Using Oscilloscopes on Vehicles

Rose

Example analysisA Dodge McLaren (3. 3L), the engine speed at 800 ~ 3000r / m in, poor acceleration. After a preliminary
inspection of oil, circuit, found no abnormalities.

When the oscilloscope is used to detect the engine, the secondary waveform shown in FIG. 8 is obtained.
In the secondary ignition waveform test, an abnormal waveform appeared in each of the first, second, fourth
and fifth cylinders, that is, before the breakdown voltage, a small abnormal peak appeared. The normal
waveform is that when the ignition power transistor is suddenly disconnected from the closed state, the
magnetic flux decreases rapidly as the primary current drops to zero, so that the voltage generated by
the secondary coil rises sharply and penetrates the spark gap before the maximum value The voltage is
the breakdown voltage.






Figure 8 single cylinder secondary ignition waveform

Rose

In Figure 8, it can be seen that before the breakdown voltage, a peak voltage of about 4 kV is generated
in the secondary circuit. The generation of this voltage, ignition coil secondary circuit should be due to
turn-to-turn short circuit.


In the secondary voltage generation process, due to local short circuit, so that part of the ignition energy
loss, and can not meet the acceleration required energy requirements. Causing the engine to work normally
at idling conditions and not working well during acceleration. After replacing the new ignition coil, acceleration
performance returned to normal.


5 Summary
The firing waveform is a window through which we can see what is happening in the combustion chamber.
The use of oscilloscope to observe the ignition waveform is one of the important methods to achieve rapid
detection and diagnosis. Among them, especially the observation of secondary waveforms, it is considered
to be a very effective comprehensive test.


If the measured secondary waveform of the engine is no problem, indicating that the ignition system, fuel
supply system and cylinder sealing are no problem.

Rose

Vehicle Diagnostics with Oscilloscope  



What is the oscilloscope?  



Unlike the voltmeter, with an oscilloscope you can not only see the mean values of the voltage in
the circuits measured, but also the change and shape of that voltage through time.

All oscilloscopes have screens on which the waveform is shown. The screen can be a cathoderay
tube type, a liquid crystal display (LCD) or in the form of a computer program. The typical
oscilloscope screen is divided into equal spaces (divisions) which allow to visually interpreting
the parameters of the signal.

The graphics shown on the monitor are called waveforms. Usually oscilloscopes show only
waveforms of the voltage. This form of visualization shows the change of voltage through time.
The divisions marked on the horizontal(x) axis allow for the time parameters to be measured,
and the vertical(y) axis allow for the voltage values to be measured.  

Rose

What purposes have oscilloscopes in auto diagnostics?

The oscilloscope helps us find the problem quicker and easier. Often the problem hasn’t recorded
an error code (DTC) in the corresponding ECU, a DTC that can be read with a code reader.
Usually a DTC is recorded when there is a broken cable or a cable has short circuited to a
positive or negative supply. But when a detector or a mechanism has stopped working in some
mid position, there is no error recorded. In this case, as when you need to find the reason that
caused an error to be recorded – the automobile oscilloscope is your most needed instrument.

With the increase of sensors, actuators and wiring diagrams built in the modern automobiles, the
automobile oscilloscope is an instrument which diagnoses irregularities in the automobile faster
and easier. The oscilloscope is a irreplaceable tool, when you have to observe output signals
from inductive sensors, whose output signals form a impulse sequence, slow-changing analog
signals, primary and secondary ignition circuits, intake manifold absolute pressure, starter
current waveforms, charging currents and etc.  

Rose

What types of oscilloscopes are there?
Analog oscilloscope
The ones with a cathode-ray tube screens. They show detailed graphics, and can usually show
high frequencies, but are not suited for observing short processes repeated through a long time
interval or relatively slow processes like the ones in an automobile.



Digital storage oscilloscope
The observed result from the digital storage oscilloscope is almost identical to the analog, but the
signal shown on the DSO can be “frozen still” on the screen, saved on the PC’s hard drive, and
used later, or printed. Further more only the current “screen” shown on the monitor can be saved,
and a sequence of many screens can later be opened and observed through time as an animation.
Any screen saved in the working fail can be printed.

There are two kinds of digital oscilloscopes: independent which are external device, and PC
oscilloscopes. The PC-based oscilloscopes are a new type of "oscilloscope" that consists of a
specialized signal acquisition board, which can be an external USB or Parallel port device, or an
internal add-on PCI or ISA card.



One, dual and multiple-trace oscilloscopes
Depending on the number of measuring inputs, both analog and digital oscilloscopes can be
divided into 3 types: one-trace, dual-trace and multiple-trace oscilloscopes.



