Diesel diagnostics for the workshop
Frank examines how following tried and tested diagnostic procedures in a consistent way will enable you to successfully find diesel faults
By Frank Massey |
Published: 11 May, 2020
I’m mindful of several recent diagnostic topics that focused on cutting edge opportunities such as noise and vibration analysis. It also reminded me of the most important aspects of fault finding; to focus on the symptoms, ask relevant questions and conduct a methodical approach based on systems knowledge, accurate data and a proven process.
All of this really boils down to training, experience, and confidence. There are no short cuts, cheap fixes or internet gurus. There are however basic steps that are easily introduced into your workshop procedures.
This brings me to the topic in hand. Can we conduct relativity simple tests on common rail diesel systems? Not only can we, but we must! Remember, the foundation rule of fault finding is a simple methodical approach. Don’t expect a magical fix-all in less than 1,000 words. However, I can provide a pathway that will illustrate the area of responsibility and potential investment in time and money.
Vital information
The first vital step is to listen and ask questions. Owners often have vital information. Remember this is not a recipe for short cuts or silver bullets for your machine gun. Your approach will always depend on the extent of problems. Will it run? are there any mechanical noises? Is there a loss of power? if so when? Is the fault intermittent and how did it start? There is an endless list of questions that will help establish a hidden history.
I often find that a physical examination or health check helps understand the way the vehicle has been driven and serviced. This will often expose basic problems especially with charge pressure circuits.
Try to explore all non-intrusive tests first. They may not be entirely logical in order of priority, but do provide results in the minimum time period. With experience, you will hone these steps into a razor-sharp intuitive process.
Serial investigation
Serial investigation is without doubt the correct first step. Do not jump to premature conclusions as serial data often shows symptoms, not cause. For example, a faulty air mass meter will cause EGR calculation error values, incorrect load and boost calculation. This is a common problem with many causes.
The volumetric efficiency relies on the intake system, swirl flap control, turbo spooling, and a free-flowing exhaust system. Please note that I keep my thoughts non-specific yet focused on all possible causes. This is a very important reaction in any diagnostic process.
Assuming a non-run condition, excluding any serial clues as often there are none, I would always check for the correct rail pressure. This can be done with a DMM. Expect around 1-1.5v with a quick rise time of 0.5-1sec. If it is slow to rise or low, check the priming system including the filter. This should be done with a gauge. Remember pressure, flow and pump current. This will depend on system type so check the schematics carefully. Most systems now prime at 5-6bar.
Isolate components
A slow rise time may be due to an internal leak or worn components within the high-pressure system. This includes the HP pump, rail limit valves, and injectors, as well as volume and pressure regulation devices. Always isolate various components and conduct a blind or proof test before suspecting the pump. They rarely fail, unless run dry or have contaminated fuel.
The PCM requires camshaft position data to sync the injectors and crank position once running. If recent belt replacement or engine repairs have been carried out, add this to your list. To check the injector sync against cam and crank position is a bit technical. To perform you will require a scope and current clamp.
Quite often the serial data identifies the incorrect timing sensor for position error. This is due to the PCM looking at the camshaft first. Slow rotation speed may be due to a faulty or incorrect battery, so check charge and health status with a suitable conductance tester. Yuasa have a fantastic free online training academy.
Next check relative compression. This is a simple cylinder balance check but when compared with current and rotation calculation will accurately predict correct compression.
Identify
A blocked exhaust or failed open EGR will prevent the correct combustion properties. Exhaust back pressure can easily be proven from the map and DPF pressure sensors. Plotting them with a scope will quickly identify intake or exhaust restrictions. The maximum DPF sensor value cranking or at idle should be 0.5-1.25 volts, 100mbar-1.5psi.
Injector type, solenoid or piezo faults will normally be identified within serial data. A single faulty injector circuit will normally shut down all fuel delivery. It is also worth noting that if a minimum rail pressure is not reached, the injectors will not be activated.
So back to priming. Leaks, faulty rail sensors will all contribute to a non-start.
If you are looking for more information, visit www.ads-global.co.uk for courses and dates, and Autoinform events.
- More product innovations from Nissens Automotive
Nissens Automotive (Nissens) is forging ahead with its intense product and service extensions, which it will showcase at this year’s Automechanika Frankfurt.
These portfolio product extensions confirm the premium quality component manufacturer’s strong commitment to expanding its footprint in the automotive aftermarket and support the needs of its wholesale distribution partners by growing their sales potential with new products, many of which will reflect the innovations that play an increasingly important role in the aftermarket with new technology vehicles.
Ongoing component electrification
Electronically-controlled system components, which offer improved efficiency and optimised control, have been an original equipment (OE) feature in vehicle design for a long time. As a result, Nissens has developed the skills and expertise necessary to introduce various new product groups that provide independent workshops with dependable, Nissens Genuine Quality, replacement components for these vehicles.
- Limited spaces still available for first REPXPERT Academy LIVE event
The first REPEXPERT Academy LIVE training day is set to take place on Saturday 16 November at Lincoln College. Spaces are still available, but techs are urged to book now to avoid disappointment. Doors open at 9.30am, and the day runs until 4pm.
