March 2020

BY Neil Currie I f I had to give one word of advice for aspiring young technicians or maybe an experienced technician looking to improve their electrical and diagnostics skill set, then it would have to be to understand what you are testing. By that I mean how it works and what you should be seeing when testing and using the correct tool to test it with. This could mean attending a training course, reading some technical information from the manufacturer or even just recording known good data and/or scope waveforms. If you can do this you are on a road to success. Not wanting to badmouth technicians or garages around the UK, but it is always disheartening to read or hear about a customer’s bad experience where they have spent hundreds sometimes thousands of pounds in an attempt to fix a car and still have the original issue and a lighter wallet in the process. In this article I am going to go over a job I had visit the workshop recently and demonstrate how understanding system operation and how to test it correctly found the root cause quickly and accurately. All-time low The vehicle was a 2009 Renault Trafic 1.9 Diesel van. The customer said there was distinct lack of power and it had been that way for some time. He had visited his local garage on multiple occasions, and had an array of new parts fitted every time, but they had still failed to fix the problem. His funds to repair the vehicle were now at an all-time low along with his confidence in garages. He explained the garage had reported that the diagnostic trouble code the van continued to log was for ‘turbo under- boost’. Despite having had a new turbocharger fitted, along with a new boost pressure sensor, boost pressure solenoid, vacuum pipework, mass air flow sensor and EGR valve, the problem still remained. Now, as we can all appreciate, those parts all add up to a large sum of money being spent, and it is clear to see the garage didn’t understand how the system worked. It is hard to imagine all of those parts failing at once. We questioned the customer some more and explained how we operate when looking at this type of fault. We then agreed to spend an hour on the vehicle to see if a root cause could be identified, with a view to giving him an accurate quotation for what was needed to fix his poorly van. Live data The symptom was confirmed during a road test while monitoring live data. This showed little to no boost pressure from the turbo. Having confirmed the customer complaint, it was now time to devise a plan to test the system and find out why it was not operating correctly. First, we need to understand how the system works. Without getting highly technical, this particular vehicle uses a vacuum diaphragm actuator operated variable nozzle turbine (VNT) turbocharger. A boost control solenoid is supplied vacuum from a vacuum pump which also supplies the brake servo. The solenoid opens and closes to vary the amount of vacuum supplied to the actuator unit which in turn moves the rod from actuator to the internal arm connected to the vanes thus changing the pitch of the 30 AFTERMARKET MARCH 2020 TECHNICAL www.aftermarketonline.net TEST! DON’T JUST GUESS! Looking back at his Top Technician victory last year and the journey to reach it, Neil points out the importance of process, with a real-life example Fig. 1