Click on the advert above to visit the company web site

Product category: Electrical and Electronic Testing and PAT Equipment
News Release from: Tektronix | Subject: Digital Phosphor Oscilloscopes
Edited by the Engineeringtalk Editorial Team on 03 August 2000

The scope for automotive electronic
diagnostics

Frank Massey of Fuel Injection Services and Christine Rickett of Tektronix look at the instrumentation in use for automotive electronic diagnostics

Most of today's automotive engine design is being driven by exhaust emission legislation A modern vehicle must be efficient to minimise pollutant production, and this level of efficiency is only achievable under the guidance of a computer-controlled engine management system

Engine performance, efficiency and durability have been improved beyond all measure thanks to the developments made in electronic control systems.

Significant advances have also been made in the location of components and the electronic sensors have become much more durable.

Moving-part components have been replaced with solid-state equivalents wherever possible, and these are almost indestructible.

In addition, voltage levels are lower, so that problems with high current levels causing interference and excess heating can be kept to a minimum.

The quality of wiring connectors has also improved tremendously, and most sockets now enjoy very good levels of waterproofing.

The downside of all this progress is that diagnosis has become increasingly complex.

Consequently, many problems are now wrongly labelled as 'system' faults when, in fact, they stem from incorrect diagnosis or poor monitoring.

Hence the requirement for investment in test equipment has increased dramatically.

The latest engine management systems include fault-coded, self-test circuitry which can be read with diagnostic equipment and is linked to warning lights on the dashboard.

Many also feature fail-safe devices whereby, in the event of a sensor failures, the engine reverts to a 'limp-home' mode where it can still be driven, albeit in a controlled way, to prevent further damage.

In addition, many electronic control units (ECUs) now boast serial communication facilities which allow technicians to observe their innermost 'thoughts'.

With the right equipment, an ECU's performance can be accurately monitored and the test results gained used to form an effective diagnosis as to the cause of virtually any engine-related problem.

The most basic requirement for automotive electronic diagnostics is the multimeter.

Voltage levels are a key factor as far as modern engines are concerned.

The electronic management systems which now control them operate by continually measuring and comparing voltages and frequencies, from which the ECU makes its decisions about the running settings.

The ability to monitor these readings is essential if adjustments are to be made and faults corrected, which is why using a multimeter can be so productive.

An electronic technician who knows his job will make many voltage checks when carrying out a diagnostic tune.

He will pay particular attention to the cranking circuit, the charging circuit, the supply to the sensors and engine management unit.

All of these are essential for building up an overall picture of how an engine is performing and where improvements can be made.

Using a multimeter is one of the best ways of developing an understanding of an engine's electrical system.

The measurement scenario is greatly eased by using commercially available break-out boxes which can be connected into the circuit using the original plugging.

This provides a very convenient and risk-free way of testing every wire in a multi-pin plug connector.

Most engine management system manufacturers have now introduced serial communication ports which allow an assortment of relevant voltage values and other calculated measurements to be extracted using a code reader or other dedicated testing equipment.

A modern automotive code reader provides information cannot be obtained by using a multimeter on its own, although the two still have to be used in conjunction with each other to draw the final conclusion when testing circuit faults.

The serial communications port can also be used to make engine adjustments.

The most important factor that can be adjusted with a code reader is an engine's fuelling, although such alterations must be supported with a gas analyser.

No code reader system yet has the ability to measure the gas flowing through an engine, although eventually exhaust sensors may do away with the need for a conventional gas analyser.

Parallel communications test equipment is also available, which is used simply to break into voltage circuitry at the ECU.

It can measure direct voltages for all the major electrical components and also has a 'snapshot' fault-finder facility.

A built-in memory allows it to record faults which occur very rapidly (including intermittent faults) which might otherwise be missed.

However, such equipment is generally computer-based, expensive and, by its very nature, a 'specialist only' installation.

The business of electrical fault finding on the modern engine is a tricky one at the best of times.

For more serious fault diagnosis, an oscilloscope will be needed.

With an oscilloscope working to its full potential, a skilled user will be able to observe quite literally any information that he chooses, relevant to the system.

