Challenging current techniques

By Frank Massey | Published:  18 July, 2017

Frank Massey looks at how you need to always keep an open mind on diagnostic methods

Accepting my reservations, I always believe in challenging any current techniques with an open mind with the possibility that a better method may exist. My reservations are based on the following reasons.

Reservations

Electronic measurements are based on sensor signatures compared with known confirmed values. Many engines now employ variable valve lift and timing. Added to this complexity, often there is no mechanical locking of sprockets to shafts. Confirming actual mechanical camshaft position regarding the crankshaft would require sensors monitoring all camshafts and all sprockets.

With exception of highly advanced engines this is not the case. So how can we predict actual valve functionality before opening the spanner drawer?

Functionality

Recently we filmed a detailed diagnostic process with Pico exploring a reliable method. Determining electronic plausibility is relativity easy. The answer to actual valve operation is reliant on accurate pressure evaluation in the cylinder. This must be overlaid in real time with the timing sensor signatures. There is still an element of error as you will need to calculate camshaft rotational angle against the crankshaft profile. The Pico software helps by converting the oscilloscope base line into the 4-stroke rotational position. So far so good, but can poor mechanical condition be predicted more easily? Well yes it can, remember the vacuum gauge. Volumetric efficiency is directly proportional to correct valve position.

Hang on a moment; Where should the valve position be? It’s not simply a function of a chain or timing belt. The computer now has an opinion on optimum timing angle. It’s not that easy anymore. However, I do support the premise of predictive evidence.

Boxter

Let’s visit an actual event in our workshop from very recently; a Porsche Boxter 3.4 supported with Bosch Motronic med control. With no obvious cause or warning it developed a drop in power and throttle response. No definitive DTC. You didn’t think I was going to make it that easy for did you?

Serial data confirmed two key symptoms. Misfire count and rich fuel trim more than 20% on one bank only. It is a low mileage car with no obvious signs of poor care. Removing the spark plugs confirmed excessive soot as the symptoms of incomplete combustion. Note my words carefully. Further examination excluded the unlikely cause as poor ignition energy. Back to basics, the vacuum gauge confirmed too high pressure in the intake system specific to the offending bank. The next action was to attach the pressure transducer to each bank in turn comparing the pressure profiles.

Direct measurement

To achieve this, direct measurement from each bank is required. Running the engine with the pressure sensor directly in one cylinder from each bank confirmed a differential in pressure between banks. It must be a mechanical problem then. The next decision is more intuitive than evidence driven as this engine has variable valve timing adjustment via electro-hydraulic control.

Serial data could not provide actual camshaft position data though, as the sensor is only motoring the sprocket position. This is the very point I made earlier. We could get clever and monitor control duty cycle and current through each actuator. Unfortunately, the computer does not monitor a problem, so there will not be any correction made.

Complete restoration

Examination of the service history confirmed long life servicing. This prompted us to flush the engine and replace both hydraulic camshaft control actuators. This resulted in a complete restoration of performance; obviously, we reset the adaption values to fuel trim, throttle angle and misfire count.

As is often the case, a combination of hard evidence and common sense prevailed. I often say to entry level diagnostic students, the most complex problem in diagnostics is you.

Further information
Please contact Annette on:
01772 201 597 or email enquiries@ads-global.co.uk for further information on upcoming training courses and events.

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  • Glowing, going, gone! 

    I decided to share this case study for my first article because what I expected to be a simple job turned into something a little more complex and gave me an opportunity to study a and learn about a system that until now I’d probably taken for granted.

    We were presented with a 2010 Skoda Fabia 1.6 TDi by a car dealer who had recently taken it in part exchange. The engine management was light illuminated, however with no other symptoms. The previous owner told the dealer that the MIL had been on for around a year and her local garage had failed to repair it. It had also recently been recalled for the ‘Dieselgate’ VAG emission software update. The dealer told the customer there were DTCs stored for the glow plugs and that they needed replacing to which she declined as she was sure they had previously been replaced. We already had a reasonable amount of vehicle history to start with, and were ready to take a look.

    Voltage and current
    A code read revealed DTCs for all four glow plugs being open circuit and a glow plug module communication fault. A quick inspection of the engine revealed that the glow plugs were not that old and also there was a new glow plug module fitted, plus an old one found in the boot.

