 printable version
Use of the oscilloscope to observe signals in chains of control systems
DMMs are ideal for testing are in the static state of electrical circuits, as well as for fixing slow changes monitored parameters. During the same dynamic checks performed on the engine is running, as well as in identifying the causes of sporadic faults completely indispensable tool becomes an oscilloscope.
Some oscilloscopes allow you to save the waveform in the embedded memory module and then print the results or pumping them in support of a personal computer is already in a hospital.
The oscilloscope allows to observe periodic signals and measure the voltage, frequency, width (duration) of rectangular pulses, as well as levels of slowly varying voltages.
The oscilloscope can be used to:
?� Identifying failures unstable nature.
?� Check the results of corrections made.
?� Active Control Lambda probe control system of the engine equipped with a catalytic converter.
?� Analysis produced by the lambda probe signal parameters deviate from the norm which is by far the evidence of infringement of serviceability of functioning of a control system as a whole - on the other hand, the correct form issued by the lambda probe pulses can serve as a reliable guarantee of non-infringement in the control system.
Reliability and ease of use of modern oscilloscopes do not require any special operator expertise. Interpretation of the information obtained can be easily made by elementary visual comparison taken during check waveforms with the following typical of the various sensors and actuators of automotive control systems temporal dependencies.
Parameters of periodic signals
 | Anyone using removable oscilloscope signal can be described by the following basic parameters:
?� Amplitude: The difference between the maximum and minimum voltage (V) signal within the period;
?� Period: Cycle signal (ms)
?� Frequency: The number of cycles per second (Hz);
?� Width: rectangular pulse duration (ms, ms);
?� The duty cycle: The ratio of the width to the repetition period (In foreign terminology used reverse duty cycle parameter called duty cycle, expressed as a%);
?� Waveform: A sequence of rectangular impulses, individual emissions, sine, sawtooth pulses, etc. |
Typically, faulty device characteristics are very different from the standard, which allows the operator to easily and quickly visually detect the failed component.
DC signals - only the voltage signal is analyzed. The signals of this kind are produced by devices in the illustrations.
AC signals - are analyzed amplitude, frequency and waveform.
The frequency-modulated signals - are analyzed amplitude, frequency, waveform, and the width of the periodic pulses. The sources of these signals are the devices shown in the illustrations.
 | Signals, pulse width modulated (PWM), - analyze the amplitude, frequency, waveform and duty cycle of periodic pulses. The sources of these signals are the devices shown in the illustrations. |
Form outputted oscilloscope signal depends on many factors and can vary considerably. In view of the above, before attempting to replace the suspect component in case of a mismatch shot form the diagnostic signal with the reference waveform, you should carefully analyze the results:
Voltage
The zero level of the reference signal can not be regarded as absolute reference value - "zero" of the real signal, depending on the particular characteristics test circuit may be shifted with respect to the reference within a certain allowable range.
The total amplitude of the signal depends on the supply voltage the circuit under test and may also vary with respect to the reference value within certain limits.
In DC amplitude signal voltage is restricted. As an example of the stabilization circuit idling speed (IAC), a voltage signal which does not change with engine speed.
In AC circuits, the amplitude of the signal already unequivocally depends on the frequency of the source, so that the amplitude of the signal output from the crankshaft position sensor (CKP) will increase with increasing engine speed.
In view of the above, if the amplitude of shoot with an oscilloscope signal is too low or high (up to the upper levels of the cut-off), you just switch the operating range of the device by clicking on the appropriate scale of measurement.
When testing equipment with electromagnetic control circuits (eg, system IAC) when the power surges can occur, which can be safely ignored in the analysis of the measurement results.
Do not worry as the appearance of the waveform deformations as mowing the bottom of the leading edge of the rectangular pulse, unless, of course, the fact that flattening of the front is not a sign of infringement of serviceability of functioning of the inspected component.
Frequency
The repetition rate of the signal pulses dependent on the operating frequency of the source signals.
Form removes the signal can be edited and brought to a convenient analysis mind by switching on the oscilloscope time scale scan of the image.
When observing signals in AC circuits of the oscilloscope time base depends on the frequency of the signal source defined by the engine speed.
As mentioned above, to bring the signal to a readable kind enough to switch the time scale of the oscilloscope sweep.
In some cases, the characteristic signal changes are deployed on a mirror with respect to reference dependencies, due to reversibility polarity of the corresponding element and, in the absence of a ban on changing polarity can be ignored in the analysis.
Typical signals of the engine control system components
Modern oscilloscopes are usually equipped with only two signal wires, coupled with a set of different probes, enables the connection of the device to almost any device.
Red wire connected to the positive pole of the oscilloscope and is usually connected to a terminal of the electronic control module (ECM). Black wire must be securely connected to the earthed point (weight).
Injectors
Management composition of air-fuel mixture in the modern automotive electronic fuel injection systems is accomplished by adjusting the duration of timely opening of the solenoid valves of injectors.
Length of stay of injectors in an open state is determined by the duration of the control unit generated electrical pulses applied to the input of the solenoid valves. Pulse duration is measured in milliseconds, and usually does not go beyond the range of 1 - 14 ms.
