April 2019

BY John Batten I f you’ve read my technical articles previously then you’ll know that the endgame for our technical training is straightforward. Quite simply our goal is to develop technicians so that they use a repeatable process, carry out root-cause analysis, diagnose the vehicle first time in a timely manner, and ensure that it does not return for the same fault. Tick the box on those five points more often than not and you’ll have a happy technician, a happy boss, and a satisfied customer. For this to be a regular occurrence though the right elements need to be in place. Essential components? So what’s required? Obviously a skilled technician, and the right information are essential ingredients, but what about tooling? Can you get by with a scan tool, multimeter, and a copy of Autodata (other technical references are available)? Or is an oscilloscope an essential tool? In this article we’ll take a look how to diagnose a misfire, and whether a scope plays a pivotal part or not. Line up your ducks The offending vehicle in this instance is a 4 cylinder 1.8 petrol Vauxhall Insignia, although this procedure could apply to any similar petrol vehicle. To say it’s sick would be an understatement. It’s only running on three cylinders, and quite honestly sounds a little sorry for itself. A couple of questions spring immediately to mind. Which cylinder is it? And what’s the overarching cause? Normally a problem like this will be attributed to a mechanical issue, fuelling issue, or ignition related fault. Our purpose at the outset is to quickly identify which of those areas deserves our attention, and to do that we need to carry out some initial high-level tests. Before we get into what’s causing the problem I like to identify which cylinder is causing the issue. Once I’ve identified that I’ll then drill down to find out why. You’ve quite a few options on how to achieve this, although my favourite wherever possible is to carry out a cylinder balance test. This is done using a serial tool to deactivate an injector whilst idling and monitor the RPM drop. If there’s no change in rpm for a given cylinder then you’ve found your culprit. On this vehicle, it was identified that cylinder 4 was having little input, and that’s where our focus should be. Now we know the offending cylinder you’ve three areas to test. On a personal level, I’ll choose a quick mechanical integrity test but the question is: “What’s the quickest way to achieve this? Understanding what cranking speed sounds like on a good car is a benefit, and I’ll normally use a scope to support this with a relative compression test. Using a current clamp ( figure 1 ) to identify a poorly sealing cylinder is a quick test that can give immediate diagnostic direction, but in this case we can see that current draw is equal across all cylinders, and as cranking sounded normal I decided that my time would be better spent looking elsewhere. Next steps With a quick mechanical integrity check undertaken my gaze turned to ignition. Ignition related misfires are commonplace and there are a number of ways to complete this part of the diagnosis. I could dive in with a scope although I’ll normally look at spark performance with a gap check first, and drill down a little deeper with an oscilloscope if it fails that test. Figure 2 shows the tool typically used for such a test. The secondary ignition output from all coils was good and equal across all cylinders. If this had not been the case then a scope would have been used to identify why, but in this instance a quick output test showed that all was well and the scope would not be required. With our previous tests all but 18 AFTERMARKET APRIL 2019 TECHNICAL www.aftermarketonline.net TO SCOPE OR NOT TO SCOPE? THAT IS THE QUESTION John Batten takes a look at whether an oscilloscope is a necessity for efficient diagnosis or just a nice to have Figure 1