The oscilloscope is connected to the car using a selection of leads, and their placement is obviously important.

For dedicated engine analysis, leads will be connected to the battery, the coil and the IF circuit.

This will enable the user to extract all the signals relative to the ignition and cranking system.

There is also generally an auxiliary lead which can be connected to special components like fuel injectors or a crankshaft sensor.

Making sense of the image on the oscilloscope screen is an involved business calling for much experience and understanding.

The subtleties of the profiles, the 'cleanness' of the line and any distortion present are all vital factors for consideration.

It is not simply the peak voltage values that are of prime concern: it is also what the line does on the way to and from these peaks which speaks volumes to the informed operator.

Complex analogue waveforms are common in vehicle electronics, and can be generated by coil outputs, signal generators from the crankshaft, and signals from the distributor.

The profile of these curves is as important as their actual amplitude or height.

As a result of these requirements, automotive diagnostic engineers are looking beyond standard real-time analogue or digital storage oscilloscopes, both of which have limitations in looking at complex, rapidly changing signals or intermittent events.

The real-time 'feel' of an analogue oscilloscope is fine for observing real-time events, but these instruments lack the ability to store waveforms for later analysis or comparison purposes.

Digital storage instruments, on the other hand, suffer from inherent limitations in the digital sampling process which can cause some elements of complex signals to be missed or inaccurately recorded.

These problems have been solved with the advent of the Digital Phosphor Oscilloscope (DPO), which uses a combination of very high-speed sampling and parallel processing to produce a real-time analogue-type display in a digital instrument.

As a result, the DPO could revolutionise the way in which tests on automotive systems are carried out.

The instrument's ability to detect small transient signals in an electrically noisy environment will make it invaluable to service and repair organisations, who, as indicated above, often struggle to pin down the causes of intermittent problems.

The advent of the DPO highlights the fact that many problems with car electronics are caused by parameters that might be within the manufacturers tolerance limits but which drift out of specification through wear and tear, and then interact with other parameters to cause potentially catastrophic problems.

For example, one major manufacturer defines a 'glitch' as lasting a minimum of 10 ms, but in fact signals far shorter than this can cause major problems.

Computer-based systems, as mentioned above, are often limited in terms of sampling rate and bandwidth, and can miss the sort of transient faults that can cause problems.

They are also, often by their very nature, confined to permanent workshop-based installations, and cannot be used for in-vehicle testing.

Conventional oscilloscopes have been used for some time within the automotive industry to give a closer insight into the nature of the 'cause and effect' signals in car electronics.

However, users have often been frustrated by their inability to detect transient events in the presence of noise, or intermittent events over a long period of time.

Now, however, the DPO offers the answer to both these problems.

It can look at high-speed repetitive signals over a period of time, identify anomalies, and zoom in on a particular section of waveform for detailed examination.

The DPO's unique display, which combines the storage and analysis capabilities of a digital storage oscilloscope with the real-time 'feel' of an analogue instrument, make it easy to not only identify problems but also examine and compare waveforms in detail.

The colour display on the DPO also makes it easy to separate and compare different signals, so that a newly captured waveform can be compared with an existing 'standard' signal.

The powerful trigger capabilities on the DPO are also advantageous, since the instrument can be set up to trigger on particular combinations of events that might be missed by a conventional oscilloscope.

Another key benefit is that these instruments are easily portable and can be battery operated.

They can therefore be used for in-vehicle testing on the road, which is sometimes the only way of finding the sort of problems that can occur in real life.

As the electronic content of cars increases, particularly as new legislation imposes limits on emissions and energy efficiency, so the automotive industry's need for instruments such as the DPO will grow.

The increased use of parallel bus structures such as CAN networks will also make it essential to look at multiple signal interactions, and it is certain that the DPO has a big future in such applications. Request a free brochure from Tektronix ...

• Tektronix: contact details and other news
• Email this article to a colleague
• Register for the free Engineeringtalk email newsletter
• Engineeringtalk Home Page

Search the Pro-Talk network of sites

Put your news on Engineeringtalk  |   Advertise  |   Get the free Engineeringtalk newsletter  |   Engineeringtalk Home
About Engineeringtalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK