Strange brew

Neil gives his brain some exercise thanks to a most perplexing Mercedes-Benz problem

Fig 1

Published:  20 September, 2022

By Neil Currie

This job was certainly an interesting one. The strange symptoms and faults the vehicle had all made sense when the cause of the problem was found. When you get to the end you will see how it reinforces what I say in most articles I write; The importance of having technical information, a good process and following a test plan to find the root cause.
The vehicle in question was a 2007 Mercedes GL320, and I had been asked to take a look at it for another garage. The customer had recently purchased it as a spares-or-repair project and dropped it straight off the recovery lorry at their workshop. The main complaint was that the vehicle would not shift out of park. When started, the dash dials would start intermittently flashing along with multiple different warning lights illuminated including the engine management light, airbag light and ABS along with red and orange triangles to name but a few. The dash needles were also going up and down by themselves like it was possessed!

Network topology
So, where to begin? As always, I confirmed the complaint and as reported, upon starting the engine, the vehicle’s dashboard started misbehaving along with multiple warning lights illuminated. When I attempted to move the gear selector from park while pressing the brake pedal, nothing happened. I then decided it would be a good idea to plug in a scan tool and carry out a global fault code report to see what was going on. However, the scan tool would not communicate with any module on the car. At this point I tried another scan tool. I don’t know about you, but I always blame the scan tool at first. Unfortunately, it had the same issue and even trying EOBD allowed no communication. Something was definitely amiss and we had our first fault to tackle. Pulling up a wiring diagram allowed me to see what vehicle communication networks this car used and how and where the diagnostic socket was connected into. This is what is called a network topology.

On this vehicle as well as most modern vehicles, the data link connector (DLC) is connected to a gateway module. “What is a gateway module?” I hear you ask. Well, in an attempt at a brief description and not waffling on for too long, there are now multiple communication networks. These range from a high-speed bus to a medium speed bus to a LIN bus, Flex ray, MOST etc. All these networks operate at varying data speeds with different language protocols. As different systems require different levels of data transfer, they cannot talk between one another when required.
The gateway, which can either be a separate module or built into another module most commonly contained within the instrument cluster (but not always; Technical information should always be consulted if in doubt) is connected to the different networks on the vehicle and to the DLC (see Fig.1). It acts as a link to allow communication between the different buses and to pass data when required. Think of it almost like an interpreter; It can take one language in and then change it and transfer it back out in the language another module on a different bus requires so it can be understood. For example, vehicle speed from the ABS module on a high-speed bus required by the navigation system on a MOST bus. It also allows a diagnostic scan tool to talk to any data bus. This is a very important module fitted to the vehicle.
With this new-found knowledge we can now continue. For testing on older vehicles, you could connect straight to the different bus networks directly at the DLC. However, with a gateway fitted now, all you will measure is the diagnostic bus which allows the scan tool to talk to the gateway. To access a particular network to measure the signals, let’s say the high speed can bus network, you would need to connect into the wiring at either a module multi plug or a section of wiring between the gateway and a module within that network. You could opt for the engine ECU depending on location and ease of access. However, if it is hidden, the ABS ECU is more easily in sight and on the same network. This means it would be more practical to connect there. With this Mercedes, as I can’t talk to just any module, I wanted to take a look at the signal from the diagnostic bus from the gateway. Upon connecting my oscilloscope to pins 6 and 14, I had 0.6v flat lines on both wires. What we should see is mirroring signals with data packets as seem in Fig.2, so we have an issue. This explains why we cannot talk to any of the modules on the car but how the car can still start and run, albeit with other issues present.

My next step was to access the gateway itself and do some checks from there. We could possibly have a wiring issue or gateway issue itself, so technical data was checked and the gateway was found to live under the passenger seat. After some stripping (of trim, steady you lot), and the removal of the seat, I noticed the carpet was wet with water. I suspected the gateway would not be dry and then upon lifting the carpet and gaining access to the module I was met with the sight as seen in Fig.3. Yes, that is a floor well full of dirty water and both the gateway module on the left and parking aid module on the right were submerged. Upon removing the module and inspecting it internally, it was found to be heavily corroded and required replacement along with the wiring connector as it had turned blue with corrosion.

At this point I left the garage to contact the customer and advise on costings to repair the damage, fix the water leak and advise of possible further issues; It was impossible to fully assess the vehicle without being able to communicate with all the fitted modules. I can report that once the water leak was repaired and the gateway and parking aid modules and corroded wiring were replaced and coded, all faults cleared except the gear selection issue. This was traced to the gear selection unit on the side of the auto box which was then also replaced and programmed, allowing the vehicle to be driven and put back on the road.


Fig 2

Fig 3

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