Using Oscilloscopes on Vehicles

WisdomAugust

Q: How will the oscilloscopes be used to diagnose a stalling vehicle?

-Customer Inverview
A: From the outset, engine stalling (like all diagnostic procedures) needs to be evaluated by an interview with your customer as they have all the answers and more importantly the history/sequence of events
A basic inspection must never be overlooked before any diagnosis as it is all too easy to attack vehicles with an array of test equipment when in fact our concern could be contaminated fuel!
Confirmation of the customers concern is paramount and ideally with a scan tool attached to gather valuable data during the fault condition. This is not always possible as faults are often intermittent and dependent on driving conditions and so again, the customer interview will assist

-Road Testing
Diagnostic trouble codes are always the first port of call along with any serial data from your chosen Scan tool. Road testing the vehicle with your scan tool connected whilst trying to simulate the condition will also assist to provide target areas for further diagnosis and testing and this is where oscilloscope becomes invaluable.

-Measure Components with an Oscilloscope
Armed with all the information above the technician will be required to follow a fault code flowchart if a relevant fault code is present, or, armed with his knowledge of the system will begin to test and measure components of the engine management with an oscilloscope that could be responsible for the customers symptom.
To give a typical example, let’s take a gas engine stalling when stopping at junctions but no fault codes evident. A description of the engine idle control system is required and a list complied of possible components that could produce the staling. From this list the technician may decide to test the, oxygen sensor, airflow meter, and throttle position sensor. The order of tests depends on the ease of access to these components but with a 4 channel scope all these inputs can be measured and monitored in real time on one screen.
Using the oscilloscope gives the technician the advantage to monitor (in real time) the actual live signals and compare them to the processed data from the scan tool.

WisdomAugust

rlmac

Aqui no Brasil fizemos um gabarito para usar com o Hantek junto com um transdutor de compress?o, veja este vídeo: https://www.youtube.com/watch?v=GGf19nQvmbc

WisdomAugust

Self-induction back-voltage - This is back-voltage produced by self-induction. This induced
electromotive force opposes the change in current, restricting it if the current is increasing and
enhancing it if the current is decreasing.
If the rate of change of the magnetic field in a solenoid (relay solenoid, solenoid injector,
ignition coil, inductive sensor for rotation detection) the self-induction back-voltage can reach up
to thousands of Volts. The magnitude of the back-voltage mainly depends on the inductiveness
of the coil and the rate at which the value of the magnetic field is changing. In electromagnetic
executive mechanisms the value of the magnetic field changes the fastest when the field fades
away after a quick shutdown of the powering voltage. In some cases the self-induction effect is
undesirable and precautions are being made in order to reduce it or to remove it. But some
electric circuits are designed to produce the maximum self-induction back-voltage, for example
the ignition system of gasoline engine. Some ignition systems can produce a back-voltage of
self-induction up to 40kV– 50kV. Such voltages can be easily measured with an automobile
oscilloscope by using capacitive pick-up.  

WisdomAugust

What is self-induction?

This concept is not directly connected to the work principle of the oscilloscope, but it is
important to understand why when activating the inductive executive sensors on a 12V car
voltage we get voltages ranging from 60V to 200V and in the primary ignition chain up to 400V-
500V.

Self-induction - This occurs when the current in an inductive circuit changes and the magnetic
field cuts the wires; this induced electromotive force opposes the change in current, restricting it
if the current is increasing and enhancing it if the current is decreasing.  

WisdomAugust

Duty cycle

The duty cycle is the fraction of time that a system is in an "active" state. For example, in an
ideal pulse train (one having rectangular pulses), the duty cycle is the pulse duration divided by
the pulse period. For a pulse train in which the pulse duration is 1 μs and the pulse period is 4 μs,
the duty cycle is 0.25. The duty cycle of a square wave is 0.5, or 50%. This period is one of the
PWM signal parameters (Pulse Width Modulation).

The PWM signal is used to control some executive mechanisms. For example in some engine
control systems the PWM signal operates the electromagnetic idle speed valve. Furthermore
PWM signal is also generated by some sensors that transform the measured physical parameter
into direct correlation with the period of ignition.

WisdomAugust

Pulse width

The width of the impulse – this is the time lapse, during which the signal is in active state. The
active state is the level of voltage that triggers the executive mechanism. Depending on how the
actuator is connected the active state can have different voltage levels: 0V, +5V, +12V.
Practically the level can vary around these values. For example: the active state for the injector
control signal in most engine control systems has a voltage of 0V, but can practically vary in
range from 0V to +2.5V and more.  

WisdomAugust

Frequency

Frequency is the number of occurrences of a repeating event per unit time. The SI unit of
frequency is the hertz (Hz), defined as one cycle per second. In the automobile electronics the
number of rotations of the engine is measured per minute – (RPM). Using the waveform of the
voltage of a periodical signal we can easily calculate the frequency of the signal. In order to do
this we need to measure the period of the (the duration of a complete cycle of the signal). The
value obtained can be recalculated into frequency using the respective formula.

Let us examine the following example. A sensor generates 1 voltage impulse per crankshaft
rotation. The time lapse between 2 impulses is called a period. In the case given 2 consecutive
impulses are separated by 7.4 divisions of the screen of the oscilloscope. The scale of the screen
used for visualizing this signal is 1 division equal to 100 ms or 1/100 of the second, thus the
period of the signal is 0.74 seconds. Knowing the length of the period of the signal we can
calculate how many cycles per second are there, hence the frequency of the signal in Hz. When
converting period to frequency we need to divide the time period chosen (in our case 1 second)
by the length of the period of the signal (in our case 0.74 seconds):
1/0.074 = 13.5 Hz

If in this case we calculate the number of repeated periods per minute we will get the frequency
of rotation of the crankshaft in RPM. When converting the period in frequency in RPM we need
to divide the time period chosen (60 seconds) by the length of the period of the signal (0.74
seconds)
60/0.74 = 81 RPM

Such calculations can be made using all kinds of waveforms with different scales of
division, but some oscilloscopes can directly show the results in RPM.  

WisdomAugust

Analog and Digital signals

All the signals examined until now are analog signals, they are uninterrupted signals. The values
of their voltage changes through time by some rule or randomly. As an example for a complex
analog signal the lambda sensor (O2 sensor) signal can be mentioned.

A waveform of digital signals switches between two voltage levels representing the two states of
a Boolean value (0 and 1), even though it is an analog voltage waveform, since it is interpreted in
terms of only two levels – high and low, on and off. Such voltage levels are called Logic voltage
levels. In most cases, the logical levels have constant voltage values: +5V and 0V for example.

Digital signals are generated by switches. These switches are represented by transistors
switching between “open/closed” states. Sometimes digital signals are generated by mechanical
switches, electromechanical relays. An example of digital signals in the automobile electronic is
the Hall sensor, the throttle end position sensors; the closed throttle position sensor (CTPS), the
wide open throttle (WOT) sensor and the data transfer signals between the different ECUs. As
the analog signals digital signals can be periodical and non-periodical.  

WisdomAugust

Slow-Variable SignalOne of the biggest advantages of the digital oscilloscope is the ability to show waves of
processes with a large period, meaning signals that change slowly through time. An example for
such a signal is the waveform on the following picture.



Slow-Variable Signal

WisdomAugust

There is another type of complex signals, which can be both periodical and non-periodical.
These signals are such that include more than 1 frequency. An example of such a signal is shown
in the following picture:




For such a signal to be synchronized on the screen, the oscilloscope needs a specialized function
called “Trigger hold off”. (* Look up in the Synchronization section)