Cut to the chase

Damien Coleman from Snap-on discusses how preparation can lead to easier diagnostics

Published:  28 August, 2018

Many modern systems, such as common rail diesel injection, can appear to be so complex that they seem to operate by magic. Over time, such systems are only going to become more and more complex, so understanding them means you can gain a head start on their repair.
    
You can be presented with a seemingly endless amount of data relating to fuel pressure feedback, fuel pressure control, cam/crank synchronisation, measured mass airflow, injector flow correction feedback, and many other areas.
    
However, if you prepare yourself with a fundamental understanding of the system and all data available pertaining to the fault, a systematic approach to the fault-finding procedure can be carried out.  
Data overload

Figure 1 shows  the live data returned from a common rail diesel injection vehicle with an EDC16 engine management system.
    
There is an enormous amount of data available from these data parameters, which can allow you to ascertain the nature of the fault. The actual operation of the fuel system can be compared to the desired system operation and using the data, a decision can be made on the condition of the system and where a fault (if any) may be.
    
An oscilloscope is another important tool when investigating a fault with such a complex system. Figure 2 shows an oscilloscope waveform from an Audi with the 2.0L common rail engine. The yellow trace is the fuel rail pressure sensor voltage (feedback) and the green trace is the current flow through the inlet metering valve (command). The waveform was captured during a wide open throttle (WOT) condition.
    
This image alone tells us that the fuel inlet metering valve is a normally open valve. The engine control module (ECM) decreases the duty cycle when the required fuel pressure is increased. This allows less current to flow through the solenoid and the valve is allowed to open, which increases the fuel pressure measured at the fuel rail.

Full analysis
When the fuel pressure demand decreases, the duty cycle control from the ECM increases. This allows more current to flow through the solenoid which results in a reduction of the fuel pressure. Duty cycle is often referred to as pulse width modulation (PWM) control.

The duty cycle control on the ground side of the fuel inlet metering valve can be analysed using an oscilloscope, as seen in Figure 3. The waveform below displays the fuel rail pressure feedback voltage (yellow trace) and the fuel inlet metering valve duty cycle control from the ECM (green trace).
    
The oscilloscope is connected to the control wire for the fuel inlet metering valve. The technician must be mindful that this is the ground control circuit. System voltage on this wire indicates open circuit voltage. The diagram in Figure 4 shows the best method of connecting this set-up.
    
By careful analysis using serial (scan-tool) and parallel (oscilloscope) diagnostics you will now be in a position to identify the area of concern accurately and in a timely manner. Knowledge, together with the right equipment and experience therefore benefits technicians by leading to a reduced diagnostic time and an easier fault finding method, rendering these complex systems much less so.

TT Archives:  Top Technician issue ten 2016 | www.toptechnician.co.uk

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  • Engine management: Past and future  

    I have long accepted that nothing stands still for long in this industry. Just when you think you have a grasp of the subject something is sure to upset it. Nothing illustrates this more than powertrain diagnostics. Initially this was called fuel injection, and later became engine management. Now I’m afraid it’s even more complex.
    I find myself fortunate to have been there at the beginning; Bosch l Jetronic, a 25 pin ECU with if I recall correctly, only 13 pins occupied. No serial diagnostics, no specific tools. So why was I fortunate? Consider my reflection on diagnostics back in the late 1970s and see if they are still applicable today.

    Firstly, you had to understand what the system had to achieve, what components it had at its disposal, what role they played and how they interacted within that system.

    The next challenge was measurement values; what to expect under a variety of conditions, and what equipment was required to access this information. This all seems so straightforward now, but in those days it was a little like Columbus sailing across the ocean. He knew it was wet, he needed a boat, he knew which way west was, despite this being blasphemy in the eyes of the Pope, and so set off without a clue as to what was out there.

    Hardly a logical diagnostic process, however I was writing the rule book and did understand the meaning of the words test don’t guess. So, what’s changed that undermines these basic principles?

    Acessibility
    With even the most basic of vehicles now relying on a level of technology that makes accessibility almost impossible, OE manufacturers totally forbid any intrusion within the wiring loom and I am sure this explains the why design and manufacture precludes access as a high priority. However, we are brave, and have the Starship Enterprise at our disposal for our journey of discovery.
    The problem is one of integration. Systems don’t function in isolation any more, and Columbus now has to map the Americas and Australia at the same time. In order to conduct an accurate assessment of a function it must be in its natural environment and be observed when functioning normally.

    Complexity
    This is not restricted to a physical state. It also includes software, algorithms, and predictive response, correction or adaptive action. Systems now change their mode of operation based on environmental influences, affected by a very wide range of changing influences. Cylinder select or dynamic stability comes to mind. The driver selects an option from a long list of choices, engine, transmission, and chassis. I used to say that for a function to occur it must have a command followed by response. In today’s world,  the command may be a software decision followed by a constantly changing response, stratified and homogenous fuelling, infinitely changing camshaft timing and variable valve lift to name a few.

    Test options
    Manufacturers are driven by non-intrusive process dictated by guided diagnostics. Pre-determined test plans more often or not end with a pass or fail result, foregoing any data reveal.  Is this due to a control of process and cost, or a mistrust in their techs? Actual evaluation of circuits, voltage, current or complex profile is getting ever more difficult. Attachment of gauges in order to measure pressure and flow is often restricted by sealed transit hoses or internal ducting within castings. Serial data has become so much, more powerful and trustworthy, however it does not and will not replace the functions available from an oscilloscope. Specialist mechanical tools and assembly techniques prohibit casual examination, due to cost or the ever more common single fitment parts.

