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

Published:  13 May, 2019

If 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 eliminating ignition and mechanical faults, it was time to take a look at fuelling faults. The problem on this particular vehicle meant that the cause would be isolated to one cylinder, this made the probability that it’d be a fuel supply issue to the rail less likely. With this in mind it makes sense to use a scope and carry out comparative checks on individual cylinders looking for anomalies that could be caused by a fuelling fault. Access to primary and secondary ignition was less than ideal due to the coil pack configuration so the ignition profile could not be used for fuelling evaluation.
    
Injector supply, ECU control and circuit current were inspected across all cylinders and while there were small differences nothing was conclusive, until we took a look at rail pressure using the Pico WPS500x pressure transducer. Using this it was plain to see that upon injector number 4 being commanded to open and deliver fuel that there was little drop in rail pressure compared to the other cylinders. This definitely warranted further inspection so the injectors were removed and  a flow test was completed in our test bench. Number 4 injector was found to be delivering significantly less fuel than expected. Bingo, we’d found our misfire.

To scope or not to scope?
Effective and efficient diagnosis is all about using the right tool, for the right test, at the correct point in your diagnostic routine, and as this vehicle has shown the oscilloscope plays a critical part in serving up the answers that whilst possible via other methods are often more time consuming to obtain.
    
If an oscilloscope isn’t playing a major part in your day to day diagnosis then there’s no time like the present to blow the dust from it and start seeing the benefits that this amazing tool will bring to your workshop.


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