Cut to the chase
Damien Coleman from Snap-on discusses how preparation can lead to easier diagnostics
Published: 28 August, 2018
Many modern systems, such as common rail diesel injection, can appear to be so complex that they seem to operate by magic. Over time, such systems are only going to become more and more complex, so understanding them means you can gain a head start on their repair.
You can be presented with a seemingly endless amount of data relating to fuel pressure feedback, fuel pressure control, cam/crank synchronisation, measured mass airflow, injector flow correction feedback, and many other areas.
However, if you prepare yourself with a fundamental understanding of the system and all data available pertaining to the fault, a systematic approach to the fault-finding procedure can be carried out.
Data overload
Figure 1 shows the live data returned from a common rail diesel injection vehicle with an EDC16 engine management system.
There is an enormous amount of data available from these data parameters, which can allow you to ascertain the nature of the fault. The actual operation of the fuel system can be compared to the desired system operation and using the data, a decision can be made on the condition of the system and where a fault (if any) may be.
An oscilloscope is another important tool when investigating a fault with such a complex system. Figure 2 shows an oscilloscope waveform from an Audi with the 2.0L common rail engine. The yellow trace is the fuel rail pressure sensor voltage (feedback) and the green trace is the current flow through the inlet metering valve (command). The waveform was captured during a wide open throttle (WOT) condition.
This image alone tells us that the fuel inlet metering valve is a normally open valve. The engine control module (ECM) decreases the duty cycle when the required fuel pressure is increased. This allows less current to flow through the solenoid and the valve is allowed to open, which increases the fuel pressure measured at the fuel rail.
Full analysis
When the fuel pressure demand decreases, the duty cycle control from the ECM increases. This allows more current to flow through the solenoid which results in a reduction of the fuel pressure. Duty cycle is often referred to as pulse width modulation (PWM) control.
The duty cycle control on the ground side of the fuel inlet metering valve can be analysed using an oscilloscope, as seen in Figure 3. The waveform below displays the fuel rail pressure feedback voltage (yellow trace) and the fuel inlet metering valve duty cycle control from the ECM (green trace).
The oscilloscope is connected to the control wire for the fuel inlet metering valve. The technician must be mindful that this is the ground control circuit. System voltage on this wire indicates open circuit voltage. The diagram in Figure 4 shows the best method of connecting this set-up.
By careful analysis using serial (scan-tool) and parallel (oscilloscope) diagnostics you will now be in a position to identify the area of concern accurately and in a timely manner. Knowledge, together with the right equipment and experience therefore benefits technicians by leading to a reduced diagnostic time and an easier fault finding method, rendering these complex systems much less so.
TT Archives: Top Technician issue ten 2016 | www.toptechnician.co.uk
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- A reality check
This year’s summer was good, but as usual, was over too quickly – so back to work and a reality check!
However, during my summer travels some of today’s necessities of life were conspicuous by their absence. I hired a car, only to discover that the USB connection I needed to use to charge my phone and link to my favourite music playlists didn’t work. The local radio station’s dubious choices in music didn’t help relive the tedium, but when I got to the hotel my woes were compounded when I discovered that they wanted to charge a ridiculous amount to use their wi-fi – I mean seriously, who in their right mind can justify charging hotel guests for basic wi-fi – unless the hotel is run by Ryanair (who seem to want to charge everyone for everything), which it wasn’t.
So, with no wi-fi in the hotel room, I had some time on my hands, so I started thinking about the connections we expect in today’s connected world and in turn what connections are needed to run today’s workshop. This got me thinking about the problems it would face if these connections were either expensive, were restricted, didn’t work as they should or didn’t exist at all.
Form over function
Back in the 1990s I remember well being handed a new portable diagnostic tool which could connect to the internet via the mobile phone networks. Subsequently, it was able to conduct remote and bi-directional diagnostics on a vehicle anywhere in the world, when the vehicle was also connected to the internet – effectively ‘PC anywhere’ technology. However, I also clearly remember complaining to the development engineer within a couple of minutes because the functionality was too slow. He was visibly shocked and was clearly offended by my negative feedback on what was his pride and joy. Then I realised what had made me comment negatively – it was not the impressive technology, but the speed of use and the corresponding ability to run the diagnostics I wanted to conduct. In IT terms, this is referred to as system ‘functionality’ and ‘non-functionality’. Simply, the ‘non-‘functionality’ is the design of the system and the ‘functionality’ is what it can deliver. It might be easier to remember this in layman’s terms as being ‘Form over function’.
When applied to the workshop, this directly applies to a wide range of electronic connections that are needed to support your day-to-day business, and if these connections do not work as needed, how this can quickly and detrimentally impact your business activities.
Don’t miss the ‘bus’
The ubiquitous Universal Serial Bus (USB) connection is a good example. A ‘bus’ within a PC are wires that transfer data between components inside the computer, or between the computer and its peripheral devices. We have all come to use this connection for a wide variety of tasks, from using it as an auxiliary power source for many different gadgets, to a vital communications port for various functions such as printers and other data transfer requirements. However, if it does not work correctly, physically or electronically, then simple tasks suddenly become major issues.
