Technical SL R129

Mercedes SL (R129) ETS and ABS Fault Light Illumination – Diagnosis

I started up the Mercedes SL (R129) and reversed it out of the drive and as I straightened up to drive forward both the ETS and ABS amber lamps illuminated on the dashboard. This was shortly after completing the front road spring swap posted earlier on this site.

Mercedes SL (R129) ABS fault

Apart from indicating more work to do, I began to wonder if I had disturbed anything when I had replaced the front springs or even earlier, the suspension top mounts.  I went about my business on that day driving the car with the fault lights illuminated, they would pop straight on as soon as the key was turned and would not clear down on any number of engine re-starts.  I had noticed disappointingly, that I had also lost 5th gear (5 speed version of the 1996 R125 with electronic control overdrive gear) and also cruise control operation.  This underlined it was a speed signal fault but I would have to investigate further to narrow the field.

Mercedes SL (R129) ABS fault 1

Mercedes SL (R129) ABS fault 2

I connected the car to my Carsoft code reader kit.  This vehicle being the 1996 model SL it had the Mercedes proprietary diagnostic communication system, accessed from the 38 pin diagnostic socket inside the ECU box next to the bulkhead (firewall).  You can read more about early MB diagnostic systems in this post.  The first read of the ABS system revealed a recorded fault code with the rear left wheel speed sensor (C1106). Rear wheel sensor plausibility.

Mercedes SL (R129) ABS fault 3

On removing the road wheel and checking the cable routing, all looked well. Often the older series of Bosch wheel speed sensors have a habit of collecting large amounts of iron debris on the sensing tip over time, causing impulse read issues at low speeds.  This was in my mind as being the fault, though it was unusual that on this occasion it had not reset the warning lamps on restart and re-triggered them only when the car started moving.

I removed the sensor from its mounting hole on the rear hub carrier and inspected the tip.  It was not too bad and looked in good order.  I had previously measured the sensors static resistance of 1300 ohms (1.3k) and this is in the range I would expect across the two sensor wires. 1.2k to 1.8k is the norm, anything outside this and I would be suspecting the sensor or wiring itself.  A quick check to chassis ground, from either of the two sensor wires just proved that there is no chafing of cables to the vehicle chassis.  It all looked electrically good, so investigations continued.

Mercedes SL (R129) ABS fault 4

Just a point on removing the sensor from its pocket on the rear carrier – invariably it will be very tight or even bound into the hole, unlike the front sensors where they have a large clearance into the hole, the rears are a ‘push fit’ for about one inch and any corrosion causes them to bind woefully into the hub.  The technique here is to remove the single hex cap head fixing from the sensor and spray penetrant all around the sensor. Push a screwdriver into the clearance hole of the sensor tab and begin to work it, levering very slightly to and fro, very carefully.  Continue application of penetrant. Tap the tag gently from the direction of the drive shaft then insert a thin screwdriver through the tab hole into the casting tapped hole and lever the sensor back to its aligned position.  keep doing this, rotating the sensor many times while spraying more WD40 or similar.  Eventually the sensor should be able to worked out from its hole.  If you have not done so already, unclip the cable from the three snap clip mounting points behind the backplate to facilitate easy removal.

Mercedes SL (R129) ABS fault 5

I shone a torch into the hole where the sensor is mounted to reveal the reluctor ring. This is a slotted (punched) continiuos steel band that is held around the drive shaft outer CV joint body as it passes through the hub bearing centre.  Its purpose in conjunction with the sensor, is to provide measured impulses as the wheel rotates, each slot passing the sensor tip as it turns.  This passing (gap, steel, gap, steel and so on) induces a small AC voltage in the sensor coil and this is passed in the form of electrical impulses, through its connecting cable to the ABS controller, allowing it to determine wheel speed.

I prodded around inside the sensor hole and discovered that the reluctor ring, sometimes called a ‘tone-wheel’ was damaged.  It was split (like a letter ‘C’)and no longer gripped the circumference of the CV joint/driveshaft.  Obviously it was just spinning and not providing the correct road speed signal in comparison to that measured at the controller from the other three wheels, flagging a fault code. My heart sank… Although tapping on a new tone wheel is quite simple, this is a large job to undertake, as to gain access it involves removing the hub and withdrawing the drive shaft from the bearing centre – this usually destroys the hub bearing in doing so and it then has to be replaced, more £££ and time I just do not have at the moment.  Although driving the car without ABS was no major concern on my normal commute, what did matter was that 5th gear was disabled and fuel costs as they are, this would have made my toes curl with the additional cost over any length of time.

