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Frank Massey looks at how to get to grips with ignition combustion diagnostics which is an increasingly complex subject

Ignition primary good earth path

Published:  22 October, 2018

Let me begin with the simplest of all overviews: There are four possible causes of combustion failure or malfunction; and please note my refusal to use the word misfire. The word is meaningless without a definitive confirmed diagnosis. The four possible causes are:

  •  Insufficient delivery of ignition energy
  •  Incorrect fuel air composition
  •  A mechanical defect
  •  A control error


I also have a very firm opinion as to the selection and reliability of diagnostic tools when looking at ignition. Serial diagnostics has improved so much that I am prepared to use it, with the qualification that it is for guidance only. For example, identifying a cylinder, the number of misfire events, ignition timing angle, coil saturation time, and in some cases, spark burn duration, ignition set back etc.

I am excluding for the moment other vital supporting data such as exhaust oxygen sensors feedback, exit temperatures, load request data, etc. what I am focusing on is it possible to accurately assess the spark energy delivery characteristics other than with an oscilloscope? The answer is no.

Critical characteristics
The reality is that given the complexity and integrated reliance on other system components and function, it is often a mandatory requirement to apply several tests to accurately confirm not just failure symptoms , but cause too. The four  critical characteristics of good ignition energy delivery are:
Burn time. Critical observations; Exposure time, slope, and spark turbulence are all vital in understanding the transition of energy across the spark delivery path. Expect smooth transition at idle, turbulence on load.

 Ignition primary current profile. Critical observations; Peak value fall time of inductance, vital when confirming if the problem of poor energy delivery is a primary or secondary issue. Expect near vertical transition with undershoot.

Coil ringing. Critical observations;  This is the electrical resonance that is characteristic when switching current within an inductor. It determines the winding insulation integrity, and predicts a breach or shunt that will reduce the spark burn time and lead to eventual ignition failure. Expect three events.

 Firing line voltage. Critical observations;  Strictly speaking this is normally viewed in primary, however can be assessed in secondary where access is not available to pin 1. The shape or ignition profile would more normally resemble a wardrobe on it side rather than stood up! It represents the circuit load from the coil tower and includes the spark plug. Expect direct ignition 30v, waisted spark 40v, rotating ignition 50v.

I am not aware of any actual measurement data existence within manufacturers TBs, or be available serially. This is something I looked at over 30 years ago and was ridiculed by many.

Evaluation  
Incorrect air fuel composition is an equally complex subject,  however  this can be in part successfully complemented with the use of serial data. Monitoring live data during a dynamic drive cycle can reveal possible symptoms, but not the cause of a problem.  Air fuel ratio, load request, fuel trim, fuel delivery status, ie homogenous or stratified, all of which should be evaluated against request, actual, and corrected data.

It doesn’t end there though. The hydraulic and mechanical dynamics of fuel delivery through the injector must be fully understood and tested. Intake air turbulence, swirl flaps, tumble and drumble characteristics, are all dramatically affected by valve and intake carbon deposits,  especially on direct injection systems that do not have the advantages of dual port injection. Mechanical defects are probably as complex to confirm as disassembly or removal is often needed to fully examine the serial or oscilloscope evidence. Many engines now require spark plugs to be precisely torqued to ensure the ignition energy is delivered to an exact position within the cylinder. How would you diagnose that?  Ignition energy across the plug would be normal, load request data would match, air fuel ratios normal.

Anomoly  
Incomplete combustion or an anomaly as I call them could be the result of mechanical or cooling defects. These can  include piston crown mapped oil cooling jets, cylinder cooling, valve seat irregularities, valve to piston timing, injector flow and atomisation issues, or ignition energy incompatibility with the internal cylinder condition.

The only useful evidence may include, cylinder misfire count, oxygen sensor irregularities, excess oxygen and increased HCs in the exhaust stream. Any of you guys out there remember the good old days when 4-gas exhaust analysis was the craft of every professional engine tuner?

So here we are around 800 words to just touch on an incredibly complex subject. In the past the cure would have been relatively straightforward. Today it will destroy the engine, with prestige manufacturers like Porsche currently suffering multiple major engine failures. Techs need to think smarter.

Ignition primary poor earth path

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    Have you ever wondered why it is that some technicians have an aptitude for complex diagnosis? You know the type of tech I mean. They take the seemingly unfixable, dive headlong into diagnostic battle and emerge triumphant time and time again.
        
    Not only that, but they’ll often do so in a time that makes other techs look on in awe! What’s their secret? And more importantly, can you emulate their success? Well, I’ve got some great news for you. You can, and knowing what to do is easy.  All that’s required is that you look to the past. History is a great teacher.
        
    I turned 50 this year, and one of the few benefits of increasing age is the ability to spot patterns, and patterns of actions that when followed culminate in your success. Patterns for success surround us, but sometimes you can be a little too caught up in the urgency of the now to spot them.
        
    I’ll show you the patterns great technicians use to triumph in the world of technical diagnosis, and how you can do the same. It’ll be your blueprint for success.
        
    You’ll like the blueprint. You’ll appreciate its simplicity, recognise the logic, and in all probability nod along as you read, agreeing with the steps that need to be followed.
        
