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.
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.
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.
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
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.
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.
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 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.
Digital Storage Oscilloscope
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