204 C-Class No Start CAN Failure
This car was brought to a dealer as a no crank no start, the dealer recommended the steering lock be replaced (common problem). The owner brought the car to an independent shop (our customer) for repair. The indie shop replaced the steering lock and all of that repair process went fine (teaching the immobilizer etc), but afterwards the car wouldn't start and there were a lot of communication faults. I read over the quick test and my general feeling was that body modules were online but nothing high speed was communicating. So I looked at the structure of the network. See Figure 1.
Just to confirm, I asked the tech to try to communicate to each module on the network individually and made notes. Figure 2 shows modules that communicated in GREEN and modules that wouldn't communicate in RED.
Some general background on this vehicle (and it's GLK SUV sibling)'s communications; the baseline vehicle has about 20 modules, but depending on the equipment, options, etc, about 70 different modules could be fitted to the car. There are 7 possible CAN networks. The main networks are the Chassis CAN which mainly covers chassis, safety, and drive systems and the Interior CAN which covers body modules. The transmission modules are on their own Drivetrain CAN network with the fuel pump module, the engine module acts as a gateway between the Chassis and Drivetrain CAN networks. The Cluster and Ignition Switch modules are on Chassis AND Interior networks but do not act as gateways. This vehicle uses a central gateway which is built into the Front SAM. Some network wiring passes through the Front SAM physically without interacting with it. There's an optional Front End CAN with high end headlamps and a Telematics CAN with optional entertainment equipment. The Comand unit is also a gateway between the Interior CAN and the Optical MOST network. The DLC is connected to the central gateway via the Diagnostic CAN, which also connects to the Teleaid module if equipped.
As far as structure goes - the network uses several CAN junction blocks which are mostly located under the door sill area, under the carpet, in black plastic casing. This location makes them very convenient to access but also a frequent victim of water intrusion. The Central Gateway hosts the Diag and Front End CAN. On the Diag side it has a 60 ohm terminating resistor. On the Front end side it has a 120 ohm resistor and each of two headlamp modules has a 240 ohm resistor. The CGW joins the chassis and interior CAN networks. The ESP module has its own CAN (Vehicle Dynamics), which has a 120 ohm resistor in it. The other participant on that network, the ESP Sensor Cluster also has a 120 ohm resistor. There are a variety of ESP configurations but only the one with Distronic (Active Cruise Control technology) has another module. The can junction blocks all have 60 ohms of terminating resistance. It's kind of a star and star network configuration. This configuration is typical of later MBs.
Enough of the boring stuff. The quicktest and communication attempts made it clear the chassis CAN (and the Drivetrain CAN by extension) were down. So we pulled the CAN junction X30/30 and checked the voltage. 2.0 volts on high and 2.0 on low. No good. It should be 2.5 +/- .1 V. So we switched to measuring resistance and disconnected the battery. About .2 ohms.
Now a game of divide and conquer. We unplugged the modules from the X30/30 CAN junction one by one and re-checked the resistance. On the third or fourth plug, it went instantly to 60 ohms. Bingo! He turns the key...... the engine fires right up!
The plugs aren't labeled so there is no way to tell what you just unplugged. Or rather, I believe on some models (221??) they may be labeled, but not this one. So, what brought the CAN down? We ran a quicktest and the tech went looking around the car for something that wasn't working. The steering wheel buttons weren't functioning. The steering column module didn't show up on the quick test. The engine module and others complained of no comm with the steering column module.
So either the SCM CAN lines were damaged or the module shorted them together internally. Since he just replaced the steering lock and had the column out, we figured it was wiring. I called him back the other day and he said the wiring slipped underneath when the column was bolted back up, crushing the CAN wires and shorting them out. He said the way the wiring is positioned it is very susceptible to getting trapped. I worked with the guy diagnosing the car, not the one who repaired the steering lock. FWIW I don't blame people for mistakes or point the finger.
So anyway that's a glimpse into MB CAN diag.
I hope to add some more information about diagnosing CAN problems on Mercedes in future posts. It's exhausting work In the future I'll try to elaborate on the use of DMM voltage readings for CAN diagnosis. Don't take anything here as a spec.
Good write up Andrew! Being able to get to X30/30 makes diagnostics easier! Have you had problems finding the correct wiring diagrams and component location on X204's? The last 3 X204s we had with electrical problems were murder trying to find the correct diagram even using Startekinfo. Mike
That is a great case study! And very helpful in teaching navigational concepts in approaching problems like that. Thanks for sharing.
Knowing topology on CAN and Flex Ray will be absolutely necessary. Good write up.
Hi Andrew, You should be able to evaluate which module was disconnected by going to actual values in the EIS module (I haven't confirmed that, but most chassis do it that way). What you would be looking for would be the specified and actual CAN members. If it isn't there it is somewhere. You probably should do that as a first step as it also will tell you whether the can is stable on both lines.
That's true Steve, Here are some shots from the 203 and the 204. The most helpful thing is that the EIS or CGW is coded with its as-built configuration of modules. My tool is in demo mode so the values are defaults. You can also figure out what modules should be talking based on the data card (see the SA codes in the following post), or by the presence of no-communication DTCs. But these actual
Here's an almost complete list of the modules on this car, there are probably a couple of mistakes or omissions because of how complicated it is. Keep in mind the C-Class is a low-line Mercedes so the base equipment is pretty minimal, but there are a lot of options. Looking through our records it seemed most of them were very close to the base configuration, but that's also influenced a bit by