To Scope or Not to Scope? That is the question

John Batten takes a look at whether an oscilloscope is a necessity for efficient diagnosis or just a nice to have

Published:  13 May, 2019

If you’ve read my technical articles previously then you’ll know that the endgame for our technical training is straightforward. Quite simply our goal is to develop technicians so that they use a repeatable process, carry out root-cause analysis, diagnose the vehicle first time in a timely manner, and ensure that it does not return for the same fault.
    
Tick the box on those five points more often than not and you’ll have a happy technician, a happy boss, and a satisfied customer. For this to be a regular occurrence though the right elements need to be in place.

Essential components?
So what’s required? Obviously a skilled technician, and the right information are essential ingredients, but what about tooling? Can you get by with a scan tool, multimeter, and a copy of Autodata (other technical references are available)? Or is an oscilloscope an essential tool? In this article we’ll take a look how to diagnose a misfire, and whether a scope plays a pivotal part or not.

Line up your ducks
The offending vehicle in this instance is a 4 cylinder 1.8 petrol Vauxhall Insignia, although this procedure could apply to any similar petrol vehicle. To say it’s sick would be an understatement. It’s only running on three cylinders, and quite honestly sounds a little sorry for itself. A couple of questions spring immediately to mind. Which cylinder is it? And what’s the overarching cause? Normally a problem like this will be attributed to a mechanical issue, fuelling issue, or ignition related fault. Our purpose at the outset is to quickly identify which of those areas deserves our attention, and to do that we need to carry out some initial high-level tests.
    
Before we get into what’s causing the problem I like to identify which cylinder is causing the issue. Once I’ve identified that I’ll then drill down to find out why.
    
You’ve quite a few options on how to achieve this, although my favourite wherever possible is to carry out a cylinder balance test. This is done using a serial tool to deactivate an injector whilst idling and monitor the RPM drop. If there’s no change in rpm for a given cylinder then you’ve found your culprit. On this vehicle, it was identified that cylinder 4 was having little input, and that’s where our focus should be.
    
Now we know the offending cylinder you’ve three areas to test. On a personal level, I’ll choose a quick mechanical integrity test but the question is: “What’s the quickest way to achieve this? Understanding what cranking speed sounds like on a good car is a benefit, and I’ll normally use a scope to support this with a relative compression test. Using a current clamp (figure 1) to identify a poorly sealing cylinder is a quick test that can give immediate diagnostic direction, but in this case we can see that current draw is equal across all cylinders, and as cranking sounded normal I decided that my time would be better spent looking elsewhere.

Next steps
With a quick mechanical integrity check undertaken my gaze turned to ignition. Ignition related misfires are commonplace and there are a number of ways to complete this part of the diagnosis. I could dive in with a scope although I’ll normally look at spark performance with a gap check first, and drill down a little deeper with an oscilloscope if it fails that test.
    
Figure 2 shows the tool typically used for such a test. The secondary ignition output from all coils was good and equal across all cylinders. If this had not been the case then a scope would have been used to identify why, but in this instance a quick output test showed that all was well and the scope would not be required.
    
With our previous tests all but eliminating ignition and mechanical faults, it was time to take a look at fuelling faults. The problem on this particular vehicle meant that the cause would be isolated to one cylinder, this made the probability that it’d be a fuel supply issue to the rail less likely. With this in mind it makes sense to use a scope and carry out comparative checks on individual cylinders looking for anomalies that could be caused by a fuelling fault. Access to primary and secondary ignition was less than ideal due to the coil pack configuration so the ignition profile could not be used for fuelling evaluation.
    
Injector supply, ECU control and circuit current were inspected across all cylinders and while there were small differences nothing was conclusive, until we took a look at rail pressure using the Pico WPS500x pressure transducer. Using this it was plain to see that upon injector number 4 being commanded to open and deliver fuel that there was little drop in rail pressure compared to the other cylinders. This definitely warranted further inspection so the injectors were removed and  a flow test was completed in our test bench. Number 4 injector was found to be delivering significantly less fuel than expected. Bingo, we’d found our misfire.

To scope or not to scope?
Effective and efficient diagnosis is all about using the right tool, for the right test, at the correct point in your diagnostic routine, and as this vehicle has shown the oscilloscope plays a critical part in serving up the answers that whilst possible via other methods are often more time consuming to obtain.
    
If an oscilloscope isn’t playing a major part in your day to day diagnosis then there’s no time like the present to blow the dust from it and start seeing the benefits that this amazing tool will bring to your workshop.


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    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?
        
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    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
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    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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