There are 4 modules on a W211 that connect the engine and body CAN buses together. The ZGW is the primary connection and the only one that can pass packets between the two networks. There are 3 other modules have double connection to the CAN buses, so they can read and write each bus, but cannot bridge data between them. I reverse engineered the wiring diagrams and made a one page tree diagram of the networking in a W211 and then found the actual diagram in the Star Finder utility. But now I am on the road and can’t recall what they are (maybe instrument cluster, key switch, and front SAM?)

CAN is meant to be able to run in a degraded mode if you short or open one of the two lines. The data is passed using differential signaling, which is a fancy way of saying that each line has a copy of the data, one is positive biased and the other is negative biased. This results in high EMI tolerance as any induced voltage will affect both lines equally.

But if you dead short the CAN then you are pretty much out of luck. This is what finally happened at the second failure, the green grunge built up to the point where it completely shut down the network.

I agree that it is crazy that a short in the CAN bus will pretty much shut things down.

W210 has both CAN buses and they meet at the key switch instead of a gateway module. The W211 (and other models of that same era) introduced the ZGW as a formal bridge between the two systems and also this is where the vehicle coding is held. I also think this is where they store the fault data on CAN bus health. There is a huge Freescale processor chip in this little tiny package.

They did a couple dumb things on the design in my opinion however. First I would have put the 2 buses on different connector plugs for isolation (and at least not on adjacent pins!). Also I would have located this module differently. Having it face up with the wire bundle leading into it from above is just asking for trouble with wicking water leaks into the connector. Especially under the dash where leaks can happen from the sunroof or windshield.

I didn’t get a chance to look for a source of the water leak this weekend but will try again next weekend. At least it’s parked out of the weather at the airport now.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech