General information
DMMs are great for testing electrical circuits that are in a static state, as well as capturing slow changes in monitored parameters. When conducting dynamic checks performed on a running engine, as well as identifying the causes of sporadic failures, an oscilloscope becomes an absolutely indispensable tool.
Some oscilloscopes allow you to save waveforms in the built-in memory module with subsequent printing of the results or transferring them to a personal computer carrier already in stationary conditions.
The oscilloscope allows you to observe periodic signals and measure voltage, frequency, width (duration) rectangular pulses, as well as the levels of slowly changing voltages. The oscilloscope can be used in the following procedures:
- Detection of unstable failures;
- Checking the results of the corrections made;
- Monitoring the activity of the lambda probe of the engine control system equipped with a catalytic converter;
- Analysis of the signals generated by the lambda probe, the deviation of the parameters of which from the norm is an unconditional evidence of a malfunction in the functioning of the control system as a whole. On the other hand, the correctness of the shape of the pulses emitted by the sensor can serve as a reliable guarantee of the absence of violations in the control system.
The reliability and ease of use of modern oscilloscopes do not require any special knowledge and experience from the operator. Interpretation of the information obtained can be easily done by an elementary visual comparison of the oscillograms taken during the test with the following time dependences, typical for various sensors and actuators of automotive control systems.
Parameters of periodic signals
General information
Characteristics of an arbitrary periodic signal
Each signal captured by the oscilloscope (see illustration Characteristics of an arbitrary periodic signal), can be described using the following basic parameters:
- Amplitude: Difference between maximum and minimum voltages (IN) signal within the period;
- Period: Signal cycle time (ms);
- Frequency: Number of cycles per second (Hz);
- Width: Rectangular pulse duration (ms, ms);
- Duty cycle: Ratio of repetition period to width (in foreign terminology, the reverse duty cycle is used, a parameter called the duty cycle, expressed in%);
- Waveform: Square wave train, bursts, sine wave, sawtooth pulses, etc.
Typically, the characteristics of a failed device are very different from the reference, which allows an experienced operator to easily and quickly identify a failed component by analyzing the corresponding waveform.
DC signals
The only operating characteristic of such signals is voltage.
DC signals are generated by the devices shown below.
Engine coolant temperature sensor (ECT)
intake air temperature sensor (IAT)
Throttle position sensor (TPS)
Heated lambda probe
Air volume flow meter (VAF)
Air mass meter (MAF)
Knock sensor (KS)
Inductive engine speed sensor
AC signals
The main characteristics of these signals are amplitude, frequency and waveform.
Frequency Modulated Signals (World Cup)
The operating characteristics of frequency modulated signals are amplitude, frequency, waveform, and periodic pulse width.
The sources of FM signals are the devices shown below.
Inductive crankshaft position sensor (CKP)
Inductive Camshaft Position Sensor (CMP)
Inductive vehicle speed sensor (VSS)
Hall-effect speed and shaft position sensors
Optical speed and shaft position sensors
Digital sensors for thermometric measurement of air mass (MAF) and absolute pressure in the inlet pipeline (MAP)
Pulse Width Modulated Signals (PWM)
Operating Characteristics of Pulse Width Modulation Signals (PWM) are amplitude, frequency, waveform and duty cycle of periodic pulses.
The PWM signal sources are the following devices.
fuel injectors
Idle stabilization devices (IAC)
Primary winding of the ignition coil
Carbon canister purge solenoid valve (EVAP)
Exhaust gas recirculation valves (EGR)
Encoded square wave train
The performance characteristics are the amplitude, frequency and shape of the individual pulse sequence.
These signals are generated by the ECM self-diagnostic memory module of the engine management system.
By analyzing the width and shape of the pulses, as well as counting their number in each of the groups, the fault codes stored in the memory can be read (code 1223 - see below).
The amplitude and shape of the waveform remain constant, the recorded value will be output until the module memory is cleared.
Waveform Interpretation
The shape of the signal produced by the oscilloscope depends on many different factors and can vary greatly. In view of the foregoing, before proceeding with the replacement of the suspected component in the event that the shape of the captured diagnostic signal does not match the reference waveform, the result should be carefully analyzed.
digital signal
analog signal
Voltage
The zero level of the reference signal cannot be considered as an absolute reference value, - «zero» real signal, depending on the specific parameters of the circuit under test, may be shifted relative to the reference ([1] - see illustration digital signal) within a certain acceptable range.
