204 C-Class No Start CAN Failure

Andrew Technician Commack, New York Posted   Latest  
Case Study
Electrical
No Crank / No Start
Can Communication
No Communication
Network

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.

+4
Michael Owner/Technician
Cartersville, Georgia
Michael
 

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

+1 Ð Bounty Awarded
Maynard Technician
Elmira, Ontario
Maynard
 

That is a great case study! And very helpful in teaching navigational concepts in approaching problems like that. Thanks for sharing.

0 Ð Bounty Awarded
Steve Instructor
Irvine, California
Steve
 

Knowing topology on CAN and Flex Ray will be absolutely necessary. 

Good write up.

+1 Ð Bounty Awarded
Steve Technical Support Specialist
Gainesville, Florida
Steve
 

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.

0 Ð Bounty Awarded
Andrew Technician
Commack, New York
Andrew
 

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 values are a great place to look.

0 Ð Bounty Awarded
Andrew Technician
Commack, New York
Andrew
 

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 region/nationality.

X30/32 INTERIOR CAN

A98 Panoramic sliding roof control module (with code (413) Panoramic glass sunroof with top sliding sunroof)

N10/2 Rear SAM control unit with fuse and relay module

N22/7 Automatic air conditioning control and operating unit

N69/1/2/3/4 LF/RF/LR/RR door control unit

N73 Electronic ignition switch control unit

N10/1 Front SAM control unit with fuse and relay module

N32/1 Driver seat control unit (with code (275) Memory package (driver seat, steering column, mirrors))

N32/2 Front passenger seat control unit (with code (242) Electrically adjustable front passenger seat with memory)

A2 Radio (with code (510) Audio 20 with CD changer or with code (523) MB Audio 20 radio)

A40/3 COMAND controller unit (with code (512) COMAND APS with DVD changer or with code (527) COMAND APS (with navigation system))

N70 Overhead control panel control unit (with code (414) Power glass tilting/sliding sunroof)

N28/1 Trailer recognition control unit (with code (550) trailer hitch) (VIA REAR SAM)

A1 Instrument cluster

-> X30/33 INTERIOR CAN

N62 Parking system control unit (up to 28​.​02​.​2011 with CODE 220 (PARKTRONIC), as of 01​.​03​.​2011 with CODE 230 (Exclusive parking assist))

N62/2 Video and radar sensor system control unit (with CODE 233 (DISTRONIC PLUS), CODE 237 (Active Blind Spot Assist) or CODE 238 (Active Lane Keeping Assist))

N110 Weight Sensing System (WSS) control unit (USA)

N69/5 Keyless-Go control unit (with code (889) Keyless-Go)

N88 Tire pressure monitor control unit (with code (470) Tire pressure monitor (standard))

B84/3 Reversing camera (as of 01​.​06​.​2012 with CODE 218 (Reversing camera))

N121/1 Liftgate control unit (with CODE 890 (EASY-PACK liftgate))

TELEMATICS CAN X30/35

A2/30 Navigation module (with CODE 509 (Navigation box))

A2/56 Radio with auto pilot system (with code (511) Audio 50 APS with DVD changer or with code (525) MB Audio 50 APS radio)

A40/8 Audio/COMAND display

A40/9 Audio/COMAND control panel

VEHICLE DYNAMICS CAN X30/28

A89 DISTRONIC electric controller unit (with CODE 233 (DISTRONIC PLUS), CODE 237 (Active Blind Spot Assist) or CODE 238 (Active Lane Keeping Assist))

N30/4 Electronic Stability Program control unit

N62/2 Video and radar sensor system control unit (with CODE 233 (DISTRONIC PLUS), CODE 237 (Active Blind Spot Assist) or CODE 238 (Active Lane Keeping Assist))

VEHICLE DYNAMICS CAN

B24/15 Yaw rate, lateral and longitudinal acceleration sensor

FRONT END CAN X30/49

B29/4/5 LR/RR bumper intelligent radar sensor system (with CODE 234 (Blind Spot Assist))

N66/2 Reversing camera control unit (with CODE 218 (Reversing camera) and CODE 498 (Japan version))

