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
- Workshop robot in development: Johnny 5 is alive?
Technicians could soon be able to concentrate on the more complex diagnostic-related jobs the workshop, if a prototype drone technician proves to be as effective as its developers claim.
- Would you like to diagnose more vehicles first time?
As we reach March, 2019 is well and truly underway. In fact by the time you read this one third of the year will have whizzed by never to be seen again. Now, I’m not one for New Year’s Resolutions (they’re so last year), but I am the type of chap that likes constant progress when it comes to developing a technician’s career.
There’s so much to be said for small steps taken everyday that on first look appear don’t appear to make a difference, but when gazed back upon over a 12 month period have a staggering affect on your capability to diagnose a vehicle first time, in a timely manner.
Pitter-patter of tiny feet
Small steps are all well and good but where do you start? After all, you don’t know what you don’t know, and you’d like to start your journey to diagnostic success off on the right foot. In this instance I’d start with the end in mind and reverse engineer the outcome you desire. It’s a logical process that works, and can be replicated time and time again in your diagnostic routine.
Your ‘end in mind’ in this instance is a vehicle where the fault no longer exists, that won’t appear back across the threshold of your workshop anytime soon. But how do you guarantee that?
One test to rule them all
I love nothing more than when the delegates working through our training programs have a technical epiphany. This happens at many points on their path of learning, but none more than with bypass testing.
Bypass testing is step nine in Johnny’s diagnostic circle of love (our 15 step routine), and often the key element in the first time fix. The good news for you is that it doesn’t require mythical creatures to forge their magical powers into an object that only one technician can possess. It’s something that every tech can learn, and become a diagnostic wizard.
What is bypass testing?
Quite simply it’s fixing the vehicle before you fix the vehicle. Let me explain.
Wouldn’t it be great if you suspected that a Mass Air Flow sensor was at fault and you could prove that you were right before you fitted a new part, or spoke to the owner of the vehicle. If you could do that then the positive effect it would have on you and the business you work for would blow you away.
Picture this: Your customer has reported that the vehicle is low on power. You’ve diligently questioned them, experienced the problem with them on a road test, and the bought the vehicle into the workshop.
You’ve pulled codes and found none present, followed by taking a look through serial data to hunt for diagnostic clues. It doesn’t take you long to identify that the MAF sensor frequency looks a little low at 1.5 Khz and your fuel trim data is incorrect and making a positive corrections. You’ve seen a bunch of these before and know that 1.85 Khz is a suitable value for this vehicle.
You’re keen to prove that the serial data is leading you in the right direction so confirm the sensor output with your oscilloscope. The oscilloscope frequency mirrors that of the serial tool and your starting to get that warm fuzzy feeling that an you’re onto something.
Steady the buffs
You’ve been close to success before though, only to be thwarted in the final moments so you’re keen not to be caught out twice. You know that documenting the reasons that the MAF output could be incorrect is the way to go, and duly make a list of tests required to confirm your theories.
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- And the worst MOT tester in the UK is… YOU
To save money and raise efficiency, the DVSA has turned to automation. They no longer need an army of Vehicle Examiners wandering from MOT bay to MOT bay. Instead they are collecting data all the time.
Let’s say I am the boss and my business is low on revenue. I beat up the manager and he in turn influences the tester to fail everything coming through the door. The customer is now stuck with no MOT and I have some simple high yield repairs.
Here’s where it gets interesting. The DVSA computer is monitoring individual tester behaviour and looking at averages. The pattern is really easy for a computer at the DVSA to see because it’s just not possible that lots of cars fail on the same items every day. The DVSA’s fix is to target garages where data shows they are hunting for work and send in a VE to crosscheck. He needs only to wait nearby until our tester issues his favourite fails and then arrive to retest the car.
We all, as testers, now have access to our TQI. Lots of testers that I speak to have the sentiment that this data is all rubbish but, here is the rub. The DVSA have a team of very capable data processors looking at this data and writing algorithms that alert them to trends that need investigation.
Take my example of one of my longest-serving testers and allow the DVSA computer to tell me every car that he has tested in the last two weeks of November for the last seven years and add in that we only want to know about cars tested after 4:30pm. We find only one car; a Y reg (2001) BMW 320i convertible, always tested after 5pm with a longest test time of thirty-two minutes and shortest of twenty-seven. Guess what, it’s my guy’s brother-in-law’s car!
For me the horror is that the car has never failed an MOT. It’s also never been in the workshop for any repairs. It looks absolutely dogged out and is on around 180,000 miles. Worst still my guy has never once even advised anything on this car. The VE would assume Barry’s guy is prepared to let things slide at the end of the day, so maybe he plans to visit me after 5pm on a Thursday.
Conflicting vehicle locations
This is a fun story from a close and trusted friend. My guy is at a DVSA IVA check and overhears a conversation by a couple of Vehicle Examiners. It goes like this; VE no.1 is suspicious of an MOT bay offering fraudulent MOT tests. He parks down the road from an MOT bay in Kent and checks which vehicle is logged on and being tested. He takes the registration number of the vehicle in question and calls the DVLA, identifies himself and asks if the vehicle has been seen on the DVLA camera system anywhere in the last half hour. The car was last seen on the M25 twelve minutes ago near Watford in Hertfordshire over 70 miles away.
So, our VE is in Kent and the car is in Hertfordshire. If this works today in a manual sense how long will it be before computers can do this to every single test? Talk about an easy way to stop fraudulent MOTs, just using computers that the government already own.
- Autofirst and RAC team up for new garage network
Autofirst Network has joined forces with the RAC in a bid to create the biggest independent garage network in the country.
