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Mercedes older seriesdiagnostic fault code reading.W124 R129 & others

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Mercedes older series diagnostic fault codereading. W124.R129 1980’s – 1990’s era

As you are reading this you will probably be wishing to extract afault code or clear a fault indicator lamp from the dashboard ofyour older Mercedes Benz model vehicle. You will have alsolikely been frustrated by reading a multitude of confusinginformation, following dead web­links and generally readingseveral different takes on the same subject – mostly from well­meaning members on forums or online motor clubs whounintentionally ‘muddy’ obtaining the most correct andtechnically accurate information.

Well the alternative information source you have been looking

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for is here, hopefully clear and accurate enough for yourneeds. I have collected the following information together in oneplace to save the frustration of trying to locate and find most thatyou would ever need to tackle this job. I will not repeat what hasalready been expertly written on this subject, you can find linksbelow to these sources to expand on what I have noted here. What I have done is written about the important parts andincluded some vital pieces of information that you may need torefer to – you may find this data at the other linked sources too,but at least it will remain available in another Mercedes resourceshould any of it disappear from the web, as a great deal on thissubject already has. Enjoy!

Carsoft Interface connected to R129

MB Diagnostic History

Back in the day when Mercedes first ventured into the world ofon board diagnostics in the era of the 1984’s W124 and itsstable mates they began to introduce electronic codediagnostics. The first being either through an 8 pin (X92) or 16pin (X11/4) square block fastened to the bulkhead. There wereactually only14 diagnostic pins on some 16 pin X11/4 models asslot 2 was a small momentary push button and slot 4 was asmall red LED used to read out fault codes flashes. All wasneeded was a paperclip or short banana plug jumper to ground

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the required pins and simply watch the number of flashes.Codes are resolved from comparison of flashes to fault codes ona written list.

Counting flashes activated by grounding individual pinscontinued through the square block connectors to the 38 pinround X11/14 diagnostic socket to about 1994. This wascommonly located within the ECU box and was accesses byremoving a sealed access cap. Although 38 pins were used onlythe pins relating to build options were loaded and as such gavediagnostic flash / blink access to the cars individual modules.

USA and some EU versions around 1994 were equipped withboth a 38 pin X11/14 and the now common 16 pin oblong OBDport we use today. The reason behind this was that MB at thetime had to comply with export requirements that necessitatedaccess to emission data through OBD standard protocols. Atthis time Mercedes added the OBD port under the dash, insome cases near the steering column just to give access toemission related diagnostic codes, all the other electronicsystems were dealt with through the 38 pin socket as before. Sovery much a hybrid situation was in existence – seemingly doneas a quick fix to continue to sell cars in the US.

Mercedes were in parallel developing their own digital diagnosticsystem and not following the industry standard of OBD1/2. From

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August 1995 production date Mercedes phased out theanalogue type of flash/blink diagnostics in favour of theirpropriety digital code reading system. Without going into toomuch detail this new system used ISO9141 communicationprotocol to access the vehicle electronics via a commoncommunications module. This used both a K­line and L­lineconnection and Mercedes own communications language toaccess the system. Even though the communications standardof ISO9141 is the same as used on OBD2 systems the way inwhich it talked to the modules was bespoke – and this is whyyou cannot use traditional OBD2 readers on systems of thistype. Fortunately Mercedes saw the error of their ways and onlyran with this for about a six year window opting to take on theOBD2 standard across the board from 1999 (Thank goodness!)

Types

So given the above history there are three primary diagnosticcode reading methods (actually four) that you may come acrossin the model year window of 1984­1999 (Please note all datesstated are approximate and for guidance only) What is a littleconfusing is that Mercedes in their wisdom used the 38 pindiagnostic connector for both the analogue and digital systemsmaking it very difficult to visually identify what system you aredealing with – they even used the same pin number allocationsto reflect the connection to individual system components !

The diagnostic systems that were available during this periodare:

…………………………………………………………………………………………………………………..

Flash / Blink – circa 1984 – 1994 Flash / Blink with OBD X11/22 Under­dash for US Markets circa1993 – 1997 (OBD Port for engine management only) Mercedes Proprietary Digital Diagnostics 1995 – 1998 OBD2 Standard 1999 – on

…………………………………………………………………………………………………………………..

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Useful links that have great information and shouldbe used for reference.

http://www.mbcluster.com/Media_Diagnostics.htm http://www.202tech.com/howto/leddiag.html http://www.onboarddiagnostics.com/page03.htm http://k6jrf.com/mb_scnrs.html

Here is some important information that budding MBSL R129 / W124 diagnosticians will need.

To build and use a simple Mercedes Flash / Blink decoder, hereis the circuit diagram.

Here is the link to the Cs1000 Baum Tools Manual code readerthat contains most of the detailed information you may everneed regarding, connections, pin outs etc specifically forMercedes Benz models. Importantly it contains the analogueflash / blink error code fault decoder information.

To read codes:

1.Connect the code reader cables to pin 1 for the black orground 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 socketdiagram and connection pin­out table. 2.Turn ignition on, engine not running.

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3.Press momentary switch for 2­4 seconds, wait and thencount flashes. Note down the number of flashes on a pieceof paper. Repeat the process until the first flashed coderepeats. Note:A single flash means – no fault stored

To clear codes:

1.After reading a code, press the momentary switch for 6­8seconds then release. 2.repeat this for all stored codes.

Mercedes SL R129 38pin Diagnostic connector pinout diagram and function list.

Please note not all pins are loaded in every model, the quantityof available pins depends on model, version and fittedequipment.