    Here’s the deal though: You’ll need to implement it. Knowing the blueprint is easy, but knowing what to do doesn’t get the job done. It’s all in the implementation, and that starts with you taking small steps to achieve positive changes each day. Don’t forget one of my favourite sayings: “Progress NOT perfection.”
        
    I’m as much a fan of the latest technical gadget as the next man. I also love “cool” test techniques, but I’ve noticed that myopic focus on these can often be to the detriment of the long-term technical success of a technician. I’m not saying that you shouldn’t explore “shiny” elements in our craft, but you’ll find huge benefits in building a solid foundation that can be executed on every diagnosis. What do you need to “do well” then? Just these five steps.

    Step one – Systemise to win
    There’s always a right and not so right way to attack any given fault. One fundamental element is to have a defined system that all technicians use.  Without a rigorous system to follow, your diagnosis could be doomed before you start. Here’s an outline of our diagnostic system that just works;

    1 – Thorough questioning of customer, establish change point
    2 – Confirm and experience fault with customer
    3 – Visual inspection for obvious issues
    4 – Retreive fault codes, and gather data on what’s required to raise them
    5 – Inspect serial data. Note what looks wrong
    6 – Research technical bulletins and any technical information required for accurate testing.
    7 – Document what’s wrong and possible causes
    8 – Form plan and prioritise relevant tests
    9 – Carry out tests and draw conclusions
    10 – Bypass test to prove the conclusion where applicable
    11 – Repair as required.
    12 – Carry out postfix operations i.e. component coding.
    13 – Carry out tests to confirm repair

    Use our process and you’ll definitely be putting your best foot forward.

    Step two – Sound electrical knowledge
    Now you know what a great process looks like the next part of your blueprint is your understanding of automotive electrics. How quickly you can decide what to test, what tool to use, and what the answer should be is an essential skill that pays huge dividends once learnt. Key elements include:

    1 – Becoming comfortable with relationship between volts, amps and ohms
    2 – Using voltage drop to accurately find circuit faults
    3 – Series and parallel circuit diagnosis
    4 – Interpretation and use of wiring diagrams
    5 – Fundamental mechantronics test knowledge

    Armed with these, you’ll be able to find wiring faults, diagnose sensor and actuator circuits as well as build entry-level bypass tests to confirm your theories. These are skills you’ll use on the majority of diagnostic repairs. Learn these and you’ll reap the rewards for your entire career.

    Step three – Oscilloscopes; One tool to rule them all
    A little dramatic I know, but understanding how to use an oscilloscope competently is a game changer. It will bring to life all that has been learned in Step two (auto electrics), and when used skilfully will display this in a way that can confirm or deny faults in vehicle circuits, sensors and actuators.
        
    As an example, take just one quick connection (less than a minute on most petrol cars) to the switched side of a manifold injector and you’ll know;

    1 – That power supply to the injector is not open circuit
    2 – The ECU has control of the injector and is commanding fuel delivery
    3 – Time taken for fuel delivery to commence (injector opening)
    4 – Integrity of injector ground circuit
    5 – Time takes for fuel delivery to cease (injector closing)

    Add some additional test points for injector power supply, current and rail pressure (another couple of minutes) and you’ll confirm the integrity of the positive supply to the injector, the injector winding, and a great test for a quick look to ensure the injector is delivering fuel once open. Like I said - It really is one tool to rule them all!

    Step four - Generic systems knowledge
    With steps one through three in place you’ll now have the foundation knowledge to explore vehicle systems. This can be a little intimidating as there are so many systems and so much to see, which is why we advise attacking this in bite-size chunks. Your goal here is to become familiar with generic items that broadly apply to a wide cross-section of vehicles. While there’s no substitute for formal training, taking a few minutes on a regular basis to self teach is invaluable. Here’s some things for you to try:

    1 – Pick one system to start with. E.g. petrol engine management
    2 – Select a book or watch a video for some foundation learning
    3 – Focus on one part of a system. E.g. Loads sensors
    4 – Inspect serial data for MAF and MAP sensors across various load and speed ranges
    5 – Scope MAF and MAP sensors across load and speed ranges
    6 – Record your results and repeat on different vehicles on the same components
    7 – Repeat points one through six on different components

    Do this on a range of vehicles and systems and you’ll become incredibly familiar with what good looks like, as well as raising many questions that we’ll answer when you attend our training.

    Step five – Manufacturer information and tooling
    There’s one final piece to this part of the puzzle and that’s using the using the best information and serial tools.
        
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    1 – Select one manufacturer initially
    2 – Become intimately familiar with their information system
    3 – Learn to use their wiring diagrams
    4 – Explore their technical service bulletins
    5 – Use their repair procedures
    6 – Substitute a generic serial tool for the O.E. tool for a month
    7 – Explore all the serial tool has to offer

    We’ve been training technicians like you to use this equipment for many years. It’s had too much of an impact for those that have grasped the nettle for you not to give it a go.
        
    You now know what it takes to begin the road to technical success. All you need to do is start. Taking regular steps, and before you know it you’ll have not only reduced your stress but your time to a first time fix as well.


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