Yet another German fish tank… a tech support tale

Joseph Technical Support Specialist New York Posted   Latest   Edited  
Case Study
Network Communications
2008 Mercedes-Benz C300 3.0L (272.947) 6-spd (716.6) — WDDGF81X88F139928
No Communication

Here is a scenario that initially had me scratching my head. A little back story on this is that it was auctioned due to flood damage and, now being sold at a used car lot. So, I discovered the water ingress wasn’t due to high water levels rather leaking through the sunroof drains into the cab. 

This one was presented as follows: 

Everything was fine and operational when we got it. Meaning the vehicle was running and drivable but suddenly there was nothing. Nothing? Nothing! The diagnostic tool could not communicate to the vehicle neither could any other diagnostic tool in the shop. No crank no start. So, first thing I needed to see was if there was still moisture or corrosion present at the CAN busses (the CAN blocks). Tech states there is no moisture at the driver’s and the passenger’s carpets. Advised tech to inspect the empty slots at the CAN busses for water damage and states not preset. Next thing was to do a quick reference Voltage check at the CAN low and CAN high of the powertrain, chassis and interior CAN busses. For this C-class W204 chassis they are going to be roughly, CAN low 2.4VDC and CAN high 2.6VDC. As far as that is concerned, we were able to verify these values at each of the mentioned CAN busses using a DMM with the COM lead at solid ground and “V” lead making contact at the exposed pins of the CAN connector. 

That’s were the head scratching started. So I moved on to the X11/14 (datalink connector) 

Was power and ground present? Yes, power and ground present. Final thing to check at the datalink connector was to check CAN. CAN at the datalink connector goes straight to the Central Gateway (an internal module of the Driver's Signal Acquisitions Module on the W204 Chassis) and then gathers the information from all other modules. Knowing this, we pulled out the diagnostic scope. Found can lines at the datalink connector was providing a flat line signal. Hmmmm, so we connected the Diagnostic tool and try to communicate under the assumption that a recognizable signal may only be present with a Diagnostic tool actively trying to communicate to the Vehicle. Still nothing. Using the Scope, since we had it out, we went to look at the drivetrain and chassis CAN signals, and we had activity.

Determined the modules are communicating within themselves. Determined the Central Gateway was down. Moving back onto the history of the car, determined it is worth opening the Driver's Signal Acquisitions Module to inspect for water damage and sure enough there was plenty of corrosion on the electrical board

Repair here is to replace the Driver's Signal Acquisitions Module and program. 

Josh Technician

Nicely explained though process. Thanks for sharing.

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Jason Technician
New Jersey

Thank u for ur share Joseph. Exactly what do u mean when u advised tech to check the empty slots of the can busses. When I think of CAN I only think of pin 6 and 14 at the data link connector. U also have that u checked at X11/14 at the DLC. If u can open up my brain to this terminology u speak of Sir. Thank u

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