September 2021

I have written a number of articles for the magazine now, and while looking through pictures on my phone and saved scope traces from the past for a possible new article, I found an interesting job I had a while back and thought it would make an interesting case study. As before, it shows the importance of having a test plan. Initial complaint The vehicle was a 2004 Renault Master van and the customer’s initial complaint when phoning the garage to book the vehicle in was that it was a crank no start. They had used the vehicle the day before and all had been fine, parked up for the evening and in the morning when they went to use the van it would not start. After further questioning the customer confirmed no recent work had been carried out either, so it was booked in and would be recovered to the workshop for further investigation. When the van arrived, I had a look and carried out my normal routine to establish what was going on. Firstly, we needed to replicate the customer’s concern, so the engine was cranked and would just spin over but not fire, all dash warning lights were operating as expected and we had an engine management light (EML) illuminated. Why does this matter? Well, over the years I have learned that a good quick tell-tale sign that the engine ECU is awake and talking is that the EML is illuminated. My next step was to plug in a diagnostic scan tool and look at fault codes and some live data to gain some direction to form a plan. As we can see in Fig.1, I had one fault code stored for camshaft/crankshaft coherence which reading further into the description could possibly have been be due to no signal or incorrect signal from the camshaft sensor, an engine speed inconsistency or the two sensors are out of synchronisation. Important to understand Why does synchronisation matter? Well as my previous articles have explained, it is important to understand what you are testing and how it works so for those who do not know, As the engine is timed via a belt or chain, the control unit looks for signals from both the crankshaft and camshaft sensors which should align correctly. This is done with a point in each signal denoting a reference point for the ECU to know where each measured shaft is position wise in the engine cycle. Depending on vehicle manufacturer and the engine management system fitted to the vehicle, the crankshaft signal can be anywhere from approximately 120° on older vehicles to 50° before TDC on modern systems. This is down to the processing time required within the engine ECU, it takes time, albeit very little, when you imagine how 34 AFTERMARKET SEPTEMBER 2021 TECHNICAL www.aftermarketonline.net RENEGADE MASTER 2019 Top Technician winner Neil Currie’s mantra has always been ‘plan, test, and test again’, an approach which proved its worth once more with a recent van job fast the engine is spinning for the control unit to be told where the engine is position wise and then act on that information quick enough to command the injectors to fire at the correct time depending on engine load etc. This is something I recommend remembering, as many technicians think this reference point is actually TDC. If it was, the ECU would not have the required time to process the data and the engine would not run. The camshaft signal uses a square wave signal. The control unit looks for a rise or fall in the signal to denote cylinder synchronisation, and which cylinder requires fuel for the injector on that cylinder to fire. If this signal is missing, the ECU can struggle to determine which injector to fire. Variable Here is one variable of which you should be aware. On more modern vehicles, if the camshaft sensor fails, the control unit can determine where the camshaft is from the crankshaft signal, and the engine will start. However, an extended cranking symptom will be apparent as the control unit decides where everything is and then commences injection. The smaller angle closer to TDC on the crankshaft signal on newer vehicles is the result of technological advancement. With emission levels being strictly monitored, the ECU needs to know information as quickly and closely as possible as to what is happening inside the engine. Doing so, it can be much more accurate, thereby maintaining smooth running to prevent excessive emissions.