Mercedes Diagnostic Information

Computerized fault diagnosis capabilities, accessible through the Diagnostic Data Link Connector, X11/4 or X11/22, have been part of Mercedes' standard equipment since the eighties. Beginning with basic analog flash trouble codes, and with the industrial development of the OBD standard, digital K-line diagnostics and the OBDII standard.

Early Nineties
Mid Nineties

X92 - 8 Pin
X11/4 - 16 Pin
Diagnostics ports
X11/4 - 38 Pin
Diagnostics ports
X11/4 - 38 Pin
X11/22 - 16 Pin
Diagnostics & OBDII ports
X11/22 - 16 Pin
OBDII port

Analog Diagnostics
Digital Diagnostics

The diagnostic system (X11/4 and X92) should not to be confused with the analog terminal block TD (X11, X11/2, and X11/3), which is used to measure spark, RPM and On/Off Lambda readings. Those are real analog signals which tap off the systems themselves. The diagnostic system is meant to be a computerized high level fault detection system that will give you information generated by the ECUs.

The term analog diagnostic system is a bit misleading, as it implies a varying voltage value. It is, however, the commonly used terminology. It's really a digital signal, in sense of having only two values, and also called an impulse code or flash code. This system is borrowed from the OBD I standard, which involves flashing the Check Engine light. Mercedes simply expanded the concept to give different modules their own impulse codes, on a proprietary connector. The digital K-line signal, in contrast, is defined in the ISO 9141 standard, and is referred to as a digital diagnostic system.

Analog Diagnostics

The analog diagnostic system is proprietary OEM, and consists of a series of pulses which count out the particular numerical value of the fault code involved. The impulse train consists of a square wave of zeros and ones, represented by 0V and 12V, and has a low frequency roughly of 1 second, which makes it easily human readable. The fault can then be looked up in the fault list of the particular module being examined, and translated to the actual fault being triggered.

8-Pole Diagnostics Connector
Models 201, 124, 126
16-Pole Diagnostics Connector
Models 124,129
38-Pole Diagnostics Connector
Models 124,129,140,170,202,208,210

The fault listing for modules in your vehicle, which decodes the fault into plain English, is a bit difficult to find. The dealership will obviously have this list, but also, some diagnostic tool manufacturers list the error codes in their product owners manuals, as can be seen in the CS1000 Fault Code Scanner manual, made by Baum Tools Unlimited Inc :

Architecting a fault diagnosis system for a vehicle is arguably a difficult task for any manufacturer. The layout Mercedes built consists of all accessible computer modules linked together via the engine CAN network, to a central diagnostic module called the N59. The N59 breaks out to the 38-pin port, which the user has direct access. Faults are relayed independently by the various modules to the N59 and can be temporary, or permanently stored depending on their frequency. The Check-Engine light, in the instrument cluster, is driven by the N59.

Digital Diagnostics

In the mid nineties, when the OBDII digital standard was gaining wide acceptance, Mercedes improved its own proprietary 38-pin diagnostics capabilities by going digital. The redesigned architecture provided independent K-line access using the ISO 9141 protocol directly to each electronic system. The old flash code scanner was replaced by a more high tech serial interface scanner, known as the MB Star Diagnosis System or SDS. Although the K-line protocol is the same as in the OBDII standard, Mercedes used its own proprietary set of commands to access it. This is why normal 38-pin to 16-pin adapters made to link standard OBDII tools do not work. It was also an unfortunate decision to utilize precisely the same physical 38-pin port as the analog system, as there is no clear physical distinction between the analog and digital interfaces.

The 38-pin digital port was still similarly connected to the same modules as defined by the analog port as seen previously. However, as is always the case with Mercedes, depending on options and model year, the precise diagnostics layout depends on vehicle. For example, as can be seen below, the specific digital diagnostic pinout varies from year to year for the W210.

W210 to 97
W210 to 99
W210 from 00

The beginning of the digital diagnosis era also meant the end of simple user diagnosis. Computer interfacing and proprietary instruction sets began phasing out the independent mechanic capabilities. Although the SDS system can be purchased independently from Baum Tools Unlimited Inc, it has the price point of $20K and a $7K/year license. Eventually tool manufacturers caught on to the new system and started developing aftermarket interface products. Tools are now accessible to the public, and quite inexpensive. A Swedish company called Carsoft launched their product line in the late nineties, capable of fully interfacing with the 38-pin port. The Carsoft tool, comes with a price point of $3K. Chinese manufacturers sell knockoff versions of the original Carsoft design, with a price point of around $100.

Once the scan tool can be acquired, determining the scan code can still be a challenge. The Carsoft tool does have the fault code lookup once the actual value has been extracted from the module. But as technological complexity has increased, so has the amount of fault information. The TCM (Transmission Control Module) has hundreds of possible error codes, depending on how the transmission has generated the fault. When relying on an aftermarket tool, one should try to cross-reference the actual code itself with STAR TekInfo, or with Alldata. The following document describes the TCM including fault codes, downloaded from the benzworld forums:

Mercedes eventually dropped the 38-pin port altogether in the early 2000s, and finally adopted the OBDII port as its sole diagnostics interface.

Built-in Self Diagnostics

A few in-vehicle systems are designed to have self-diagnostic functionality built into them, to allow convenient and tool-free diagnostics capabilities. These systems were introduced as early as the eighties, for example, with the power window automatic switch learning. The instrument cluster is another system, and is described in a bit more detail in the FAQ.

The climate control system is also another such system, with easy access via the pushbutton module. Debugging the air conditioning system in the W210, for example, is easily done by scrolling through the sensor data list and evaluating defective components. Error messages stored in the AIRCO ECU can be reviewed with an option to clear for DIY testing, all without the need of a scan tool. The following two documents describe the built-in testing procedure for the climate control, and a listing of the body DTCs which could be generated by the pushbutton module.


