‘Live data’, and all the diagnostic benefits it offers, is available to anyone who owns a good quality scantool, like Omitec's OmiScan. Unfortunately, a worryingly high proportion of technicians owning this sort of equipment confine its use to the retrieval of fault codes; a crying waste of functionality. Of course, even ‘live data’ won’t provide all the answers. As with most things, there are limitations associated with the live data that is vehicle manufacturer-specific. There is no requirement for any sort of measurement standard, and so car makers can present their technical data however they want. For example, the data from the throttle position sensor can be presented as a voltage or as a percentage of the total movement of the throttle. Likewise, MAP data can be expressed in kPa (kilopascals), bars, psi or voltage.
Expected values, such as the MAP value at idle and with the throttle fully open (WOT), are not always readily available but can be found. Alternatively, when vehicles are available in good working order, it is easy enough to record data values for future reference.
In this way, the canny technician will develop a valuable data resource that will prove useful for many years. But, as with all data, the true value of this sort of information can only be realised with good interpretation; if you don’t understand what you’re looking at, then you’ll get next to nothing from it. To this end, presentation can make a big difference. Good quality scan tools are able to display live data in graphical form, which brings everything to life, and makes spotting potential problems that much easier. Of course, the size of the scan tool’s screen is a limiting factor, which is why the ability to linkup with a laptop is such a worthwhile option to have.
Graphical presentation is generally ‘scaled’ to show the greatest detail possible, depending on the number of data items being displayed. The big advantage of this is that it enables the user to properly interpret the output trace, and make a reasoned judgment about what is actually happening. So, what looks like an erratic signal may, when correctly presented, turn out to be just a series of small variations that are perfectly acceptable in terms of an environment that is constantly changing.
Another factor to bear in mind is that the ‘sample rate’ of the scantool may not capture ‘glitches’ every time they occur. The engine management ECU’s primary job is to control the efficient running of the engine, and most of its processing power is devoted to this complicated task. Consequently, it can only sample the data for output as ‘live data’ at regular intervals, which means that it may take a few goes before a ‘glitch’ is actually captured and revealed on screen.
It’s this type of problem, one that occurs only under specific conditions, or on a completely random basis, which presents the biggest challenge to the diagnostic technician; these easily missed if you are not completely familiar with how best you use your test equipment. Capturing this type of fault will require the selection of an appropriate ‘time base’ or ‘window’, allowing the interruption in the signal to be displayed clearly on the screen. Viewing the data captured over a 10-minute period, for example, may well prove sufficient to catch even the most elusive electronic ‘glitch’.
The amount of live data available is purely in the hands of the vehicle manufacturer; some are better than others. However, whatever you find within the system is likely to be useful as a means of building an overall picture of what’s going on, and assisting with the decision-making process needed to ultimately pin-down which bit of the system is actually at fault.
Another useful feature introduced by the EOBD regulations is ‘Freeze Frame Data’. Every time a DTC (diagnostic trouble code) is set, related data associated with that specific fault is stored from the exact time at which the problem occurred. This is really useful as it provides a sort of performance overview that highlights what else was going on at the time. This data can then be used to help re-create the conditions needed to induce the problem – a particularly valuable ability when trying to track down an intermittent fault.
However, it should be noted that when fault codes are cleared, freeze frame data is erased as well. So, as a precaution, it’s good workshop practice to get into the habit of recording all this data before clearing the codes.
One important part of the EOBD emissions control regulations is the monitoring of emissions-related control systems within the overall control of the engine. ‘Readiness codes’ are an OBD II testing procedure that will, in the fullness of time, eliminate the need to use a gas analyser in the annual MoT test.
Although ‘readiness codes’ are not yet a compulsory item for MoTs, they do present a quick insight into the health of the control systems, emission related sensors and the catalytic converter. Accessing them can be very useful for checking the whole system if any emissions-related components have been replaced during the rectification of a problem.
The areas monitored are divided into two groups; continuous and non-continuous. Those in the first group include; misfire, fuel system and so called comprehensive components.
Those monitored on a non-continuous basis are; catalytic converter, heated catalytic converter, EVAP system, secondary air system, A/C refrigeration, oxygen sensor, heated oxygen sensor and EGR. All these factors are monitored and assessed during a ‘drive cycle’, but more about this later.
The ‘continuous monitors’, as you might imagine, run at all times, testing the system and components that directly affect emissions, as follows:
The possible result of serious misfires is the ‘death’ of the catalytic converter, so monitoring and controlling these is of great importance. Misfires can be caused by ignition or fuelling problems so, in addition to separate fault reporting of injectors and ignition systems, the acceleration rates of crankshaft position sensor pulses are monitored to identify individual cylinder misfires.