The full amplitude of the signal depends on the supply voltage of the tested circuit and can also vary within certain limits relative to the reference value ([3] - see illustration digital signal and [2] - see illustration analog signal).
In chains direct current the signal voltage is limited by the supply voltage. An example is the idle speed stabilization circuit (IAC), the signal voltage of which does not change in any way with a change in engine speed.
In chains alternating current the amplitude of the signal already unambiguously depends on the frequency of the signal source, so the amplitude of the signal generated by the crankshaft position sensor (CKP) will increase with increasing engine speed.
In view of the foregoing, if the amplitude of the signal taken with the oscilloscope is excessively low or high (up to cutting off the upper levels), you just need to switch the operating range of the device by switching to the appropriate measurement scale.
When checking the equipment of circuits with electromagnetic control (e.g. IAC system) voltage surges may occur when the power is turned off ([4] - see illustration digital signal), which can be safely ignored when analyzing the measurement results.
Also, don't worry about waveform distortion such as skew at the bottom of the leading edge of the square wave ([5] - see illustration digital signal), unless, of course, the very fact of the flattening of the front is not a sign of a malfunction in the functioning of the tested component.
Frequency
The frequency of repetition of signal pulses depends on the operating frequency of the signal source.
The shape of the recorded signal can be edited and brought to a form convenient for analysis by switching the scale of the time base of the image on the oscilloscope.
When observing signals in AC circuits, the time base of the oscilloscope depends on the frequency of the signal source ([3] - see illustration analog signal), determined by engine speed.
As mentioned above, to bring the signal to a readable form, it is enough to switch the time base scale of the oscilloscope.
In some cases, the oscillogram of the signal turns out to be mirrored with respect to the reference dependence, which is explained by the reversibility of the polarity of the connection of the corresponding element and, in the absence of a ban on changing the connection polarity, can be ignored in the analysis.
Typical Signals of Engine Control Components
Modern oscilloscopes are usually equipped with only two signal wires, coupled with a variety of probes that allow you to connect the instrument to almost any device.
The red wire is connected to the positive pole of the oscilloscope and is usually connected to the terminal of the electronic control module (ECM). The black wire must be connected to a properly grounded point (mass).
Fuel injectors
The control of the composition of the air-fuel mixture in modern automotive electronic fuel injection systems is carried out by timely adjustment of the duration of opening of the electromagnetic valves of the injectors.
The duration of stay of the injectors in the open state is determined by the duration of the electrical impulses generated by the control module and applied to the input of the solenoid valves. The duration of the pulses is measured in milliseconds and usually does not go beyond the range of 1-14 ms.
Fuel injector opening control pulse (typical waveform)
Often, a series of short pulsations can also be observed on the oscillogram, following immediately after the initiating negative rectangular pulse and maintaining the injector solenoid valve in the open state, as well as a sharp positive voltage surge accompanying the moment the valve closes.
The correct functioning of the ECM can be easily checked with an oscilloscope by visually observing the change in the shape of the control signal with varying engine operating parameters. So, the duration of the pulses when turning the engine at idle should be slightly higher than when the unit is running at low speeds. An increase in engine speed should be accompanied by a corresponding increase in the time the injectors stay open. This dependence is especially well manifested when opening the throttle with short presses on the gas pedal.
1. Using a thin probe from the kit supplied with the oscilloscope, connect the red wire of the device to the injector terminal of the ECM of the engine management system. Second signal wire probe (black) ground the oscilloscope securely.
2. Analyze the shape of the signal read while cranking the engine.
3. Having started the engine, check the shape of the control signal at idle.
4. Sharply pressing the gas pedal, raise the engine speed to 3000 rpm, - the duration of the control pulses at the moment of acceleration should increase markedly, followed by stabilization at a level equal to, or slightly less than, the idling speed.
5. Rapid closing of the throttle should lead to a straightening of the oscillogram, confirming the fact of overlapping injectors (for systems with fuel cut-off).
During a cold start, the engine needs some enrichment of the air-fuel mixture, which is provided by an automatic increase in the duration of the opening of the injectors. As the duration of the control pulses on the oscillogram warms up, it should continuously decrease, gradually approaching the value typical for idle speeds.
