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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.

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.

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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  • 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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