Often on the waveform can be seen as a series of short pulsations following directly behind the initiating negative rectangular pulse and supporting the solenoid valve of the injector in an open state, as well as a sharp positive surge voltage that accompanies the closing of the valve.
Serviceability of functioning ECM can be easily verified with an oscilloscope by visually observing changes in the shape of the control signal by varying the operating parameters of the engine. Thus, the pulse duration while cranking the engine idling to be somewhat higher than when the unit is at low revs. Increase the engine speed should be accompanied by a corresponding increase in the residence time of the injectors in an open state. This relationship is especially evident when you open the throttle quickly pressing on the gas pedal.
With the help of a thin probe attached to an oscilloscope kit, connect the red wire to the unit injector terminal ECM engine management system. Probe the second signal wire (black) of the oscilloscope properly grounded.
Analyse the form read out during the cranking signal.
Start the engine, check the shape of the control signal at idle.
Slammed on the gas pedal, raise the engine speed to 3000 rev / min, - the duration of the control pulses at the time of acceleration should be significantly increased, followed by stabilization at a level equal to, or slightly less peculiar idle.
Fast closing of the throttle must lead to a flattening of the waveform confirming the overlap injectors (for systems with a cut-off of fuel supply).
At cold start the engine needs some rich air-fuel mixture that is provided by an automatic increase in the duration of opening of injectors. As the duration of the warm-up control pulses on the waveform must be continuously reduced, gradually approaching the typical value for the idle.
In injection systems in which injector does not apply a cold start, cold start using additional control pulses that appear on the waveform of the pulsation of variable length.
The table below shows the typical dependence of the duration of operating impulses of opening of injectors on the operating condition of the engine.
Inductive sensors Start the engine and compare the trace, taken from the output of the inductive sensor shown in the illustration reference. The increase in engine speed must be accompanied by an increase in the amplitude of the pulse signal generated by the sensor.
The solenoid valve of stabilization of turns of idling (IAC)
The automotive industry uses electromagnetic valves IAC many different types of signals are output as different shapes.
A common feature of all the valves is the fact that the duty cycle of the signal should decrease with increasing engine load associated with the inclusion of additional power consumers, causes a decrease in idle speed.
If the duty cycle of the waveform changes with increasing load, but when the consumer has been a violation of the stability of the idle speed, check the circuit of the solenoid valve, as well as the correctness of the command signal issued by the ECM.
Usually in chains of stabilization of turns of idling uses 4-pole stepper motor, which is described below. Check 2-pin and 3-pin IAC valve is made in a similar manner, but they issue waveform signal voltages are totally dissimilar.
The stepper motor in response to the output from ECM pulsating control signal produces a stepped adjusting the engine idling speed in accordance with the operating temperature of the coolant and the current load on the engine.
Levels of control signals can be checked using an oscilloscope probe is connected alternately to each of the four terminals of the stepper motor.
Warm up the engine to normal working temperature and leave its working on single turns.
To increase the load on the engine Turn the lights, air conditioner, or - on models with power steering - turn the steering wheel. The idle speed should briefly drop, but immediately re-stabilized at the expense of the valve IAC. Lambda sensor (oxygen sensor)  | The section provides a waveform typical of the most commonly used in cars Zirconia type oxygen sensors, which do not use reference voltage of 0.5V. In recent years become increasingly popular titanium sensors, which signal the operating range is 0 - 5 V, with a high voltage level is issued from the combustion of a mixture of impoverished and low - enriched.
|
Connect an oscilloscope between terminal lambda probe ECM and weight.
Make sure the engine is at normal operating temperature. If the recording signal is not uneven, and a linear relationship, then, depending on the voltage level, it indicates an excessive repauperization (0 - 0.15 V) or pereobogaschenie (0.6 - 1) of air-fuel mixture.
If the idling engine holds the normal wavy signal, try several times to squeeze out sharply beadle gas - signal fluctuations should not go beyond the range of 0 - 1 V
The increase in engine speed must be accompanied by an increase in the signal amplitude, reduction - reduction.
The knock sensor (KS)
Connect an oscilloscope between knock sensor terminal of ECM and weight.
Make sure the engine is at normal operating temperature. Sharply squeeze the gas pedal and compare the form removes the AC signal to the one illustrated reference waveform. With the lack of image sharpness gently tap the cylinder block in the vicinity of the knock sensor placement.
If you achieve an unambiguous waveform fails, replace the sensor, or check the status of its electrical circuit.
Ignition signal at the output of the amplifier
Connect an oscilloscope between the terminal of the amplifier of ignition ECM and weight.
Warm up the engine to normal working temperature and leave its working on single turns.  | The screen of the oscilloscope should be given a sequence of rectangular pulses of DC. Compare the shape of the received signal with the one illustrated reference waveform, paying attention to the coincidence parameters such as amplitude, frequency and pulse shape. |
With increasing engine speed signal frequency should be increased in direct proportion.
The primary winding of the ignition coil
Connect an oscilloscope between the terminal of the ignition coil ECM and weight.
Warm up the engine to normal working temperature and leave its working on single turns.  | Compare the shape of the received signal with the one illustrated reference waveform - positive surges have to have a constant amplitude. |
Uneven surges can be caused by excessive resistance of the secondary winding and whack BB coil wire or spark plug wire.
|