Universal and specialized oscilloscopes
Depending on their purpose oscilloscopes are divided into 2 groups – universal and specialized.
In automobile repair, an ignition analyzer is used to show the spark waveforms for each cylinder.
In this way the specialized automobile oscilloscopes are also used for testing injectors, ABS, O2
sensor, quick compression tests, fuel pump, CAN Bus and much more. The Motortester is a
specialized automobile oscilloscope.  

Rose

What’s the universal oscilloscope?

The universal oscilloscope is an electronic measuring device used only for observing electrical
voltage through time. The screen of the oscilloscope shows the changes in one or more input
signals over time in an X-Y display, allowing for the amplitude and shape of the voltage to be
accounted, as well as making phase and frequency measurements of the signal.

In order for the oscilloscope to observe other physical parameters, as well as observing voltages
outside its original ranges different types of additional attachments and transformers that convert
the given input into voltage are used.  

Rose

What is the difference between a motortester and a lab scope?

The motortester is one kind of specialized oscilloscope used for auto diagnostics.
The main difference between a motortester and a universal oscilloscope is that the motortester is
capable of visualizing short-timed processes like the ignition spark process. This process is
exceptionally fast, and the period of repeat of the ignition of sparks in time is many times greater
than the time the spark itself exists. This is easily observable when testing the engine in idle
speed, when the majority of the measurements are conducted.

For example: if we observe the ignition cycle of a 4 cylinder gasoline engine, and an ignition
spark that lasts around 2ms, at 800 RPM, the time period between sparks on a single cylinder
would be 150ms. What this means is that the ‘length’ of the spark would account for around 2%
of the actual work cycle, and therefore the burning of the sparks will be seen as very thin lines on
the screen of the oscilloscope, and no information about the phases of the ignition would be seen.
Because of this many diagnostics are forced to increase the RPM of the engine thus shortening
the ignition cycle thus ‘saturating’ the waveform of the cycle.

The motortester shows all the cylinders simultaneously, and allows for detailed observation of
the time period that includes: dwell period, drilling voltage, burn time and turbulence of voltage.
Most motortesters can show the cylinders graphs next to each other, or under one another,
excluding the long time periods between sparks, this method is also known as “parade”.

Another distinctive feature of the motortester is that it can show its time divisions on its
horizontal (x) axis in milliseconds as well as in degrees – up to 720 degrees.  

WisdomAugust

Features that allow PC-based oscilloscopes to be used in auto diagnostics.

Lower cost than a stand-alone oscilloscope, assuming the user already owns a PC.
Easy exporting of data to standard PC software such as spreadsheets and word processors
The software of the device can be directly installed on a PC and upgraded via CD or directly
download from the Internet without having to send the device back to its manufacturer.
Use of the PC's disc storage functions, which cost a lot extra when added to a self-contained
oscilloscope.

PCs typically have large high-resolution color displays which can be easier to read than the
smaller displays found on conventional scopes. Color can be utilized to differentiate waveforms.
PC-based USB oscilloscope get their power supply from a USB port, so no external source is
required.

The USB oscilloscope as any other USB device can be turned on/off without having to turn
off/on the computer.  

WisdomAugust

How many input channels are needed when conducting measurements in auto diagnostics?

When observing signals from sensors, valves, primary ignition chain, secondary ignition chain
etc. no more than one channel is needed. The first kinds of motortesters, which were analog,
needed more than one channel in order to show uniform signals simultaneously on the screen, so
they could be compared with each other. But whit PS based DSO’s this became unnecessary
because a standard waveform can be saved on the PC’s hard drive and opened at any time for
comparison with the currently observed.

Two channels are needed when the sequence in time between 2 signals has to be measured and
how many milliseconds are between them. In other words the second channel is used when the
phase difference between the 2 signals has to be observed and measured. An example for such a
measurement is when simultaneously observing signals from the Crankshaft Position Sensor and
the Camshaft Position Sensor.

Using more than 2 channels is more convenient in some cases but pointless from a functional
point of view.  

WisdomAugust

Where can scope waveforms used for comparison be found?
Most programs that contain automobile technical information have sections whose headlines
have “waveform”, “pattern” and “trace” in them. In other words these sections contain sets of
standard waveforms. Such programs are Autodata, Vivid Workshop, and many others that
contain technical information about automobiles. When looking at a “waveform” on the screen
you must not forget that this is just an ordinary coordinate system like the one everyone has
learned about at school. Like any coordinate system it has a horizontal (x) axis and a vertical (y)
axis. The vertical axis (height) represents voltage, and the horizontal (width) represents time.
The scale of both axes can be changed.