In injection systems that do not use a cold start injector, during a cold start of the engine, additional control pulses are used, which appear on the oscillogram as variable length pulsations.
The table below shows a typical dependence of the duration of the control pulses for opening the injectors on the operating state of the engine.
Engine condition |
Control pulse duration, ms |
idle |
1.5—5 |
2000-3000 rpm |
1.1—3.5 |
Full throttle |
8.2—3.5 |
Inductive sensors
1. Start the engine and compare the waveform taken from the output of the inductive sensor with the given reference.
An increase in engine speed should be accompanied by an increase in the amplitude of the pulse signal generated by the sensor.
Idle speed solenoid valve (IAC)
There are many different types of IAC solenoid valves used in the automotive industry, producing signals of various shapes as well.
A common feature of all 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, causing a decrease in idle speed.
If the duty cycle of the waveform changes with increasing load, but when the consumers are turned on, there is a violation of the stability of the idle speed, check the condition of the solenoid valve circuit, as well as the correctness of the command signal issued by the ECM.
Normally, the idle speed control circuits use a 4-pole stepper motor, which is described below. The 2-pin and 3-pin IAC valves are tested in a similar manner, but the waveforms of the signal voltages they produce are completely different.
The stepper motor, in response to a pulsed control signal from the ECM, adjusts the engine idle speed in steps according to the operating coolant temperature and the current engine load.
The levels of the control signals can be checked using an oscilloscope, the measuring probe of which is connected in turn to each of the four terminals of the stepper motor.
1. Warm up the engine to normal operating temperature and leave it running at idle.
2. To increase the load on the engine, turn on the headlights, air conditioning, or, on models with power steering, turn the steering wheel. The idle speed should drop for a short time, but then immediately stabilize again due to the operation of the IAC valve.
3. Compare the captured oscillogram with the given reference.
Lambda probe (oxygen sensor)
The Section contains oscillograms typical of the most commonly used zirconium-type lambda probes in cars, which do not use a reference voltage of 0.5 V. Recently, titanium sensors have become increasingly popular, the operating signal range of which is 0–5 V, and voltage is issued during the combustion of a lean mixture, low - enriched.
1. Connect an oscilloscope between the lambda probe terminal on the ECM and ground.
2. Make sure the engine is warmed up to normal operating temperature.
3. Compare the oscillogram displayed on the screen of the meter with the given reference dependence.
4. If the recorded signal is not wavelike, but is a linear relationship, then, depending on the voltage level, this indicates excessive over-depletion (0—0.15 V), or reenrichment (0.6—1 V) air-fuel mixture.
5. If there is a normal wave-like signal at idle speed of the engine, try to sharply squeeze the gas pedal several times - the signal fluctuations should not go beyond the range of 0-1 V.
An increase in engine speed should be accompanied by an increase in signal amplitude, a decrease - by a decrease.
Knock sensor (KS)
1. Connect an oscilloscope between the ECM knock sensor terminal and ground.
2. Make sure the engine is warmed up to normal operating temperature.
3. Slowly depress the accelerator pedal and compare the waveform of the AC waveform to the reference waveform shown.
4. If the image is not clear enough, lightly tap on the cylinder block in the area where the knock sensor is located.
5. If the waveform cannot be unambiguous, replace the KS sensor, or check the condition of its circuit wiring.
Amplifier output ignition signal
1. Connect an oscilloscope between the ECM ignition booster terminal and ground.
2. Warm up the engine to normal operating temperature and leave it running at idle.
3. A sequence of rectangular DC pulses should be displayed on the oscilloscope screen. Compare the received signal waveform with the reference waveform shown, paying close attention to matching parameters such as amplitude, frequency, and pulse shape.
With an increase in engine speed, the frequency of the signal should increase in direct proportion.
Primary winding of the ignition coil
1. Connect an oscilloscope between the ECM ignition coil terminal and ground.
2. Warm up the engine to normal operating temperature and leave it running at idle.
3. Compare the waveform of the received signal with the given reference waveform - positive voltage surges should have a constant amplitude.
Uneven throws can be caused by excessive resistance of the secondary winding, as well as a malfunction in the condition of the BB wire of the coil or spark plug wire.
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