulse signal developed by the sensor.
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Condition of the engine |
Duration of the operating impulse, ms |
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Single turns |
1 ÷ 6 |
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2000 ÷ 3000 rpm |
1 ÷ 6 |
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Full gas |
6 ÷ 35 |
Lambda probe (oxygen sensor)
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Oscillograms typical for the most often used on cars a lambda probes of zirconium type in which the basic tension of 0.5 Century is not used are provided in this subsection. Recently the increasing popularity is gained by titanic sensors which working range of a signal 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.
Compare the oscillogram displayed the measuring instrument with reference, given on the illustration Lambda probe.
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.
Ignition signal at the ignition module exit
Connect an oscillograph between the ignition module plug on ECM and weight.
Warm up the engine up to the normal working temperature and leave it working at single turns.
The sequence of rectangular impulses of a direct current has to be given for the screen of an oscillograph. Compare a form of the accepted signal to reference, paying close attention to coincidence of such parameters as amplitude, the frequency and a form of impulses.
The operating ignition module signal
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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 and weight.
Warm up the engine up to the normal working temperature and leave it working at single turns.
Compare a form of the accepted signal from the reference, given on an illustration Primary winding ignition coil, - 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 In/in coil wires.