    While checking the resistance of the glow plugs may tell us something, measuring the voltage and current with an amps clamp paints a much clearer picture. The oscilloscope was connected and the ignition was cycled. The screen capture revealed a healthy 12 volts for around 10 seconds then pulsed at random, however there was zero amps flowing (on all glow plugs). It was clear the plugs had gone open circuit for some reason so they were removed for inspection. It was then we noticed that the heater plugs fitted were rated at 4.4 volts, so now we know why they burnt out! Could they be the wrong glow plugs? Could it be the wrong control module? We checked and found the part numbers were correct.

    At this point it was crucial that we understood exactly how the system is wired and how it should operate. By studying a wiring diagram we were able to plan how we were going to test the system (see image 1). Starting with the power supplies and ground, it is always best to test a circuit in its normal environment which means we really need the current load of working heater plugs. If we were to fit new heater plugs at this point there was a high risk of them being damaged which is expensive so we substituted four headlamp bulbs instead. The fuse rating for the circuit was 50A so with a quick bit of maths we calculated the current required for four bulbs was safe. The main live feed, ground and ignition switched live were all good so we moved on to the two communication wires that link directly to the PCM.  

    If the PCM can log individual codes for each glow plug then we know that it must have a two-way communication system. Scoping both wires with the module connected and disconnected showed us that there was clearly a command signal from the PCM and although it was random and rather messy (see image 2), the glow module responded directly by activating the glow plugs at the same rhythm.

    The second wire had totally different digital signal which had to be the feedback to the PCM. The noise and irregularity of the command signal was clearly an issue so we checked the wiring back to the PCM and with the aid of the good old-fashioned wriggle test the fault was identified as a poor connection in the PCM harness connector. The connection was cleaned and the system retested which revealed a much healthier scope pattern and the communication DTC was cleared (see image 3).

    Reliable repair
    At this point we could have fitted new glow plugs but to save unnecessary expense we wanted to make sure it was a reliable repair so we decided to monitor the system with the faulty glow plugs still installed and the leads connected to the bulbs. We started by monitoring all four glow plug voltages on the oscilloscope. Using the scan tool to activate the glow plugs showed us that the 4.4 volts is achieved by pulse width modulation at a duty cycle of around 13% with a frequency of around three times per second. What was more interesting was that all four plugs were individually triggered in a sequence (see image 4) so there is never more than one glow plug energised at any one time. The logic behind this is that it makes a substantial reduction in power consumption.

    Our next test was to observe the control strategy of the PCM from a cold start and warm-up phase. The objective here was to ensure that there was no software related issues. From the point of key on there is a 1.5 second supply phase to heat the plug as fast as possible then temperature is maintained by the 13% duty control.

    Decade box
    Of course, after a period of time, once the engine starts to warm up the system turns off and the communication wires go quiet. If you want to test it more than once then you’d have to wait for the engine to cool so to save time we connected a decade box in place of the engine coolant temperature sensor and by observing the coolant temperature in serial data on the scan tool we were able to select a variety of resistances that would represent low temperatures and fool the PCM into commanding glow plug activation.

    The decade box has proved to be an extremely useful tool really is a must in any diagnostic technician’s tool box. It is great for substituting in place of certain sensors and components to check the integrity of a circuit or to observe an ECU responding to a variation in signal (resistance).

    The final test was an observation of voltage over current on one glow plug. The other interesting thing we noticed was the simplicity of the digital feedback signal. By unplugging each glow in turn you could see the pattern in the signal change and when all were connected and working it was a regular pattern.

    Summing up
    Clearly more time was spent on this job than necessary and the labour charge remained fair. In a busy workshop it is hard to find spare time for these situations but my point is that sometimes sacrificing a lunch hour or staying behind for half an hour gives an opportunity to learn so much which can only aid you in speeding up diagnostic time and process on future jobs.

    Winning the Top Technician 2017 competition was unexpected. It has not only introduced me to some very inspiring, like-minded people, but has also taught me you can never have too much training, whether it’s self-training like in this instance or on a professional training course. There are some fantastic training companies offering a variety of courses available now. Also, some of the best and most respected all regularly write for Aftermarket!  





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    ABS, dynamic stability, hill start, audio volume, navigation, self park, all wheel drive, active steering assist, electronic handbrake etc. Sharing this data on a high speed can network ensures very accurate vehicle motion dynamics.