The advent of the OBDII standard was intended to consolidate many different industrial communication protocols into one unified automotive diagnostics connector. The 16-port connector contains enough bandwidth and precision to itemize any DTC a vehicle can generate. The following diagram depicts the universal OBDII port with all pins specified :

In the mid nineties, Mercedes adopted the OBDII protocol, which was being internationally mandated at the time for all automotive manufacturers. Some manufacturers embraced this technology eagerly and based their entire fault diagnosis system on this standard. Mercedes was, conversely, slow to accept it, and began by implementing only a stripped down version. They implemented only the K-line ISO 9141 protocol, which is defined as a dual signal K-Line with L-Line wakeup, but made it single ended. A new module called the N59/1 was introduced which handled communication with the OBDII port, but only provided high-level error codes. Low level, and more useful codes, could only be accessed through the 38-pin digital port. As can be seen below, the line in red connects the X22/11 and X11/4 together and only talks to the N59/1. This is the only information you can pull out of the OBDII port, which is just a generic fault code. The blue line represents the OEM K-line signal, which has the real meaningful information you want, because it's talking directly to the system involved.

In the 2000s, a European version of the OBD standard, called EOBD was developed, and Mercedes then abandoned the 38-pin port altogether in favor of the EOBD. This port is fully pinned out and accesses all the vehicle modules combined, as can be seen in the table below.

Tools for Analog Diagnostics

The scan tool required to interface to the analog diagnostic system is quite simple, and can be found online or even made by the average DIYer. K6JRF's Page has information on how to build a simple flash code reader, as can be seen below.

Interfacing is done this way :

  • Connect push button (PB) interface tool to desired diagnostics pin
  • Key inserted, ignition on
  • Press the PB for 3 to 5 seconds. If there are no faults in the system, the LED will flash once indicating no faults stored in the system. Any number of 'flashes' greater than one indicates the DTC stored in the system.
  • Press the PB again for 3 to 5 seconds. If there are further faults in the system, the respective DTC will be displayed. If there no additional faults, the previous DTC will reappear. Repeat this step until the first DTC is displayed.
  • To erase the code you've just read, within 10 secs, press the PB for 8 to 9 seconds. Then turn the ignition off for 15 secs.

The system works quite simply, all diagnostic pins are normally tri-stated when idle and active-low when in use. When the user presses the PB, it grounds the pin and actives the N59 module. The module then looks up the next fault stored in its memory pertaining to that pin. It then serializes the fault number, using an active-low response, broadcasts the fault, increments an internal counter and cycles over and over. Although very easy to read, the drawback is that each error must be sought consecutively, and slowly, across each accessible module.

To help access pins on the 38-pin port, a breakout module exists which proves easy access to all pins.

Tools for Digital Diagnostics

The digital diagnostic tool is a bit more complicated than the analog, but essentially works in the same way, by probing the individual system involved. The Carsoft system was the first aftermarket tool available, and has gained a lot of support from the online community earning it an excellent reputation. The price point still weighs in at around $1000, but it has full ECU reading, writing and erasing abilities, and multiplexes all the K-lines together in one package.

The Chinese knockoff counterpart weighs in at a very attractive price point at around $100. Because it's a pirated copy, it comes with no support, and many of the features don't work. If you can actually get it to work on your laptop, it does support basic reading and erasing abilities on the X11/4 port, which is all you need. The X11/22 port interface adapter it comes with does not work, as explained above, and for the adaptive erasing abilities, probably wise not to do it.

The system we tested the knockoff Carsoft on, was a Dell Latitude 810 running Windows XP, and a real serial port, running smoothly. The Carsoft application does not have a friendly logging capability, and only records data in its own native PDA binary format. It can't copy to clip board or save as text file which means you can only do screen shots. We wrote a quick perl script to convert the binary file into a text file, which is more useful. Here is the code (right click and download), it runs in DOS, using perl for DOS.

Tools for OBDII

Many tools exist to read and clear DTCs using the OBDII port, depending on price point. We purchased the Autel MaxiScan MS509 OBDII/EOBD scanner and are very pleased with the performance. Not only reading and erasing codes, live data scanning, computer interfacing, and manufacturer specific libraries are all built in, all weighing in at a price point of well under $100.

Unfortunately the high level OBDII DTCs gives you no real no information as to the actual cause of the fault. This information is still only accessible through the MB SDS, which is done at your local dealership. There exists also knockoff versions of the SDS C3 and C4 systems, and can be purchased for under $1000, as can be seen on Ebay :

Digital Storage Oscilloscope

Undoubtedly, a Digital Storage Oscilloscope or DSO, is one of the coolest tools in the toolbox. It measures the actual voltage waveform of the system in question, whether it be a sinusoid, square wave, ignition spark, or anything that's not a simple DC waveform. The traditional price point of a DSOs was in the low thousands, but that too has come down over the years. The UNI-T UTD2102CEL as pictured above is a Chinese made scope, simple, powerful, and having a price point of under $400. It has two channels for simultaneous measurement, USB capture and storage, and a measurement speed of up to 100Mhz.

W140 Spark & TNA Capture
Approved Documented Waveform

Using the UTD2102CEL we captured the spark and RPM signals coming from the analog terminal block TD (X11, X11/2, and X11/3). There is simply no other way to verify if the quality of spark is acceptable, other than measuring it directly. Problems like intermittent spark, or corrupt VSS or TNS signals can easily be caught with direct measurement. The spark as measured above is within specification, as determined by the OEM literature that can be downloaded from STAR TekInfo :

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