In recent years, general workshops and dealerships alike have been forced to become familiar with electronic diagnostics, not only as a repair method but also as a means of understanding what is going on under the skin of a modern motor car. With today’s vehicles bristling with electronic control units it’s become a requirement for survival that workshops are able to work efficiently and effectively with this sort of technology.
But it’s not all been plain sailing. The trade has dragged itself, falteringly, into the electronics age, with a good many independent workshops trying to resist making the necessary investment in test equipment and training. Such cost-saving measures really are a false economy though, only serving to reduce everyday working potential, and increase the reliance on expensive, freelance diagnostic consultants.
The general situation hasn’t really been helped by a common misconception that has been around for years, and which remains to this day. It particularly afflicts those about to invest in test equipment, or who are new to the business of electronic diagnostics. People falsely believe that modern diagnostic testers will give them all the answers, quickly and easily. The impression often given by over-eager salesmen is that even the most subtle vehicle management problems can be located and confirmed with a short sequence of keystrokes; if only things were that simple!
In reality, no matter how good the hardware you are working with, it is still only going to point you in the right direction; that really is the best you can hope for. Users still have a vital role to play in terms of interpreting the information presented to them. Diagnostic equipment isn’t good enough yet to make reasoned judgements about the likely causes of problems, so the idea that an all-singing, all-dancing machine is going to hand you the solution on a plate is well wide of the mark.
While budget-priced testers are likely to be limited to accessing and clearing system fault codes, higher-specification machines will offer the potential for truly great things. They will open the door to a range of data that can incorporate operational information from every control system on a vehicle. However, while having the data is one thing, acting on it in a meaningful way is another.
It is always worth remembering that the few pounds of grey matter that we all have between our ears, is still be best diagnostic tool on the market. With it we have the processing power to think logically and intuitively, which is just as well as far as diagnostics is concerned. Remember, all testers and scantools are only capable of telling you what they are being told by the ECU they’re plugged into; it’s like an data interpreter. It’s up to you to make judgements about what you find, which is where the skill comes in.
Those who lack this ability are the ones who tend to be disappointed by the diagnostic equipment they buy. The combination of a false expectation of how easy the tool will make the job, combined frequently with a poor understanding of how the tool actually works (people still don’t read the manuals!), is NO recipe for success. It’s been suggested that the majority of owners, for whatever reason, only ever use about 20% of the functionality of their equipment. This is a great shame because those who make the extra effort, and dig a little deeper into the remaining 80%, frequently unearth the diagnostic crown jewels!
Originally, it was the vehicle manufacturers’ need to deal with their own electronic management systems that prompted the development of hand-held diagnostic testers. But the reduction in the restrictions that the car makers had managed to build for themselves, and the introduction of emissions-related EOBD (European On-Board Diagnostics) regulations, cleared the way for the aftermarket to begin developing similar equipment for the independent motor trade. The EOBD regulations also dictated that manufacturers should make available, over a common communications protocol, a standard set of data from which reasoned diagnostic decisions could be made. One of these sets is ‘live data’.
A vehicle’s electronic management system relies on the transfer of information. Data is gathered by the countless sensors around the car and sent to the ECU for processing. The results of that processing – a series of command outputs – are then sent off to the various motors and solenoids that actually control how the vehicle operates. All of this data is transferred in the form of electrical signals, which, on their own, have very little meaning. Thankfully, though, the control units contain processors that convert these electrical signals into useful, real-world values.
So, the voltage from the coolant temperature sensor, for example, is transformed into a value expressed in degrees centigrade. Similarly, the output pulses from the crankshaft position sensor are converted into an rpm figure. Gaining access to this ‘live data’ is the job of the scantool, which plugs into the system, communicates with it and then presents the information for all to see.
Inconveniently, the regulations didn’t impose a standard set of measurement criteria to be used when displaying ‘live data’, so different manufacturers present things in different ways. For example, outputs from the throttle position sensor (usually a voltage potentiometer), can be presented either as a percentage of the total output available, or as a voltage level. So, you might see a half-throttle setting being expressed as ‘50%’ or ‘1.63V’. Likewise, MAF data can be expressed in ‘kg/h’ or as a straightforward voltage value, while the MAP sensor’s signal might be in Bars, volts or kPa (kilo Pascals).
There are generally two levels of live data available – EOBD and the vehicle manufacturer’s (VM) ‘live data’. The EOBD represents the minimum level, while the VM is the maximum available (refer to table). These two lists vary from manufacturer to manufacturer, and are also dependent on the equipment fitted. For this reason, it’s not unusual to find a data category quoted in the list, but no actual value given for it. This is usually because the equipment level varies from the control unit software’s capability.
The amount of data shown on the screen can vary too (depending on the scantool being used) but, generally, it‘s viewed in the order shown in the lists provided here. Some scantools have the facility to show the data in a graphical form on their screen but others, such as the OmiScan with OmiTechcenter, manufactured by Omitec Ltd., have software that can be loaded on to a laptop PC. This allows the scantool to act as an interface between the EOBD socket and the laptop.