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Later MB Proprietary Digital DiagnosticCommunications – 1996 MY (R129) SL

I have discovered that this period model (The one I have!) wasfitted with a combination of both analogue (Blink / Flash) andDigital Mercedes Proprietary diagnostic systems. I do not havean OVP module (voltage controller) I have a Base Module. Thebase module can be accessed with the blink/flash code reader,faults read and cleared on pin 8, while other systemcomponents can be accessed with the Mercedes Proprietaryreader from the same socket!

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← Mercedes SL (R129) Front CoilSpring Replacement

Mercedes SL (R129) ETS andABS Fault Light Illumination –

However that is about the extent of any function accessible bythis method. As stated earlier above to read engine codes orABS codes etc the unit has to be connected to a capableelectronic code reader. The Autel OBD2 reader I use for mostof my modern day Mercedes diagnostics does not work – evenwith the OBD 16 pin to 38 pin adapter. This is because theprotocol or access language is specific to Mercedes.

About the only system capable of reading and accessing themajority of codes through the Mercedes proprietary system isthe Carsoft V12 system. This is a Mercedes single modeldiagnostic tool made in Belgium. Its kit consists of a 7.2interface 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 postshortly.

For those fortunate to own post 1999 vehicles of the R129stable your car should be equipped with a fully compliant OBD2diagnostic system, 16 pin standard diagnostic port and thecapability to be interrogated by most any off the shelf OBD2reader. (Lucky you!)

Air conditioning system – self diagnosis (R129)

The AC control unit in the R129 Mercedes SL model along witha few other MB models of the time (outlined in the CS1000manual) could be accessed to diagnose system problems byretrieving fault codes directly from the AC control unit console inthe car. Instead of counting flashes, the AC console displayedfault codes on the two digit LCD temperature display and werecleared by a combination of pushing buttons. Information for thereading and clearing of the diagnostic codes of the AC unit forthe R129 can be found on page 90 of the Cs1000 Baum ToolsManual. Information on other period MB models that had similarAC control consoles is included in the above publication.

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Diagnosis →

8 Comments on “Mercedes older series diagnostic faultcode reading. W124 R129 & others”

Frank O Donnell says:December 12, 2015 at 8:55 pm

Hi,Carsoft V12 connected via the 38 pin port to a 1996 SL 320( R129 ) . On total diagnosis it only responds with 4 results,the rest failed to communicate, having selected the 38 pindigital. It look like this software is extremely limited inscanning this particular car.

Reply

Steve Ball says:December 12, 2015 at 10:00 pm

Hi Frank,Like all computer software it is atemperamental interface and you have toget things just right. It is most certainly notlimited (but I suppose is to the MB tools) butis probably one of the best available toolsfor older diagnostics. You will have to playwith the settings a little to get the most outof it (the settings for the port comms arecritical) and indeed you have to run it acouple of times on occasion to connectcorrectly. Did you get their interface or use aclone. The only system that it cannot accessis that of the roof controller which can becode red/reset via basic flashing LED tool.ABS, SRS and engine systems are covered.Let me know how you get on Frank. Carsoftdo have a good support line if youpurchased legitimate product with a serial(most do not ;)) and will always refund you ifnot satisfied. Apart from Star Diagnosticstool there is little to choose from to readvehicles from around this era.All the bestSteve

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Richard Lankford says:December 31, 2015 at 9:05 pm

need help!1992 mercedes 400e 38 pin diagnostic connectorwhen i connect blinker code reader to pin 8 (which is forbase module) the blinker reader lghts up immediately an icannot get it to clear nor read any codes. i am havingissues with very intermitent tach needle jumping. havechanged throttle body, maf, coils, distributors, rotors, andplugs. i assume if vacuum leak, it would be consistent.got code 2 and 10 on pin 7, and have ordered cruisecontrol/idle speed control module but has not arrived yet.would base module light staying lit indicate anythingtowards my problem?

Reply

Steve Ball says:January 1, 2016 at 2:40 pm

Hi Richard,

I would be lifting the cover that houses themodules by the 38 pin connector andchecking the fuses in the base module.There are two or three hidden fuses directlyon top of the base module under the boxcover. The instrument cluster could be anindicator that the base module has a dryjoint and causing an intermittency. If you arehandy with a soldering iron the base modulecan be unplugged and taken to theworkbench. Here you can open it up andreflow the solder connections to theconnector pins. Dry or fatigued joints in thebase module or OVP relay are a verycommon problem in pre 1996 MB models.See here for OVP module repair:

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Mercedes OVP relay ...

Hope this helps you out.All the bestSteve

Reply

Richard Lankford says:January 1, 2016 at 3:12 pm

according to what i candetermine, there is not an ovprelay on my 1992 400e v8.could it be that the cruisecontrol speed control modulecould be faulty as dtc 2 and 10on pin 7 indicate?

Richard Lankford says:January 1, 2016 at 3:14 pm

by the way, ohmed fuses in base module and they are ok(only 3 in my 400e)

Reply

Steve Ball says:January 1, 2016 at 4:04 pm

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Hi Richard,See here and here, it could help locate theOVP, overprotection is provided by the basemodule it seems in this instance. I would pullthe base module from the backplane andcheck it over. Power distribution to modulesincluding the Cruise control I suspect, will allbe powered through the base module. Anydry joint or intermittency here could be anissue. This would be my No. 1 look­at first.

All the bestSteve

Reply

Richard Lankford says:January 1, 2016 at 7:10 pm

thank you for response. alreadypurchased cc/isc module butwill check base module as well.will let you know how i makeout after receiving module.

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