Aftermarket November 2022

NOVEMBER 2022 AFTERMARKET 21 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. www.aftemarketonline.net Fig. 3 Fig. 4