The Omitec OmiTechcenter system can display up to four data items in graphical form with the same time scale, so that all can be viewed together and usefully compared. This method of display gives the user the ability to select and display related data, enabling the interaction between the two to be viewed in real time. Data presented in this form gives great insight into what the control unit ‘thinks’ it’s seeing, and what its reaction is to data changes. Also, with the data presented on a laptop like this, the user is also able to observe performance during a road test, which is essential when checking for an intermittent fault.
A great many electrical faults on modern cars start off intermittently – triggered as the car goes over a bump, for example. So being able to ‘capture’ relevant data values during an actual road test, and then study then later back at the workshop, provides a tremendously powerful diagnostic tool.
The VAG group – VW, Skoda, SEAT and Audi – has a unique method of presenting its ‘live data’. Instead of a straightforward list, the designers decided to divide things up into related groups of four. Some of the data in these groups is digital information; there may be a sub-group of 0s and 1s, ranging in length from three to eight digits. These indicate the status of specific items – either on or off. Although this system is very comprehensive, it’s necessary to have relevant documentation so you can select which group (or groups) of data to view and interpret. The data groups are linked directly to the engine code, and the documentation lists the groups by engine code. VAG documentation is included in the Omitec OmiTechcenter set-up.
So let’s consider now a few examples of just how useful being able to access ‘live data’ really can be.
The pre-catalyser, listed as sensor 1, oxygen sensor input to the control unit, if shown in graphical form, should show a waveform that is switching between approximately 100mV (lean) and 900mV (rich) at a frequency of about 1Hz (once a second). The actual shape of the waveform is not specifically important but a very ‘noisy’ waveform should be investigated, the main point is whether or not it is switching. If a post-catalyser is fitted, listed as sensor 2, the waveform should be a steady signal, not switching, and should show a slightly rich mixture, i.e. slightly higher than about 500mV. If both are switching this is an indication of a defective catalytic converter. However, the important thing to remember is that the sensors do not operate effectively until they are correct temperature, therefore, do not condemn them unless you are certain the exhaust gases are hot enough (approximately 350°C).
A ‘map’ that inputs the engine speed, coolant temperature and load, and outputs the basic duration of the injector pulse, determines the basic quantity of fuel required per cylinder. The short-term fuel trim has a very fast reaction to modify the pulse duration (indicated in % terms), controlled by the input from the pre-cat oxygen sensor. The waveforms of the fuel trim and oxygen sensor data values shown graphically should show that as the oxygen sensor signal shows ‘rich’, the short-term fuel trim should show a low % trim. Then, as the signal goes ‘lean’ the trim should increase the % level. The two signals should show peaks in opposition to each other.
The long-term fuel trim is a method of correcting the ‘map’ value to account for gradual changes in the engine performance, and will usually be steady at 0-1%, depending on the age and mileage of the vehicle. A high long-term fuel trim may be an indication of a fuel delivery problem.
As the throttle is pressed from idle to the wide-open position (WOT), the output signal should show an uninterrupted increase. Any ‘glitch’ along the way indicates a probable track error on the sensor. The signal may be shown as a voltage or as a %, dependent upon the manufacturer. This signal can be tested with the ignition switched on, but the engine off.
This is easily viewed non-graphically, as the value changes slowly over a period of minutes rather than seconds. However, if a heat-related fault is suspected, this can be clearly shown by graphing the suspect component against temperature, and the point of change can be accurately related to the temperature. The temperature also affects the fuel level, so that with a cold engine the short-term fuel trim should indicate a higher % level, dropping as the temperature rises.
The signal from the MAP/MAF represents the volume of air entering the intake manifold, which is an indication of the load, and is the main signal used to calculate the injector pulse timing. When this signal is graphed against the throttle position sensor signal, the two should rise and fall in unison.
This data shows the amount of ignition advance applied at any instance and, particularly at idle, will change rapidly and often as it is one of the main controllers of idle speed.
Diagnosing problems within any electronically controlled system is not for the faint-hearted. But if a basic set of rules is followed, and as much data as possible is gathered, it is perfectly possible to arrive at a reasoned cause of the trouble. However, it’s important to appreciate that interpreting ‘live data’ becomes easier with experience, and is always enhanced by previous knowledge gained from work with general vehicle engineering diagnostics.
Technicians involved in this sort of work should be constantly building a database of information from good and bad cars, so that they can start to develop an intuitive feel for the sort of values to be expected. So much time will be saved by knowing instinctively roughly what a particular output should be, rather than having to look it up. But reaching this level of knowledge isn’t a quick process; it requires effort and tenacity. The regular ‘Electronic Diagnostics’ articles published in CM detail component data where available for the vehicles being featured, and the internet can be a good source of information too. But, in the case of the web, the good information will certainly take some searching out.
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.