    Older variable reluctance sensors (VRS) rely on a coil generating an alternating voltage when rotation occurs. The problem is they are not directional sensitive and cannot report motion at very low speed. Air gaps were critical as they affect signal amplitude. They are often referred to as passive sensors. So, the introduction of digital or active sensors was inevitable.


    Principles
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    Subtle differences
    There are two very subtle differences in the digital outputs. They can be called pull up or pull down. The sensor supply voltage will be slightly lower than battery voltage this is due to the different internal resistance values. However, it will be around 10.5/11.5v.

    The ground or return signal value will vary between 0v or 1.4/1.8v. You could have a sensor or circuit fault; let me try and explain the subtle differences, and how to prove which is which. Remember the golden rule if in doubt compare a wheel circuit that works normally.

    First unplug the sensor and measure both circuits in the loom. With no load applied the supply voltage should jump up to NBV

    Next check the ground circuit if its true ground then it’s a pull-down type and the signal will be on the power line, and may only be around 200mv

    If a small voltage exists then it’s a pull up type and the signal will be on this wire not the supply. The digital signal will be very small when the wheel rotates. It could be small around 200/400mv, or as high as 0.5/1.8v, depending on the manufacturer variant

    Common sense would dictate the serial route is easiest, however how would you determine an intermittent fault? It could be a faulty sensor, faulty encoder, or a circuit error. The only way is using a scope. Should we measure voltage or current though? Both change in the circuit. Unless you have a very special current clamp, go for voltage and select a AC coupling.

    The specific question I am often asked is current measurement, well I can tell you in a pull-down circuit its around 7-15 ma with a 400mv voltage change. The pull up type will produce around 6/13ma with 0.2/0.35mv.     However, these voltage values can vary due to the value of the two parallel internal sensor resistors these are normally 1.4k ohms, with a much higher resistor in the meg ohm range, within the ABS pcm.

    I hope this helps. The pico image was taken from a VW Golf 1.4 TSI. The easy bit is replacing the wheel sensors. Ever since metal housings were replaced with plastic they never corrode in the housings
    do they…?

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  • The good and the great 

    Being part of Top Technician for the last few years, I have seen many technicians succeed and develop new skills. Typically all are good rounded technicians and have great knowledge, but what makes the difference and makes the good into the great?
        
    It’s not just that they are lucky. The difference is that a great diagnostic technician will have a well-defined diagnostic process (or procedure) that they stick to every time.

    Process
    Some technicians start their diagnostic procedure with a well laid-out and defined process that they have normally learnt, often from training courses. As with any new process, it starts slowly as theory is put into practice until it becomes natural.
        
    Many technicians typically revert ‘back to type’ during the early stages, as their older method seems to make the diagnostic process shorter. As a result they believe it could make them more money. Yes, in the short term they may be right. However, normally in the longer term a well-defined diagnostic process proves to be infallible especially when the fault is difficult to diagnose or a vehicle that has been to several garages and the fault is still apparent.
        
    Many technicians also try to shortcut the process, taking out some of the steps that don’t seem to help in finding the answer. Sometimes a simple fault is made more complex by the technician overlooking the obvious in the second or third step, jumping from step one to step four because that’s where they feel comfortable. In this series of articles I’ll be covering the 10 steps that make up a well-planned, well organised, tried and tested diagnostic process. Use the process and refine it within your business, it works.
        
    Many businesses use a similar structured process and base their estimating/costing model on it
    as well.

    Meaning
    Let’s start at the beginning, with the meaning of diagnosis. Most technicians will look at the word and think it only relates to a computer controlled system and they have to use a fault code/scan tool to be able to diagnose a fault. This is not the case. Diagnosis can relate to any fault, whether that is electrical or mechanical. Therefore, the diagnosis can relate to an electronic fault by the malfunction indicator lamp (MIL) indicating a fault exists or a mechanical fault that exists within a clutch operating system.
        
    The meaning of diagnosis is: ‘The identification of a fault by the examination of symptoms and signs and by other investigations to enable a conclusion to be reached.’
        
    Or alternatively: ‘Through the analysis of facts of the fault, to gain an understanding which leads to
    a conclusion.’
        
    Both can relate to various professions.
        
    With this in mind, what have celebrity chef Paul Hollywood, your doctor, the green keeper at the local golf course and a automotive technician all  got in common?
        