So what was I to do?

I had noticed that the tone ring was in very good condition and the surface where it once was attached was clean and oil free. I could just see its inner edge as it passed the rear of the hub, so I selected Neutral and rotated the hub so the broken edges could be seen – they were clean and nothing was missing – just a straight, clean fracture. Taking a selection of pick tools and jewellers screwdrivers from the toolbox I proved it was possible to wedge the tone ring onto the drive shaft by forcing the thin tools into the gap between the ring and hub casting around the accessible 1/3 that was visible of the ring.  The gap in the split ring was forced closed, all but for less than half a millimetre gap.

Mercedes SL (R129) ABS fault 6

Mixing up some quality epoxy two part adhesive, using a small screwdriver I built up the edge of the tone wheel to the CV joint body, pushing epoxy into the punched holes of the tone ring, then cleaning off the excess. Allowing this to dry hard at the repaired split section, the hub was rotated and further epoxy was applied to the complete circumference of the ring, filling in the slots, pushing and forcing the glue into the ring as I went.  Making sure I cleaned any excess that could interfere with the clearance between ring and sensor. Finishing the job, allowing it to fully harden before testing the clearance by inserting a screwdriver into the sensor hole then slowly rotating the hub confirming the lack of high spots – there wasn’t any so the sensor was refitted and the road wheel replaced.

Mercedes Benz Code Reader


Later the Carsoft reader was connected and the ABS fault code cleared once more, a road test proved the repair to be sound and reliable.  The 5th gear worked, cruise control functioned as it should and importantly the ABS was once more available should I ever need it. (Even better no warning lamps!)

I do realise that this repair is only a temporary fix to keep me going until I revisit the rear hub to replace the tone wheel/reluctor ring, but it worked perfectly and for me is a perfect short term solution.  The repair to the ring is not stressed, it merely rotates in front of a sensor with an air gap between the two components – if it does fail again at some point before I get around to the proper fix, I will at least know exactly what the problem will be!  Two weeks on – no problems to date, but I will say they are expected to return again at some point!

Mercedes SL (R129) ABS fault 8

Mercedes older series diagnostic fault code reading. W124 R129 & others

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!

Carsoft Interface connected to R129

Carsoft Interface connected to R129

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.

Mercedes SL Diagnostic Socket

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.

Simple Mercedes Flash Code reader

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.

Mercedes SL (R129) Front Coil Spring Replacement

A slight difference in ride height on the drivers side front was noticed from when I purchased the SL over Christmas, so it was no real surprise the spring refresh needed doing.

Mercedes SL R129 Front spring swap 0

I put the spring job off (waiting for slightly warmer weather) and it jumped the queue slightly when a torsion bar hanger snapped, obviously because of the constant tension the weak spring was placing on the hanger arrangement.

Mercedes SL R129 Front spring swap 1

I ordered up a new pair of the front torsion bar hanger brackets and clamps from the local Mercedes dealer at £13.00 each. (Part numbers: A129 323 0026, A129 323 0126 and two clamps A124 323 1140) and a pair of 15mm diameter OEM replacement road springs.

The job itself is pretty straightforward and requires a Mercedes specific ‘special tool’ to compress the coil spring on the car to enable you to lift out the old spring and simply slot in the new one. With the specially designed tool, the job is very quick, though if you want to separate the ball joints and remove the torsion/roll bar then I have read some reports of being able to do a spring change without the spring compressor, undoubtably by levering the lower wishbone down – Personally for the cost of the tool, (£50) its not worth making the job ‘ten times bigger’ with the increased risk of doing an injury to yourself or someone else – Get the tool!

Mercedes SL R129 Front spring swap 2

Jack the car at the front from the jacking point and remove the road wheel. Slide into the spring the top plate of the compressor tool, locate the bottom plate and thread through the hole in the wishbone the telescopic compressor bar. Locate the upper bar tangs through the top plate by turning the bar until the top plate locks into its slots. Pay attention to align the bottom plate correctly as you begin to ratchet up the compressor bar. (19mm socket) Make sure all is located correctly with the compressor and the tool top plates, recheck as soon as you begin to feel tension being taken from the spring.  Double check before proceeding to tighten and compress the spring further. It will be under a great deal of tension and you need to be super sure everything is latched and locked before you proceed.