    Data extraction
    This may lie in a multitude of directions; Physical extraction, camshaft timing, fuel quantity per stroke via the serial port or fuel pressure rise time via the scope. We are forced to monitor not just a physical value, but not how the PCM is adjusting or adapting a value. How do we know the parameters of operation when VMs are removing more and more data in favour of the pass-fail flags from a software automated test profile?

    SENT
    Rieve gauche, no not a walk along the Left Bank, but a completely new protocol for data and diagnostic transmission. SENT has been developed specifically for automotive applications, rather than being a black-market hooky copy from other engineering developments. SENT stands for single edge nibble transmission, and is a uni-directional out-only data line to the PCM. SENT is essentially a serial interface, used predominantly with throttle position, air mass and temperatures. The basic unit of time is the tick, with a minimum data unit nibble. 0Data transmission speeds over fast or slow channels, where bitrate can also vary: 1xtick= 3us. In essence it is very similar to a single channel can transmission, where the function includes synchronisation, calibration, CRC and checksum.
    How am I to challenge the authenticity of data? For example, sensor error may come from power or ground discrepancies, range error, environment influences, calibration error or simply a genuine condition fault. Its design is of course intended to provide an autonomous diagnostic platform via the serial port, excluding any assessment by the techs.

    Full circle
    What does this mean for the industry? I suspect it will go full circle back to the 1970s, when part swapping was the norm for Christopher Columbus frauds.

  • Would you like to diagnose more vehicles first time? 

    As we reach March, 2019 is well and truly underway. In fact by the time you read this one third of the year will have whizzed by never to be seen again. Now, I’m not one for New Year’s Resolutions (they’re so last year), but I am the type of chap that likes constant progress when it comes to developing a technician’s career.
        
    There’s so much to be said for small steps taken everyday that on first look appear don’t appear to make a difference, but when gazed back upon over a 12 month period have a staggering affect on your capability to diagnose a vehicle first time, in a timely manner.

    Pitter-patter of tiny feet
    Small steps are all well and good but where do you start? After all, you don’t know what you don’t know, and you’d like to start your journey to diagnostic success off on the right foot. In this instance I’d start with the end in mind and reverse engineer the outcome you desire. It’s a logical process that works, and can be replicated time and time again in your diagnostic routine.
        
    Your ‘end in mind’ in this instance is a vehicle where the fault no longer exists, that won’t appear back across the threshold of your workshop anytime soon. But how do you guarantee that?

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    Bypass testing is step nine in Johnny’s diagnostic circle of love (our 15 step routine), and often the key element in the first time fix. The good news for you is that it doesn’t require mythical creatures to forge their magical powers into an object that only one technician can possess. It’s something that every tech can learn, and become a diagnostic wizard.

    What is bypass testing?
    Quite simply it’s fixing the vehicle before you fix the vehicle. Let me explain.
        
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    Picture this: Your customer has reported that the vehicle is low on power. You’ve diligently questioned them, experienced the problem with them on a road test, and the bought the vehicle into the workshop.
        
    You’ve pulled codes and found none present, followed by taking a look through serial data to hunt for diagnostic clues. It doesn’t take you long to identify that the MAF sensor frequency looks a little low at 1.5 Khz and your fuel trim data is incorrect and making a positive corrections. You’ve seen a bunch of these before and know that 1.85 Khz is a suitable value for this vehicle.
        
    You’re keen to prove that the serial data is leading you in the right direction so confirm the sensor output with your oscilloscope. The oscilloscope frequency mirrors that of the serial tool and your starting to get that warm fuzzy feeling that an you’re onto something.

    Steady the buffs
    You’ve been close to success before though, only to be thwarted in the final moments so you’re keen not to be caught out twice. You know that documenting the reasons that the MAF output could be incorrect is the way to go, and duly make a list of tests required to confirm your theories.

  • Workshop robot in development: Johnny 5 is alive?  

    Technicians could soon be able to concentrate on the more complex diagnostic-related jobs the workshop, if a prototype drone technician proves to be as effective as its developers claim.

  • Pineapple Curtains  

    Pineapple Curtains are British manufacturers and suppliers of made to measure industrial workshop curtains. The company makes workshop door curtains, workshop divider curtains, MOT bay curtains, service bay curtains, bodyshop door curtains, curtains for spray booths, auto body repair curtains, smart repair booths and much more.  They use clear and coloured fire retardant PVC to match any corporate style or branding.  Sewing is double stitched with rot proof thread for strength.  Pineapple Curtains also offer a full colour digital print service for customised curtains.
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  • And the worst MOT tester in the UK is… YOU 

    To save money and raise efficiency, the DVSA has turned to automation. They no longer need an army of Vehicle Examiners wandering from MOT bay to MOT bay. Instead they are collecting data all the time.
        
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    This is a fun story from a close and trusted friend. My guy is at a DVSA IVA check and overhears a conversation by a couple of Vehicle Examiners. It goes like this; VE no.1 is suspicious of an MOT bay offering fraudulent MOT tests. He parks down the road from an MOT bay in Kent and checks which vehicle is logged on and being tested. He takes the registration number of the vehicle in question and calls the DVLA, identifies himself and asks if the vehicle has been seen on the DVLA camera system anywhere in the last half hour. The car was last seen on the M25 twelve minutes ago near Watford in Hertfordshire over 70 miles away.
      
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