Aftermarket May 2022

MAY 2022 AFTERMARKET 37 www.aftermarketonline.net Fig.1 Fig.2 Fig.3 Fig.4 Fig.6 ECM is at fault. Looking at the wiring diagram in Fig.1, you will find that the engine ECU’s CAN data lines go directly to the UPC module (under bonnet fuse box). Therefore, my next diagnostic test is to check for the availability of the CAN signal at the UPC. A signal verified at this location will indicate a wiring fault between the UPC and engine ECU. As you can see from the scope capture in Fig.4, there are proper CAN high and low signals, exhibiting each data packet as mirror images of each other, at the UPC module. This then confirms a wiring fault between the UPC and ECM, on the CAN low data line. Using a jumper wire to connect the CAN low signal, from the UPC module, to the ECM, there is now a proper CAN low signal being measured at the ECM, as seen in Fig.5. This allows me to reconnect the ECU and start the vehicle, confirming that there is indeed a wiring issue between the UPC and ECM. Only then do I move on to the repair stage of this job. The final countdown The final step of the repair was to locate the damaged wiring between the UPC and the ECM. By removing the airbox, battery and battery tray I gained better access to the wiring harness. A closer inspection of the harness revealed where it was damaged. Please refer to Fig.6. The wiring is clearly damaged, and it is the green wire that carries the CAN bus low signal, from the UPC to the ECM. After repairing this damaged wire, communication to the ECM was re-established, allowing for the vehicle to start and run. In-depth knowledge of vehicle networking is strongly advised when dealing with these systems. As can be seen from our diagnosis path, it will streamline your diagnosis and make tackling these difficult jobs a lot easier. You will likely find it a struggle to diagnose these faults without an oscilloscope allowing you to really see what's going on. Therefore, I highly recommend using an oscilloscope when diagnosing network faults. www.aftermarketonline.net