B84/3 Reversing camera (as of 01​.​06​.​2012 with CODE 218 (Reversing camera))

N71/1 Headlamp control unit (as of 01​.​06​.​2012 with CODE 615 (Bi-xenon headlamp unit) with integrated curve illumination or CODE 616 (Bi-xenon headlamp unit with integrated asymmetric curve illumination) or CODE 621 (Intelligent Light System (left-hand traffic)) or CODE 622 (Intelligent Light System (right-hand traffic)))

B29/6/7 LR/RR bumper radar sensor (with CODE 237 (Active Blind Spot Assist))

B29/2/3 DISTRONIC (DTR) sensor, LF/RF bumper (with CODE 233 (DISTRONIC PLUS))

E1n1 Left xenon light control unit (with code (614) Bi-xenon headlamp unit or with code (622) Intelligent Light System (right-hand traffic))

E2n1 Right xenon light control unit (with code (614) Bi-xenon headlamp unit or with code (622) Intelligent Light System (right-hand traffic))

N10/1 Front SAM control unit with fuse and relay module

X30/38

N118/5 AdBlue control unit

N37/7/8 NOx sensor control unit (with engine 651)

N37/5/6 Left nitrogen oxide control unit (engine 272.98), left NOx sensor control unit (engine 276.9 with stratified operation)

N40/3 Sound system amplifier control unit (with code (810) Sound system)

N72/1 Upper control panel control unit

N87/3 Digital Audio Broadcasting control unit (with code (537) Digital radio)

N87/5 Satellite digital audio radio (SDAR) control unit (with code (536) SIRIUS satellite radio)

N89 Transmission oil auxiliary pump control unit (transmission 722.9 and CODE B03 (ECO start/stop function))

N112/2 Telematics services communications module (with CODE B54 (COM module))

N123/4 Emergency call system control unit (with code (359) TELE AID emergency call system)

DRIVETRAIN CAN X30/21

N3/9 CDI control unit (with diesel engine 642, 646, or 651)

N3/10 ME-SFI [ME] control unit (with gasoline engine M271, M272, M274, M276, or M156)

Y3/8n4 Fully integrated transmission control unit (with transmission 722.9)

N118 (N118/2 N118/3 AMG) Fuel pump control unit (with gasoline engine)

N15/3 Electronic transmission control unit (on transmission 722.6)

N15/5 Electronic selector lever module control unit (with transmission 722)

N89 Transmission oil auxiliary pump control unit (transmission 722.9 and CODE B03 (ECO start/stop function))

CHASSIS CAN X30/30

A1 Instrument cluster

N30/4 Electronic Stability Program control unit

N80 Steering column module control unit

N51/5 Adaptive damping system control unit (with CODE 483 (ADVANCED AGILITY package))

N145 Transmission Mode Control Unit (AMG)

A40/11 Multifunction camera (with CODE 238 (Active Lane Keeping Assist), CODE 476 (Lane Keeping Assist), CODE 513 (Speed Limit Assist) or CODE 608 (Adaptive High beam Assist))

N2/10 Supplemental restraint system control unit

A76 Left front reversible emergency tensioning retractor (with code (299) PRE-SAFE)

A76/1 Right front reversible emergency tensioning retractor (with code (299) PRE-SAFE)

N62 Parking system control unit (up to 28​.​02​.​2011 with CODE 220 (PARKTRONIC), as of 01​.​03​.​2011 with CODE 230 (Exclusive parking assist))

N62/2 Video and radar sensor system control unit (with CODE 233 (DISTRONIC PLUS), CODE 237 (Active Blind Spot Assist) or CODE 238 (Active Lane Keeping Assist))

N88 Tire pressure monitor control unit (with code (470) Tire pressure monitor (standard))

N68 Electrical power steering control unit (engine 274.9, model 204.002 as of 01​.​12​.​2012 with engine 651 with transmission 722.9, model 204​.​002/302 as of 01​.​12​.​2012 with engine 651 with transmission 711.6)

+2 Ð Bounty Awarded