This monitor looks for excessive fuel corrections – large changes in the short-term and long-term fuel trims that have a direct affect the air fuel ratio.
Emission-related components, which cover both sensors and actuators, are checked for integrity and circuit problems.
The ‘non-continuous monitors’ function is run to check systems that are not always controlling emissions during a drive cycle, and include the following:
This component doesn’t become operative until it has reached its optimum temperature and, consequently, some vehicles make use of a secondary air injection system during the warm-up period. This ensures that the catalytic converter gets as hot as possible in the shortest time.
Not many vehicles have heated cats, and will be listed by the monitor as ‘Not supported’.
A full EVAP system pressurises the vapour recovery system to test for possible leaks. This isn’t usually supported on European specification vehicles.
Not a common system, but one that operates only during the warm-up stage, to inject air into the catalytic converter and accelerate its chemical reaction.
Not always supported, this monitors the air conditioning system.
The monitoring of this sensor is important; it’s the main controller of fuel correction used to maintain the stoichiometric ratio at 14.7:1. The correction control of fuel, by input from the oxygen sensor, only occurs when the fuel system is in ‘closed-loop’, and this state occurs when the engine is running at idle, or at any other time when it’s in a steady state (not accelerating or decelerating).
Although there are some non-heated sensors, the majority nowadays use a heater to raise them to their operating temperature as quickly as possible. The heater operating current is monitored to indicate the status of the circuit.
Exhaust gas recirculation is another system not always supported but, when fitted, only functions within specific engine operating parameters within a drive cycle.
As we’ve already mentioned, the clearing of the fault code memory also causes the ‘readiness codes’ to be wiped as well. These can only be re-established by either carrying out a specific ‘drive cycle’ or following a period of one or more normal driving operations (ignition on, use the vehicle normally, ignition off etc).
But in the testing environment, the drive cycle has to be performed to a set method if it’s to be successful in meeting the system requirements. It begins when the ignition is switched on, and ends when it’s turned off. What happens in between is carefully structured to pass the monitor tests.
Unfortunately, these vary from maker to maker, but the basic format of the cycle is as follows:
The ideal drive cycle process can be difficult to achieve under busy urban driving conditions – the tests were originally designed for the US market, where it’s probably easier to achieve. Nevertheless, it’s perfectly possible with a bit of determination and careful route planning!
Finally, one other very useful function offered by scantools such as the OmiScan is the ability to actuate certain components that are normally controlled by the main ECM. Once again, though, the number of actuators that can be operated in this way is determined by the vehicle manufacturer, and so varies from system to system. Injectors, for example, can be forced to operate by selecting ‘actuators’ from the scantool’s menu, then pressing the appropriate key. The injector will give an audible click if all is well. However, it’s important not to hold this for too long otherwise you’ll run the risk of damaging the control module and/or putting excessive fuel into the engine.
This could, in turn, threaten the catalytic converter when the engine is next cranked. Carrying out this form of actuator test will check the control module, the wiring to the injector and the injector itself. However, bear in mind that although the injector works electrically and opens and closes, the seat may still be damaged so that it’s leaking fuel under pressure.
This type of testing can also be used to check relays, the air conditioning compressor clutch, cooling fans, ABS solenoid valves in the modulator etc. – it’s a useful, practical function.
So, to sum up, although by no means perfect, the availability of ‘live data’ and the opportunity to actuate important system components certainly add some very valuable tools to the diagnostic armoury. The more options we have available, the better the chances of accurately identifying and successfully resolving the increasingly complicated electrical problems which affect modern vehicles. The primary aim should always be to avoid resorting simply to the swapping of successive components in the hope that the problem will go away. This is an expensive and generally hit-and-miss approach that very few professionals ever adopt.
Most manufacturers of quality diagnostic equipment offer training courses designed as introductions to their equipment. But surprisingly high numbers of new customers fail to take this opportunity, even when the cost of the course is included in the price of the equipment! This is a big mistake.
If you’ve spent your hard-earned cash buying a good quality scantool, then why on earth wouldn’t you want to be coached on how to use it properly? Plenty of users never get further than being able to read and clear fault codes, which is the equivalent of using a PC accounts package to control the petty cash at your workshop, rather than the finances of the whole business.
Omitec offers a one-day ‘Practical Diagnostics’ course for all of its OmiScan customers, held at its manufacturing unit in Oxford. What’s more, this course is also open to anybody who wishes to learn more about diagnostic procedures, whether or not they own one of the company’s products.
For more information, call Omitec on 01380 732000.
