It’s all very Scopetastic!

John Batten points out the magical power contained in the scope that could be helping you with diagnostic spells every day

By John Batten | Published:  02 April, 2018

It’s been an interesting few weeks here at Auto iQ HQ. After my last article discussing the merits of “growing over buying” technicians I received a few phone calls looking for my views on the most productive path to technical enlightenment.

Whilst I’m not sure that it’s possible to reach a Zen state in this industry (please let me know if you’ve attained this), it was great to discuss the various paths a technician has to choose from once they’ve decided to continue their technical advancement.

The available options are obviously specific to the individual. That being said my catch-all mantra is to always take the path least travelled. Why would I not advocate an easy path? Well, as with most things in life the route of least resistance may throw up the odd quick win, BUT it rarely delivers the long-term results that pay the kind of dividends we dream of and it’s with this in mind that I scribe this article.

Where’s the map?
Every technical adventure should start with a review of the map. Whilst our diagnostic process will make plain the waypoints on your route to the first time fix, there are many twists and turns along the way where our hero is forced to make a decision. The quality of these decisions will ultimately see their journey culminate in glory, or see them lost in the maze of misery that is misdiagnosis.

A little dramatic I know, but there’s nothing like a bit of emotion to brighten up what would otherwise be just another technical article. Back to the adventure…

If Harry Potter did diagnostics
With so much at risk, what are the critical choices our hero should make? Possibly the most important decision for a technician is to truly understand the relationship between Volts, Amps and Ohms. I’m not talking about the theoretical math that underpins Ohms law, not that it isn’t important. Just that it’s not as important as understanding the fundamental relationship between those values when it comes to using the tool that will quite frankly change a technician’s career forever.

What tool I here you cry? It’s a bit like the Elder Wand (apologies if your not a J.K. Rowling fan). Difficult to find the right one, not the easiest thing to use, but oh boy once you’ve had a bit of practice it’s possibly the most powerful diagnostic weapon in your arsenal. If you’ve not guessed it so far then you really need to get out more. I am of course proclaiming the need for a scope.

Is it really that important?
In one word, yes! Think of it this way. If you’ve ever replaced an electrical component and it didn’t fix the car, then quite simply there was a test that hadn’t been carried out and the tool of choice in these instances more often than not is an oscilloscope. It’s not important which one you own, just so long as you use one that’s suitable for the job at hand. After all a scope is no more complex to connect to the circuit than a voltmeter and with a little guidance and practice on your part I’ll show you how to reveal faults that can elude technicians that give this tool a wide berth.

Let’s do battle
OK. So I’ve piqued your interest and now I have to make good on my promise. See what you think to this.

The battleground for this adventure is a petrol Vauxhall Insignia and the customer has reported rough running. The MIL light is on and glowing brightly, codes have been pulled to reveal a list as long as my arm and upon lifting the bonnet I’m almost blinded by a very shiny coil pack. The vehicle is quite obviously running rough and deserving of the many fault codes. So where to start?

There are a host of options open to a technician at this point. I decided to see if the rough running was isolated to one cylinder or whether it affected multiple cylinders. I used a serial tool to carry out a cylinder balance test whilst monitoring rpm to assess the contribution of each cylinder. I found that number four was making little contribution to the overall performance and set about finding out why.

Bring out the wand
The magic here is all in the tests. The tests for a routine diagnosis like this are straightforward and quick to apply. At this point I’m looking for diagnostic direction and keen to isolate the fault down to a mechanical, fuelling or ignition issue. A relative compression test bore no fruit; neither did my low down and dirty stress test on the ignition system. The weight of probability at this point was tipping the scales in the direction of a fuelling problem. It’s worth mentioning that whilst I have a love for scopes I won’t use one just for ‘scope sake.’ It needs to be the right tool for the test at hand. It’s very interesting though when the planets align and you see Ohms Law come to life through the eyes of a scope and that’s what happened on this vehicle.

So it looked like our rough running was down to fuelling on one cylinder and I was keen to discover if this was an electrical fault with the injector circuit or something a little more sinister. I used three channels to inspect the injector. CH1 – supply voltage, CH 2 – injector switched ground control circuit and CH 3 for current in the circuit. As soon as I had a stable waveform the problem was there for all to see.

Ohms Law + a scope = The answer
Whilst I could have used less channels to draw out this conclusion each one helps to paint the picture. We can see that channel 3 (see image) on the faulty injector circuit (point B) has a lower overall current level than that of the good injector (point A). This is affecting the reaction time of the pintle, and whilst it doesn’t look like much at first glance when cursors are used it shows a 20% difference. This reduced current was caused by an increase in resistance in the injector voltage supply.

Hey presto, the answer on a plate and all in a timely manner. Not much not to like about that. Further inspection revealed that injector harness had been damaged and the vehicle was restored to good health once repaired. You do have to love it when a plan (a diagnostic plan) comes together.

Cutting your way through the maze
All being well you’ll see the benefits of a little electrical knowledge and how using an oscilloscope can reveal this kind of fault like no other tool can. This type of testing isn’t ‘electrickery’ and IS within the reach of all technicians that have the will to
take the right fork in the road. It’s all very scopetastic!


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