printable version

Application of the oscilloscope for monitoring the performance of the control system

general information

DMMs are ideal for testing electrical circuits are in a static state, and 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 when performing the following procedures:

• Identifying failures unstable nature;
• Check the results produced patches;
• Monitoring the activity of 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 of pulses emitted by the sensor 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 time-dependencies typical of the various sensors and actuators of automotive control systems.

Parameters of periodic signals

Overview

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);
• Duty cycle: the period of repetition to width (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.

Characteristics of the random signal.

Typically, faulty device characteristics are very different from the standard, which allows experienced operator to quickly and easily identify the failed component by analyzing the corresponding waveform.

DC signals

The only performance characteristics of these signals is the voltage.

Sensor Engine Coolant Temperature (ECT).

Intake air temperature (IAT).

Throttle Position Sensor (TPS).

Heated lambda probe.

Measuring the volume flow of air flow (VAF).

Throttle Position Sensor (TPS).

AC signals

The main characteristics of these signals are amplitude, frequency and waveform.

The knock sensor (KS).

Inductive engine speed.

The frequency-modulated signals (FM)

Performance Frequency-modulated signals are amplitude, frequency, and waveform of the periodic pulse width.

Sources FM signals are device.

Inductive crankshaft position sensor (CKP).

Inductive camshaft position sensor (CMP).

Inductive sensor Vehicle speed (VSS).

Working on the Hall effect sensors turns and position of the shaft.

Optical sensors turns and position of the shaft.

Digital sensors thermometric measuring the mass of air (MAF) and the absolute pressure in the inlet pipeline (MAP).

Signals, pulse width modulated (PWM)

Performance characteristics of signals of pulse width modulation (PWM) are the amplitude, frequency, waveform and duty cycle of periodic pulses.

PWM signal source device are presented.

Fuel injector.

The devices of stabilization of turns of idling (IAC).

The primary winding of the ignition coil.

Solenoid valve purge of a coal adsorber (EVAP).

Valves exhaust gas recirculation (EGR).

The encoded sequence of rectangular pulses

Performance characteristics are amplitude, frequency and shape of the individual pulse sequences.

Such signals are generated by the memory module self ECM engine management system.

By analyzing the width and shape of the pulse, and counting their number in each of the groups can be read stored in the fault memory (code 1223).

Signal DTC The engine management system self-test (code 1223).

The amplitude and waveform remain constant, the value written will be issued as long as the memory module will not be cleared.

Waveform Interpretation

The form issued by the oscilloscope signal depends on many different factors and can greatly modified. 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, should carefully analyze the results obtained.

The digital signal.

The analog signal.

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 ([1] - cm. The digital image signal) is within a certain allowable range.

The total amplitude of the signal depends on the supply voltage the circuit under test, and may also vary within certain limits with respect to the reference value ([3] - cm. The digital image signal, and [2] - see. Analog image signal).

In DC signal range is limited by the supply voltage. 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 operation 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 (see [4] - see. Illustration digital signal) that the analysis of the measurement results can be safely ignored.

Do not worry as the appearance of the waveform deformations as mowing the bottom of the leading edge of the rectangular pulses ([5] - see. Illustration digital signal), 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 timebase scale image.

When observing signals in AC circuits of the oscilloscope time base depends on the frequency of the signal source (see [3] - see. Illustration analog signal) determined by the engine speed.

As mentioned above, to bring the signal to a readable kind enough to switch out the time oscilloscope.

In some cases, the waveform is expanded relative to the reference mirror according to that reversibility is due to 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 together 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). The black wire should be connected to a properly grounded point (weight).

Injectors Injection

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 module produced by electrical pulses applied to the input of the solenoid valves. The pulse duration is measured in milliseconds, and usually is within the range of 1 ÷ 14 ms.

Impulse control opening of the injector fuel injection.

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.

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

PERFORMANCE ORDER

Start the engine and compare the trace, taken from the output of the inductive sensor with a reference. A typical waveform signal generated by an inductive sensor. 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 in turn to each of the four terminals of the stepper motor.

PERFORMANCE ORDER

Warm up the engine to normal working temperature and leave its working on single turns. To increase the load on the engine including the head lights, air conditioning or - on models with power steering - turn the steering wheel. The idle speed should briefly drop, but immediately re-stabilized by operation of valve IAC. Compare removed the reference waveform. Oscillogram control signal system of stabilization of turns of idling (IAC).

Lambda sensor (oxygen sensor)

In the waveforms are typical for the most commonly used in cars lambda probes zirconium type, which do not use a reference voltage of 0.5V. In recent years become increasingly popular titanium sensors, which signal the operating range is 0 ÷ 5 V, and a high voltage level is issued from the combustion of lean, low - enriched.

PERFORMANCE ORDER

Connect an oscilloscope between terminal lambda probe ECM and weight. Make sure the engine is at normal operating temperature. Compare the displayed waveform with the reference gauge dependence. The waveform outputted typical lambda probe. If the recording signal is not uneven, and a linear relationship, then, depending on the voltage level, it indicates an excessive repauperization (0 ÷ 0.15V) or pereobogaschenie (0.6 ÷ 1B), the 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 ÷ 1V. The increase in engine speed must be accompanied by an increase in the signal amplitude, reduction - reduction.

The knock sensor (KS)

PERFORMANCE ORDER

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 with the reference waveform. The waveform outputted typical knock sensor (KS). With the lack of sharpness gently tap the cylinder block in the vicinity of the knock sensor placement. If you achieve an unambiguous waveform fails, replace the sensor KS, or check the status of its electrical circuit.

Ignition signal at the output of the amplifier of ignition

PERFORMANCE ORDER

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 reference waveform, paying attention to the coincidence parameters such as amplitude, frequency and pulse shape. Oscillogram control signal booster ignition. With increasing engine speed signal frequency should be increased in direct proportion.

The primary winding of the ignition coil

PERFORMANCE ORDER

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 reference waveform, - positive surges have to have a constant amplitude. The waveform at the terminal of the primary winding of the ignition coil. ???�???�?????????�?????????�?? ?�???????????? ?????�?�?� ?�?�?�?? ???�?�???�???� ?�???�?�???�?????�?? ???????????�?????�?�?????�?? ???�???????�?????? ???�?????�????, ?� ?�?�???�?� ???�?????????�?????????�???? ???????�?????????? ?�?� ?????????????� ???�?�???????? ???�?? ?????�?�???????? ?????????????�.