Using Oscilloscopes on Vehicles

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 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 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

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

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

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

(6) If the breakdown voltage of secondary parallel wave less than 5kV (the waveforms of each cylinder
are the same), as shown in Figure 7, indicates that the secondary coil is leakage.




Figure 7 secondary coil leakage

Rose

(5) If the secondary juxtaposed wave from time to time there are beating phenomenon (each cylinder
waveform is the case). Shown in Figure 6, indicating the secondary circuit intermittent power outages.





Figure 6 secondary circuit intermittent power outages

WisdomAugust

(4) If the ignition voltage of the ignition waveform is low, as shown in FIG. 5, it may be caused by the rich
mixture of the combustible mixture or the spark plug leakage. When the combustible mixture is too rich,
the ionization degree is small and the breakdown voltage is high at the initial stage of ignition, but the
ionization degree is increased during the sustaining spark and the spark voltage is reduced (the waveform
of each cylinder is the same). When the spark plug leakage, the spark voltage is also reduced (a cylinder
waveform).







Figure 5 Combustible mixture is too rich or spark plug leakage

WisdomAugust

(3) If the spark voltage of the ignition waveform fluctuates, as shown in Fig. 4, it indicates that the injector
of the EFI system is working poorly, causing the concentration of the combustible mixture to fluctuate.
This failure may occur in every cylinder waveform, may also appear in a cylinder waveform.




Figure 4 EFI system injector work poorly