It CAN be done!

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

By Barnaby Donohew |

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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