Mercedes older series diagnostic fault code reading. W124.R129 1980’s – 1990’s era
As you are reading this you will probably be wishing to extract a fault code or clear a fault indicator lamp from the dashboard of your older Mercedes Benz model vehicle. You will have also likely been frustrated by reading a multitude of confusing information, following dead web-links and generally reading several different takes on the same subject – mostly from well-meaning members on forums or online motor clubs who unintentionally ‘muddy’ obtaining the most correct and technically accurate information.
Well the alternative information source you have been looking for is here, hopefully clear and accurate enough for your needs. I have collected the following information together in one place to save the frustration of trying to locate and find most that you would ever need to tackle this job. I will not repeat what has already been expertly written on this subject, you can find links below to these sources to expand on what I have noted here. What I have done is written about the important parts and included some vital pieces of information that you may need to refer to – you may find this data at the other linked sources too, but at least it will remain available in another Mercedes resource should any of it disappear from the web, as a great deal on this subject already has. Enjoy!
MB Diagnostic History
Back in the day when Mercedes first ventured into the world of on board diagnostics in the era of the 1984’s W124 and its stable mates they began to introduce electronic code diagnostics. The first being either through an 8 pin (X92) or 16 pin (X11/4) square block fastened to the bulkhead. There were actually only14 diagnostic pins on some 16 pin X11/4 models as slot 2 was a small momentary push button and slot 4 was a small red LED used to read out fault codes flashes. All was needed was a paperclip or short banana plug jumper to ground the required pins and simply watch the number of flashes. Codes are resolved from comparison of flashes to fault codes on a written list.
Counting flashes activated by grounding individual pins continued through the square block connectors to the 38 pin round X11/14 diagnostic socket to about 1994. This was commonly located within the ECU box and was accesses by removing a sealed access cap. Although 38 pins were used only the pins relating to build options were loaded and as such gave diagnostic flash / blink access to the cars individual modules.
USA and some EU versions around 1994 were equipped with both a 38 pin X11/14 and the now common 16 pin oblong OBD port we use today. The reason behind this was that MB at the time had to comply with export requirements that necessitated access to emission data through OBD standard protocols. At this time Mercedes added the OBD port under the dash, in some cases near the steering column just to give access to emission related diagnostic codes, all the other electronic systems were dealt with through the 38 pin socket as before. So very much a hybrid situation was in existence – seemingly done as a quick fix to continue to sell cars in the US.
Mercedes were in parallel developing their own digital diagnostic system and not following the industry standard of OBD1/2. From August 1995 production date Mercedes phased out the analogue type of flash/blink diagnostics in favour of their propriety digital code reading system. Without going into too much detail this new system used ISO9141 communication protocol to access the vehicle electronics via a common communications module. This used both a K-line and L-line connection and Mercedes own communications language to access the system. Even though the communications standard of ISO9141 is the same as used on OBD2 systems the way in which it talked to the modules was bespoke – and this is why you cannot use traditional OBD2 readers on systems of this type. Fortunately Mercedes saw the error of their ways and only ran with this for about a six year window opting to take on the OBD2 standard across the board from 1999 (Thank goodness!)
So given the above history there are three primary diagnostic code reading methods (actually four) that you may come across in the model year window of 1984-1999 (Please note all dates stated are approximate and for guidance only) What is a little confusing is that Mercedes in their wisdom used the 38 pin diagnostic connector for both the analogue and digital systems making it very difficult to visually identify what system you are dealing with – they even used the same pin number allocations to reflect the connection to individual system components !
The diagnostic systems that were available during this period are:
Flash / Blink – circa 1984 – 1994
Flash / Blink with OBD X11/22 Under-dash for US Markets circa 1993 – 1997
(OBD Port for engine management only)
Mercedes Proprietary Digital Diagnostics 1995 – 1998
OBD2 Standard 1999 – on
Useful links that have great information and should be used for reference.
Here is some important information that budding MB SL R129 / W124 diagnosticians will need.
To build and use a simple Mercedes Flash / Blink decoder, here is the circuit diagram.
Here is the link to the Cs1000 Baum Tools Manual code reader that contains most of the detailed information you may ever need regarding, connections, pin outs etc specifically for Mercedes Benz models. Importantly it contains the analogue flash / blink error code fault decoder information.
To read codes:
1.Connect the code reader cables to pin 1 for the black or ground connection and pin 3 for the 12v supply on the 38 pin diagnostic socket. Select your chosen test point (module connection) to test from the following 38 pin socket diagram and connection pin-out table.
2.Turn ignition on, engine not running.
3.Press momentary switch for 2-4 seconds, wait and then count flashes. Note down the number of flashes on a piece of paper. Repeat the process until the first flashed code repeats.
Note:A single flash means – no fault stored
To clear codes:
1.After reading a code, press the momentary switch for 6-8 seconds then release.
2.repeat this for all stored codes.
Mercedes SL R129 38pin Diagnostic connector pin out diagram and function list.
Please note not all pins are loaded in every model, the quantity of available pins depends on model, version and fitted equipment.
Later MB Proprietary Digital Diagnostic Communications – 1996 MY (R129) SL
I have discovered that this period model (The one I have!) was fitted with a combination of both analogue (Blink / Flash) and Digital Mercedes Proprietary diagnostic systems. I do not have an OVP module (voltage controller) I have a Base Module. The base module can be accessed with the blink/flash code reader, faults read and cleared on pin 8, while other system components can be accessed with the Mercedes Proprietary reader from the same socket!
However that is about the extent of any function accessible by this method. As stated earlier above to read engine codes or ABS codes etc the unit has to be connected to a capable electronic code reader. The Autel OBD2 reader I use for most of my modern day Mercedes diagnostics does not work – even with the OBD 16 pin to 38 pin adapter. This is because the protocol or access language is specific to Mercedes.
About the only system capable of reading and accessing the majority of codes through the Mercedes proprietary system is the Carsoft V12 system. This is a Mercedes single model diagnostic tool made in Belgium. Its kit consists of a 7.2 interface module, selection of leads (for all MB models) Operating USB security dongle and Carsoft V12 PC software.
I will cover the use of this diagnostic equipment in another post shortly.
For those fortunate to own post 1999 vehicles of the R129 stable your car should be equipped with a fully compliant OBD2 diagnostic system, 16 pin standard diagnostic port and the capability to be interrogated by most any off the shelf OBD2 reader. (Lucky you!)
Air conditioning system – self diagnosis (R129)
The AC control unit in the R129 Mercedes SL model along with a few other MB models of the time (outlined in the CS1000 manual) could be accessed to diagnose system problems by retrieving fault codes directly from the AC control unit console in the car. Instead of counting flashes, the AC console displayed fault codes on the two digit LCD temperature display and were cleared by a combination of pushing buttons. Information for the reading and clearing of the diagnostic codes of the AC unit for the R129 can be found on page 90 of the Cs1000 Baum Tools Manual. Information on other period MB models that had similar AC control consoles is included in the above publication.