Powertrain Vehicle Networking

A Renault Megane that was experiencing issues has given Ryan the chance to discuss vehicle networking

Published:  11 July, 2022

At times, we encounter troubling vehicles from other workshops because other techs have failed to draw a correct diagnosis. Failures found in a controller area network (CAN) can be as simple as an open wire or as complicated as a noisy network resulting from outside interference. Regardless, a thorough understanding is required to ensure an accurate and swift diagnosis.
The vehicle in question was a Renault Megane 2, with a 1.5DCI engine. It had quite a few local garages puzzled, keeping this vehicle off the road for over six months. Knowing how these systems operate along with the use of an oscilloscope is crucial for analysing any networking fault, including this one.
The CAN bus network is made up of various control modules, also known as nodes,  all connected via two wires which send data packets to each other. They communicate via a binary signal (signal in either a recessive or dominant state) and transmit data at an average rate of 500Kbps (which is the equivalent to 0.5Mbps). Often, the voltages of the binary signal range from the following:
CAN-HI= 2.5v – 3.5v
CAN-LOW=2.5v – 1.5v

Anything ‘CAN’ be fixed, if it’s understood
The customer complained that the vehicle would not crank when the push-to-start button was pressed. However, the ignition would come on. They also noted that the cooling fan would operate with the ignition ‘on’. They mentioned multiple scan tools had been used. However, communication with the engine computer could not be established.
I started by confirming the fault and noted that the engine management light (MIL) did not illuminate on the dash display, with the ignition ‘on’. With no-start complaints, paying attention to the MIL status during ignition ‘on’ is a great first observation, as this will often tell us whether the engine computer is online or not. I carried out a full system scan to find no fault codes present in the vehicle, but did note the engine control module was not detected on the scan. This indicated a communication problem for the engine control module. If this is not communicating with the rest of the vehicle, it will not start as the immobilizer data will not be shared between modules.
Knowing how vehicle networking operates, as described earlier, is critical and can speed up your diagnostic process. For example, I now know I cannot communicate with the engine computer via CAN, but older engine computers often have a single serial data line known as K-line, which connects directly from the data link connector (DLC) to the engine ECU.  Therefore, my next diagnostic step was to see if I could communicate via the K-line to the ECM. If so, this would confirm that the engine ECU is receiving the powers and grounds it needs to communicate.
This can be done with most scan tools by simply carrying out an emissions on-board diagnostic (EOBD) scan. The result of this was established communication. It is now likely a CAN bus-related issue for the ECM. See Fig.1.

Following the stepping-stones
Knowing I likely have a CAN Bus fault for the ECM, the next job was to access the engine computer and verify the wiring integrity. It is crucial that powers and grounds are verified, along with CAN communication line integrity, before ever condemning a control unit. Powers and grounds were verified as ‘good’. Although this was already assumed, because communication was established with the ECM via K-line, it had to be confirmed.
Next, I had to check the CAN bus directly at the engine computer. This check can only be performed accurately with an oscilloscope to truly verify its integrity. I found, with the ignition ‘on’, and the engine computer connected, the CAN high and low signals were shorted to ground, thus confirming our suspicions of a CAN Bus problem. See Fig.2
You must carry out this test with the engine ECU connected as well as disconnected. In case the ECM is internally faulted and is the root cause of the CAN bus short. If the signal, with the ECM disconnected, returns to the expected waveform, then it is likely the issue is related to the ECM, or its internal circuitry. As you can see, referring to Fig.3, with the engine ECU disconnected, the CAN high signal has returned correctly. However, there still exists an obvious issue with the CAN low signal. When checking these signals with an oscilloscope you are looking for uniformity and a mirror image of them, as you will see.

The proof is in the pudding
I have now proven that the CAN low signal leading to the 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.

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