September 2021

BY Frank Massey I thought it may be timely to revisit the subject of ignition diagnostics. It is an old chestnut I know, but I suspect it is a subject that does not receive the respect it demands. I’m mindful of the lucrative days we referred to as vehicle tuning. I was from the Crypton stable, having grown up through the 332, 440 and disastrous Cudos days. So why do I owe so much to this now archaic period? It was the birthplace of diagnostics of course, with the introduction of oscilloscopes. I must correct something here; All so-called ‘automotive scopes’ of this period, and for several years afterwards were appalling. As often happens a Phoenix rises from the ashes so to speak, as it prompted me to adopt the use of pure lab scopes for vehicle diagnosis. Ignition being the prime focus as electronic fuelling control and engine management were in their infancy, anyone remember 35 pin series 1 motronic or 25 pins. L Jetronic, (LUFT) not forgetting D(DRUCK) Jetronic. Before we get technical let us take a step back and reflect on just how much ignition systems have evolved, and there is my first error statement. Did you miss it? ignition is no longer a stand-alone system and must be taken in context with engine mechanical function, fuelling delivery, software control, environmental influences, driver demand and the stuff most drivers do not understand or respect fuel quality and servicing. Essentials The essential elements of ignition can be divided into two simple components: #1 available electrical energy; #2 delivery mechanism. I am sure most of us consider ignition energy as voltage (high tension), and possibly spark duration measured in milli-seconds. For all you scopies out there, you will have noticed spark duration periods from around 1.8-3m/s. does not however confuse ignition energy as proportional to burn time. The unit for electrical energy is the joule, flow is current and pressure or potential difference is the volt. The next important consideration is understanding the basic scope image so we can attribute a specific fault to a precise component or function. Please refer to fig.1. This can be divided into four key elements, expressed in order of priority. 1: Spark burn time or duration , measured from the point of induction to the first oscillation or coil ringing event. We will discuss ‘slope’ later. Ignition energy, and in cylinder environment (load, fuelling and turbulence) can be assessed from the burn time profile. 28 AFTERMARKET SEPTEMBER 2021 TECHNICAL www.aftermarketonline.net IGNITION DIAGNOSTICS Frank is going back to his diagnostic roots with a two-part look at potential causes for ignition issues 2: Primary current ramping , measured from coil on (start of dwell or saturation time) to the point of induction. Both ramp profile or angle and peak value are important. The condition of the primary windings can be accurately assessed from the current path profile. 3: Firing line voltage , measured vertically from the scope base line to the point of spark burn. Better viewed from the primary image where possible, it represents the electrical resistance or delivery load in the delivery mechanism including the spark plug. Not the in-cylinder load as this is represented by slope. 4: Secondary coil ringing , the number of coil oscillations following the end of the burn time, representing the dissipation of residual energy in the coil secondary windings. There are only two reasons for an error here; A shunt within the coil secondary winding; Or a leaky power switch at the point of induction. I will discuss this later. Accurate determination From these four key elements of the ignition profile, we can very accurately determine an ignition problem or a potential fuelling error, something I would rather call a combustion anomaly. This expression allows for a more intuitive thought process. Above: Fig.1. Point of induction ref 0.011. Burn time from ref 0.011 to first ringing event. Current peak value 7 amps. Firing line value 30 volts. Coil rings 4 events PART ONE