This wired technology has moved on and most of us are now connected by wi-fi in the office environment, but increasingly also in the workshop to connect diagnostic tools to the internet. Data transfer speeds depend on the technology used and the latest generation (soon to be 802.11ax) is super-fast, which becomes more important as software updating of vehicles involves the transfer of massive data files. Generally, wi-fi connections work well, but when they suddenly stop working, it is more difficult to diagnose as it is not a physical connection than can be more easily tested. This may happen after a software update and a recent experience showed me how simple a problem can be, but how difficult it was to discover, when my PC was updated and a simple setting was changed. Over three hours of technical support was needed to discover that it was a simple tick-box setting which needed to be re-enabled. These wi-fi problems move into understanding the IT environment of certificates, configurations, permissions, log-in and passwords between the router and the various connected devices, without even starting to consider the wider communications providers that connects your workshop to the wider world.
Have a cookie
This leads me onto an increasing communications requirement which has become a fundamental part of our day-to-day lives, from both the personal and business aspects – the internet. If there is ever a perfect example of living in a connected world, this is it. However, if you think about the wide-ranging possibilities that the internet supports, do you ever stop to think about the technology behind what is happening to understand the control mechanisms that are needed for it to be safe and secure? If you visit a website, not only are there likely to be cookies tracking your choices and mapping your activities, but there will be certificates being exchanged to ensure secure communication. This may extend to log-in criteria and passwords, or may be implemented by the service provider whose website you are viewing. This becomes particularly important when you are paying for something online.
In simple terms, all this is a form of coded access, but this works not only to ensure the correct access rights, but more importantly, to stop anyone who does not have the valid access rights from interfering or monitoring what you are doing.
Control
What then does all this lead to at the workshop level? In terms of the technology of the equipment, then it is developing to be both more reliable and faster, but the same cannot be said of the beloved OBD connector, which is not only restricted in terms of speed, but will become restricted in terms of access without the correct roles and rights authentication which requires certificates from the vehicle manufacturer. As the manufacturer controls this certificate, then it becomes ‘He who controls the connection, controls the function and ultimately the business’, so the workshop of tomorrow needs to worry most about a connection that they have no control over, but which will control their business.
Time then to sign up with one of the aftermarket associations and join the fight to protect access to the in-vehicle data!
xenconsultancy.com
- The shoe’s on the other foot
After my recent articles on tyres and TPMS systems, this month’s topic is wheel alignment, and the massive impact excessive toe-in or toe-out could have come MOT time.
A lot of us can spot alignment issues a mile off. I would worry if after 17 years in the trade, that a technician such as myself couldn’t tell the difference between a tyre that had been toeing-in and a tyre that had been toeing-out most of its life. If you look at a front tyre on a car and only the outer edge is bald (and the tyre has plenty of pressure in) then you walk around to the other side and discover the same issue with that tyre, it would be plain enough to see that the wheel geometry isn’t correct.
Compensate
A car with tyres like this and wheel alignment so far out must handle like it has a mind of its own - like Herbie the Beetle. However, people get used to a car in this state and will compensate to a surprising extent. When we take the car for a test drive, we may come back to the customer and say things like; “there maybe an issue with the tracking” or “the car is pulling rather a lot to one side.”
The customer then looks at us in amazement as if we are saying things just get their hard-earned money from them. We then offer a FREE wheel alignment check, and show them the virtual view on the screen. By Jove, the customer can then see with their own eyes exactly why their tyres have worn the way they have.
I first mentioned toe in and toe out. This is the way the wheels ‘point’ in relation to the forward motion of the car. Look down at your feet. Now keep your heels in position, and point your toes inwards so they point towards each other. If you try walking like this you will wear off the outside of your shoes first.
Do the same again with your heels, but point your toes outwards. This is toe-out and walking with excessive toe-out will wear the inner part of your nice new shoes and that’s the last thing you want. I’m sure you’d like it if your shoes wear evenly. The majority of track cars/race cars will be running with toe-out and negative camber. This will be topic of conversation next time as wheel geometry is a large subject.
Minutes and degrees
I also mentioned minutes and degrees earlier too. A while ago I tried explaining this to another tech and he struggled to grasp the concept of the theory behind alignment/ geometry, but he knew how to set the wheel alignment up on 99% of cars.
Getting your head around the actual maths is a different story. Think of it as a clock; there are 60 seconds in one minute; there are 60 minutes in one degree; and there are 360 degrees in a full turn. Our alignment set-up only works in minutes and degrees as it is really sensitive so no need for seconds-it would be worthless. The wind could blow and move the car slightly and that’s enough to make you panic and pick up your spanners again.
Once the alignment is set-up correctly and you’re happy with the end result, your customer will in turn be very happy. Remember, in the end they are the ones paying our wages, not just your boss. It does help if you have a happy boss too of course.
- Desk diagnostics
By Neil Currie
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