    They all use a diagnostic process within their profession. Paul Hollywood is often seen as a judge within baking competitions. He uses his experience and process to perform a diagnosis on why a bread is not cooked correctly.
        
    Meanwhile, a doctor uses a diagnostic process to find an illness. A green keeper uses a diagnostic process to determine why the grass does not grow as green as it should, while a automotive technician performs a diagnostic process to find the fault on a vehicle.

    Let’s begin to go through the steps of the diagnostic process.

    Step 1: Customer questioning

    Being able to question the driver of the vehicle of the fault is always a very important part of the diagnostic process. Using a well-structured and documented series of questions can determine how the fault should be approached. Many experienced technicians do this part very well, but when a business becomes bigger, the customer’s information on a fault can get lost  when passed between the receptionist and the workshop.
        
    A document can be developed to perform this task, and is often the solution here.
        
    A customer has often seen a ‘warning lamp’ on the dash. They can only remember that it was an amber colour and it looked like a steering wheel. The document shown has a variety of warning light symbols that they can simply highlight to let the technician know of the MIL symbol and in the circumstances that the fault occurs (driving uphill around a right-hand bend etc).
        
    Much of the diagnostic process is about building a picture before the vehicle is worked on. Trying to fix the fault by jumping to step 4 or step 5 can often neglect what the customer has to say. One of the last steps in the diagnostic process is to confirm that the fault has been correctly repaired and will not occur again (‘first time fix’). How can the fix be successfully tested if the circumstances where  the fault occurred are not replicated during the final stages of the process?
        
    The MIL illuminating again (recurring fault) when the vehicle is driven by the customer is not always as easy to fix a second time, as you need to fix the vehicle fault as well as fix the customer, who has been forced to return.

    Step 2: Confirm the fault
    Some technicians just seem to take the fault highlighted as by the job card (or similar document) and diagnose the fault without first confirming, which can take some time to complete. This step might involve a road test to confirm that the fault exists. The apparent fault may be just a characteristic of the vehicle or the receptionist/customer may have explained the fault to be on the other side of the vehicle.
        
    Therefore, it is imperative that the technician confirms that the fault exists and the situation that the
    fault exists within, all providing additional information on building
    the picture before actually working
    on the vehicle.

    Step 3: Know the system and its function
    In order to fix a vehicle fault(s) a technician will first need to understand how the system works. If a technician doesn’t know how the system works how can they fix it?
        
    Don’t be shy or foolish and indicate that a technician knows everything (even on a specific manufacturer brand). Every technician sometimes needs to either carry out new system training or just have a reminder on how a system works.  
        
    With all the systems now fitted to a vehicle, it’s not surprising that a technician cannot remember every system and its function especially to a specific vehicle manufacturer or the model within the range. A technician may just need to remind themselves on the system operation or fully research the vehicle system.
        
    Most vehicle manufacturers will provide information on how a particular system works and how that system integrates (if applicable) with other systems of the vehicle. Spending some time researching the system can pay dividends in terms of time spent diagnosing the system and it is also educational. System functionality can often be learnt from attending training courses but if these are not available the information can be sourced from various other sources such as websites.
        
    External training courses can provide additional benefits especially discovering how a system operates and understanding its functionality and how the various components work. They will also allow the technician to focus on the specific system without the distraction of customers or colleagues.
        
    Once the system is thoroughly understood, the technician may be able to make some judgements as which components are ok and those which may be faulty and affect the system operation.

    Refine
    Just to recap on the three diagnosis steps covered in this article, these were:
    Step 1: Customer questioning
    Step 2: Confirm the fault
    Step 3: Know the system and its function

    Remember to follow the process and don’t try to short circuit it. Some steps my take longer to accomplish than others and some may be outside of your control (it may be necessary to educate others). Practice, practice, practice. Refine the process to fit in with your business and its practices, align your estimating/cost model to the process to be able to charge effectively.

    Next steps
    In the next article I will be looking at the next four steps which are seen to be:
    Step 4: Gather evidence    
    Step 5: Analyse the evidence
    Step 6: Plan the test routine
    Step 7: System testing

    The last article in this series will indicate the final three steps and how to fit them all together in order to become a great technician and perhaps win Top Technician or Top Garage in 2018. Go to www.toptechnicianonline.co.uk to enter this year’s competition. The first round is open until the end of February 2018.
        
    Every entry is anonymous so have a go!


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