The best tools in life are free
Many car owners gravitate towards the easiest option and rely on expensive electronic gizmos, theorising that technology will solve their problems with neither intervention, nor effort, being required on their part. Whether prioritising a garage, based upon the level of expensive equipment it possesses, or investing in lavish kit yourself, Andy’s advice is to STOP. While acknowledging that code readers, scanners, analysers and the like have become important additions to every modern technician’s toolbox, he stresses that the user must exercise restraint and not let technology take over: “Any car repairer must rely on their natural senses,” he says. “Use your eyes and ears to identify any unusual behaviour and use logic to discover any patterns, such as trying to identify the conditions under which the problem occurs, or detecting other affected components, which might lead you to the real cause.”
Research need not cost vast sums of money. Even a cursory Google search might uncover other owners experiencing similar problems, although you should be wary of taking the phrase ‘common fault’ as gospel. Even if the statement is true, it might not be correct in your case. Diagnosing by hearsay might cost you dearly. Be critical, especially of internet forum statements unless they include data which you can apply to test a suspect part. Simply replacing supposed ‘common fault’ components and hoping for the best, without testing them first, can turn out to be a false economy; proper diagnostics is more than mere guesswork.
To prove his theory, Andy intercepted a number of discarded engine sensors from a host of garages and tested them with his students. More than half of the cast-offs were found to be serviceable. He theorised that the reason so many perfectly good sensors had been thrown away was down to relying too much on screen readouts, jumping to a conclusion and fitting a new sensor instead of investing time in fully testing the original part. While Andy appreciates that the act of replacing a sensor may solve a problem, it might have been avoided: “Problems occur when engine management fault codes are treated as
absolutes, rather than guides. A code might be interpreted as identifying a faulty sensor, but it might not be so. Its connector might be suffering from a corroded contact, for example, and the very act of disturbing the wiring might solve the problem, negating the need to replace a potentially costly sensor.”
Andy also advises that, even if a part has been identified as faulty and renewed, the diagnosis procedure should not end there, because the failing component might be a symptom of a further underlying problem. If left unchecked, the repair might not last.
A common situation involves coil packs that are placed under greater stress when the spark plugs and HT leads are worn. Mass air flow (MAF) meters can also become contaminated and fail to provide correct readings if air filter changes are neglected or if a poor quality aftermarket high-flow replacement is fitted. Some replacement parts may be of a poorer quality and, in the case of coil packs, might not feed the expected signal back to the engine ECU. Not only can poor quality components not last as long as the Original Equipment part, they can also cause further damage, including to ECUS. Data extraction Basic electrical testing involves measuring, reading and storing a set of values from the car’s electrical system. To take data readings, you can rely on expensive equipment, but a far simpler and cheaper alternative is to invest in a multimeter.
With a multimeter, a beginner can prioritise the settings that measure voltage (V) and resistance (ohms Ω). Obviously, even basic multimeters possess more advanced options, but these tend to be useful for increasingly advanced diagnostic procedures, although the hertz (Hz) setting measures how many on-off signals are registered per second, which is useful especially for measuring sensors that use that method to work (such as Hall-effect sensors). CM’S multimeter masterclasses, published in the February and March 2016 issues, explain this in much more detail.
Fortunately, basic digital multimeters are inexpensive – you need not pay more than £30, although investing in a more sophisticated device might be prudent if frequent use is envisaged. Despite tending to be far more expensive – although you can buy self-assembly kits for around £30 and interfaces for your laptop cost from around £100 – oscilloscopes measure voltage over time and present the information as an analogue graph, as shown in Step C5 on page 12. The best thing about multimeters and oscilloscopes for the beginner is that they are measuring tools and their use cannot damage any other components. Storing data Whether extracting fault codes or carrying out serial or parallel diagnostics, always record your findings. You can simply write them down on a piece of paper or else take a screengrab with a mobile phone camera. This will allow you to compare the original information with later readings.
Certain oscilloscopes will allow you to convert and save a selection of live data to a computer file. However, you will need to configure the graphical settings to ensure that the resulting file size is big enough to provide meaningful information, but not so large that it cannot be easily stored or shared with other people. There is no set rule for this, because everything depends on the part being interrogated and for how long. Fast pulse signals, such as those generated within fuel injectors and which occur many times a second, require a greater sample rate, compared to other components, such as coolant temperature or air mass meters that require slower signals. Comparing data Having retrieved data from your vehicle, the next stage is to compare your readings with the official data. If you have access to car-makers’ information,
which can be expressed in either actual figures or wave-forms (see Step D5, for example), making a comparison is straightforward so long as you are aware of any tolerances.
If you cannot access manufacturer information, all is not lost. First, look for irregular patterns. Should you inspect the flow-rate correction of four fuel injectors and discover that one injector yields considerably different figures to the others, you may have identified the culprit. If you are extracting data via the EOBD socket (serial diagnostics), the ability to read and compare several data streams simultaneously can be invaluable, especially if you know that there should be correlations between different readings. The live data graphs of voltage received at the MAP sensor, as pictured, and the MAP sensor’s pressure reading should be fairly similar, even if the official voltage data is unknown.
When looking at live data, consider what Andy describes as “known givens”: what would you expect to see? A coolant temperature sensor’s resistance, for example, can only change in one direction as the engine warms. Similarly, a throttle position sensor should respond in a linear fashion while the accelerator pedal is depressed.
This is only the beginning
The following pages give an overview of how to carry out the most common basic diagnostic techniques using a simple multimeter and oscilloscope. While a code reader is useful, it is not always necessary, and you can invariably enlist a local garage to reset the codes for you once you have solved the problem. It is also worth investing in back issues of CM to see if the popular Electronic Diagnostics series has covered your particular car in the past, especially as it includes useful data such as wiring information (you can check for specific makes and models of cars either by consulting either the Electronics Diagnostics Back
Numbers section published in certain issues of the magazine or the Index page at www.carmechanicsmag.co.uk). By following the guide, you can hopefully build your confidence to progress further and save money by not renewing parts unnecessarily.