- Government aims for new 2035 petrol/diesel ban deadline
A ban on the sale of new petrol and diesel vehicles could come even sooner than expected, with government now saying it intends to bring the deadline forward to 2035 – or even earlier if possible.
- Process, process and more process
People ask me what made the difference this year when I went from being a Top Technician finalist to a Top Technician winner, and my answer is my process. I have worked hard since last year’s final, refining my process, and learning from my mistakes and it thankfully paid off this year. This article highlights the importance of a great process not only in Top Technician, but also in everyday working life, and how a fault, which at first may seem overwhelming, can be simplified and confidently fixed.
The week after winning Top Technician, a 2016 Ford Ranger was booked in for me to have a look at from another garage. The garage’s complaint was that multiple warning lights were present on the dashboard along with multiple warning messages, the power steering was heavy and the indicators and windscreen washers didn’t work from their respective stalks.
As with every job, the first step of my process is to interview the customer and gather as much information as possible. When I questioned the garage owner, he said they had just completed fitting a galvanised chassis. He explained that the vehicle had been fully stripped and rebuilt in the process, and since the rebuild, warning lights, messages and other complaints were now present. The next step was to confirm the fault. Upon starting the vehicle to bring it into my bay, the complaint was verified. The engine management, traction control, anti-lock brake and airbag warning lights were illuminated along with multiple different messages, one of which was a steering assist malfunction warning (see fig. 1).
Complete picture
I then carried out a global scan of the vehicle to get a complete picture of what faults were present and also to see what modules were or were not talking to the scan tool (see fig. 2).
Straight away we could see that multiple modules could not communicate with the power steering control module (PSCM) and restraints control module (RCM). It was also noted that there was a communication issue between the body control module (BCM) and the steering column control module (SCCM).
As the instrument panel cluster (IPC) communicated and reported stored fault codes, I knew it was more than likely a historic fault code which wasn’t related to the issues present. Attempting to communicate directly to the PSCM, SCCM and RCM with the scan tool all returned a ‘no communication’ message, so we knew we were dealing with hard faults that were currently present. Following my process, I decided the next step was to do some research on this particular vehicle using Ford ETIS which is Fords online information portal. This allows me to access wiring diagrams, connector locations and anything and everything related to the vehicle in question. As many a clever man has said, “if you don’t know how it works how can you fix it?”
Thinking of possible causes, I decided to study the wiring for the PSCM and RCM, how the indicators and windscreen washers work and a network topology to allow me to see how all the modules communicate to each other and the diagnostic scan tool.
It was found that the steering column module controlled the indicators and washers and sent the message to the BCM to activate them. As the SCCM wasn’t communicating it now made perfect sense why those functions were not operating. Next, I found that all three modules worked on the high speed can data bus and all were powered by fuses. All the related information and diagrams were printed out and taken to the vehicle so that a test plan could be drawn up and executed.
Plans within plans
Before writing up my plan, I made a visual inspection of wiring under the bonnet and underneath the vehicle. Having had a major overhaul, something as simple as there being a connector left unplugged could cause some of if not all the faults present with this vehicle. Everything looked ok, so I laid out my wiring diagrams and proceeded to write a plan. My plan was to test the fuse for each of the modules to see if it gave me direction, then if all was ok I would look at the communication wiring and how the modules at fault linked into each other and the rest of the vehicle.
All three fuses tested fine so it was onto seeing if there was a common link. Looking through the topology, I found a page which had the PSCM and RCM joined by two connectors. This is where technical information is a must, as dealing with a fault like this it can be very easy to dive in full speed. I don’t want to go straight to a module, for example the RCM, and remove half the interior of the vehicle to find all is ok there and have to spend time reassembling everything! I speak from experience here, and I am sure some of you reading can relate to this.
Diagnostic direction
ETIS showed one connector inside the nearside front wheel arch and the other in the location of the bulkhead of the vehicle. This meant I could test the network without removing anything, saving time and gaining diagnostic direction. I inspected visually to see which of the two connectors was the easiest to access and it was the connector in the nearside wheel arch. Visually the connector looked correct and looked to be correctly latched. However, I decided to double check and upon squeezing the two sides together and audible click was heard meaning the connector was open (see fig. 3).
I then decided to scan the vehicle again to see if this had made any change and every module now communicated and it was also noted the dashboard warning lights had disappeared. I cleared all the faults codes in the vehicle. None returned, and the dashboard now had no messages or warning lights illuminated. The final checks proved the steering assistance now worked correctly and the indicators and washers operated completing the fix.
In the end then a fairly simple fix once it was established how the system worked and where everything was located but without the correct information and a well polished process this job could have taken a very different, and perhaps longer, turn.
- Top Garage 2022 winners revealed
The following businesses have been named as the winners of Top Garage 2022, as has the first ever winner of the brand-new Top Service Adviser award.
Business with 1-3 staff
A&J Fleetcare, Leeds
