It CAN be done!

Barnaby Donohew has to stick to his guns to track down the ‘simplest’ of faults

Published: 10 September, 2018

We all remember certain jobs which test our nerve but ultimately serve to strengthen our capabilities. Proper learning experiences so to speak. Unsurprisingly, these memorable jobs tend to occur when tackling novel technologies or environments which, by their nature, can be unsettling.

Some time ago a customer arrived with a MINI having persistent warning lights, instrumentation faults and bearing a new instrument cluster and engine control unit. Mindful that the expensive repair history must have included some seriously ‘in-depth’ diagnosis, I decided to get involved and see what I could do to fix the issues.

Ruling out
A system scan reported various powertrain CAN faults in the engine, ABS and instrument cluster control units, indicating a system-wide communication issue but with no systematic patterns to help isolate the fault. The MINI had a separate diagnostic bus, which thankfully permitted scan tool communication in the presence of a CAN fault. However, CAN access was not available on the diagnostic connector to aid recording of the signals. Instead, an oscilloscope was connected to the engine control unit (Figure 1) to reveal that the wires were unlikely to have shorted together, to Earth, nor to +5V, as the signals from the engine control unit were almost ideal. The fault was more likely due to circuit integrity. After powering down the CAN this was confirmed, as a 120 Ohm resistance was measured between the high and low lines (around 60 Ohms was expected).

Subsequently, the customer was called with an update and to authorise further expenditure. The next stage involved pulling the car apart to fully check the wiring and control modules. Plainly, it was unwelcome news.

Added pressures
When conscious that the meter is running, doubt can creep in and you find yourself asking if a wiring fault is too simple, alongside other related questions. This was not a good time for misinformation. The resources available (course notes and workshop information) identified the MINI’s engine control and ABS units as each having a 120 Ohm terminating resistor between the CAN pins. Subsequent measurements determined a resistance of 120 Ohms on the engine control unit but many kilohms on the ABS control unit. Was it faulty? Nerves started to fray. Following a thought process akin to James Dillon's mantra "what would you test next if the part you had just fitted did not cure the fault," basic procedures were recalled.

Firstly, on this MINI the terminating resistors actually were in the engine and instrument control modules (all were fine). Next, a series of continuity tests isolated an open circuit on the CAN-H line between the ABS and engine control units. It was located in a well-protected and tiny portion of wire, equidistant between the terminating connectors. Figure 2 shows the damage.

The process demonstrated to me how, during stressful situations, it is worth trying to adhere to basic procedures as faults are often straightforward. As it turns out, this would have been good advice for the recent Top Technician practical tasks, which proved a very similar experience – I wish I had listened! For anyone thinking of entering, I highly recommend it.

TT Archives: Top Technician issue seven 2014 | www.toptechnician.co.uk

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As I write this (in April), we had just began our fourth week of lockdown and I am doing just two mornings per week to deal emergency jobs for key workers. Very strange times indeed! However this has given me the opportunity to write up a few more interesting jobs that I’ve done over the past few months. This particular one got me thinking about a few different things and maybe considering tweaking my diagnostic process slightly.

The owner of a garage that we do a bit of diagnostic work for contacted me to ask if I could take a look at a 2015 Ford Fiesta 1.5 TDCi that was causing him grief. The complaint was that shortly after start-up and moving off, the vehicle would drop into limp-home mode with multiple warning lights on the cluster.

As always, I asked some questions to gather as much background information as possible, and so the story began. The vehicle came into their workshop with the above symptoms. The fault codes stored pointed towards a turbo wastegate fault. You’re probably mostly thinking the same as I was at this point; sticking wastegate, wastegate control solenoid malfunctioning, vacuum fault, can’t be that complicated surely?

We discussed all the tests that were carried out and what parts had been replaced. This included a new genuine solenoid valve, an actuator repair kit followed by a reconditioned turbocharger. With the fault still present the turbo supplier then recommended a trip to the nearest franchised dealer for testing and a turbo position sensor relearn. After spending some time on the vehicle, the dealership’s diagnosis was the turbocharger and recommended a genuine replacement unit. Reluctantly the garage fitted a genuine unit but guess what? I don’t need to answer that!

Challenge
The vehicle arrived and I started my plan for diagnosing this fault. I was in challenge mode now and was not prepared to be beaten. I knew my plan had to be thorough so I took some time to confirm the fault and do some research. As tempting as it is, just to clear the fault codes and carry out a fresh test – this isn’t always a good move, particularly when the fault is intermittent and may take some time to replicate. In this instance however, there was little to lose by doing this as apparently the fault was very consistent.

I took a read of the fault codes which were as follows:
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