Left carrying the CAN

Process and discipline were the watchwords when Frank recently took on a Volkswagen Touran V6 with a CAN error

By Frank Massey |

Published:  20 December, 2022

This month’s topic has been one of the most rewarding for quite some time, not just on a technical level, but also due to the process and discipline that underlined a smooth progression to a successful ending. It all came about quite by chance, during a random visit to the ADS workshop. Dave Gore, our diagnostic tech called me over to discuss an unusual and thus-far difficult diagnostic challenge.
    
The vehicle in question was a VW Touran V6 3 LTR 2015 model, engine code CVWA. The first unusual aspect of the vehicle was the fitment of a SCR additive system, which theoretically did not enter service until 2016. How very odd.
    
The owner, who we believe was not the original owner, found us online and had the vehicle transported from way down south. The problem first appeared when the vehicle had failed to start while in a car park without any previous issues or warnings that a fault existed. The vehicle would crank and run briefly and it had been to at least two other garages for repair without success or significant progress. Several trim panels had been removed from the dash as well as rear quarter panels. Prior to my involvement, David had conducted some preliminary tests to determine the nature and scale of the problem. This is how I understood the situation; CAN communication errors in the gateway module, most if not all with a common thread, no communication with engine PCM. Comms with transmission and gateway both reported no engine PCM comms. Please refer to Fig.1, which shows gateway errors. Due to a total inability to communicate with the engine PCM, a decision was taken to replace and code a S/H engine PCM, with no change in fault conditions. This was premature in my opinion but that is where we were at this point. David also discovered a vehicle tracker, which he removed.

Before I begin with the technical aspects of the journey, it is particularly important for you to understand some fundamental aspects to successful diagnostics. Many of you who have attended my training programmes over the last 30 years or so will remember my absolute belief in having a dedicated diagnostic area, and the need to always follow a methodical progressive, disciplined process. This includes uninterrupted time on task. Let me reinforce this point. Even with limited experience or confidence in your diagnostic abilities, your success rate will increase dramatically if you adopt this method. Testament to this was the fact that David had been granted limited time and physical space in the workshop due to dead cars and multiple tasks.

Joint involvement
Our joint involvement began with VCDS re-checking the CAN network communication, especially our inability to communicate with engine PCM. However, David had discovered quite by accident that unplugging the engine PCM with ignition on, then reconnecting it actually re-established communication with engine PCM. Checking through various sensor data, all seemed normal. So, the diagnostic line was okay. Cranking the vehicle then caused a total loss of comms. Our thoughts directed us to check the CAN physical layer between engine, transmission, gateway, and SCR module at the rear. Both CAN high and CAN low was normal. I should point out that cranking was disabled if trouble codes were not cleared from engine PCM. This was only made possible by disconnecting the PCM with the ignition left on, then re-connecting.
    
Please refer to Fig,2, which shows a Pico screen snapshot of the CAN gateway and PCM. This suggested that no physical wiring network errors were responsible for the issue. I took the opportunity to revisit my initial thoughts; Car cranks, and then starts briefly? Does this seem like it is being immobilised? An owner concerned enough to fit a tracker would probably fit further protection. I call it human behavioural profile assessment. My crystal ball needed a software update.
    
Despite an extensive search David could not find additional wiring or evidence of previous device fitment. Was it time to call in some second and third opinions? I then had a conversation with Steve Smith at Pico. He suggested repeating our CAN scope tests, but this time setting up a trigger on starter current inrush to confirm if RF from cranking was corrupting CAN comms.

Local problem
So, channel A/B CAN high, CAN low channel C crank angle sensor, channel D starter current. Setting a high sample rate of 10 ms/s, with a short time-base to avail the best true sampling rate, a 40% pre trigger, with single shot capture. With approximately a 100-amp threshold, we could now examine the CAN pre-post cranking, and guess what? No RF induction, perfectly clean CAN. Please refer to Fig.3, which shows a Pico scope CAN capture pre-post crank. There was only one test option left now. If the problem was not within the physical CAN network it must be due to error messaging, corrupt telegrams or packet data.

So, we selected the CAN decode option, channel A and repeated our previous tests. We immediately noticed lots of error frames with no ACK/CRC present with the error frames. We also noted most error frames disappeared when the engine PCM was removed from the network. We did not have a global network problem, just a local one between the gateway and engine PCM.
    
Please refer to Fig.4, which shows Pico CAN decode pre-post cranking. So, we have a local network corruption. I left David without a specific fault cause, repeating my thoughts about a device between the gateway and engine. About an hour later, David rang me to say he found an immobiliser in the headlining which when removed restored all comms and normal crank start. These devices were obviously unknown to the owner.
    
Diagnostics are not dissimilar to problems faced by a veterinary surgeon. You can look, you can test, but you cannot speak with the patient. It takes seven years to train a vet, two years longer than a GP, but it takes us a lifetime.


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