Mercedes SL R129 Front spring swap 3

After compressing the spring with the tool almost to the point of the spring being ‘coil bound’, tilt while pushing upward to remove the spring toward the front of the car. Lay the removed spring down and use the 19mm ratchet to unto and relax the tension on the coil spring so the tool can be removed and placed in exactly the same position on the new spring and wound up for refitting.

Mercedes SL R129 Front spring swap 4

Mercedes SL R129 Front spring swap 5

Swap over the rubber top seat that cushions the spring against the spring perch. Make sure this is in the correct position rotationally, so the springs ‘end of coil’ seats in the moulded slot made prominent by the previous spring.  If these rubber spring mounts have split or deteriorated – now is the time to replace them.

Once the new spring is compressed it can be orientated back into position on the car, making sure you clean the spring seat on the lower wishbone of all debris and corrosion.
If you are lucky, you will still have the soft metal seat crescents that afford a little abrasion resistance for the springs final turn onto the wishbone. In most cases these parts will be corroded, worn away or missing, it is optional that you may wish replace these parts before spring reinstallation.

Mercedes SL R129 Front spring swap 6

Once the spring is positioned in the upper perch cup correctly and the bottom turn of the spring is located in the ‘pressing formation’ on the lower wishbone, begin to relax the spring compressor. Check the position of the spring and ensure it is seating correctly as you fully undo the spring compressor tool. Once fully undone, the tool bar can be twisted and removed then the plates taken out from the newly fitted spring.   Isn’t that quite easy..!

Mercedes SL R129 Front spring swap 7

At this point in my case I replaced the torsion bar hangers and brackets (one 17mm bolt and locator bracket into the chassis), not fully tightening the two clamp bush bolts until the spring job was completed on both sides and the vehicle lowered to level ground. A quick nip up with a 13mm spanner and matching socket swiftly completed the torsion bar fixing.

Both road wheels were refitted and the vehicle road tested. Great job done!

Interestingly once the spring is removed it gives a great opportunity to test out the bushings in the lower arm/wishbone and more importantly the lower ball joint. This item is very hard to detect wear in when under road spring tension. In the case of R129 and W124 series lower ball joints are virtually impossible to decern very small amounts of play in, as unusually the ball is being ‘pulled’ by the spring arrangement of the suspension. So check everything out in detail while you have the spring out. In my case all was tight as a drum, although a little play in the near side wheel bearing was noticed. (I will deal with this later!)

Animated Mercedes Front Suspension

I found the above wonderful animated diagram of how the front suspension is set up and how to apply force to test the ball joints. This great animation is credited to Chistian K and has been published on several Mercedes forums. It is by far the best explanation of the front suspension that I have seen to date. Many thanks for sharing this work Christian!.

Mercedes SL (R129) Front Strut Top Mount Replacement – Video


After almost 20 years of being fitted, I thought it was time to replace the front strut top mount bushes on the Mercedes SL R129.  This is part of the SL’s front end refurbishment that is taking place pretty much as I have time to tinker.

Mercedes SL R129 Front Suspension mount 1

Mercedes SL R129 Front Suspension mount 2

The top bush or rubber mount that is fitted between the chassis and the damper/shock absorber leg is a metalastic bonded rubber bushing and as such is susceptible to cracking over time and should be inspected regularly as it is possible for the mount to fail in such a manner that the damper rod can push up into the bonnet/hood with the full weight of the vehicle, if it were to give way completely. £££ !

Mercedes SL R129 Front Suspension mount 1

The video below hopefully documents the simple procedure to replace these bushes, there is no need to remove the road wheel although careful and accurate use of a trolley jack to raise and lower the vehicle (keeping the tyre on the ground at all times to control height) to remove the strut top will be needed.  Just make sure that the brake flexible hose to the caliper is not stretched or pulled during this process.

Mercedes SL R129 Front Suspension mount 2

The only other tricky thing is to replace the protective bellows that are attached to the rubber lip on the lower part of the strut top bush, familiarise yourself in how this is fitted before removing the old bush. It will be a fiddle to re-install, and once you develop a technique you will find it quite simple to do.  I have even read that some people when replacing the damper / shock absorbers too, fit the gaiter on the bench before installation.  This may be worth considering if you are doing a complete suspension job.  The best technique I found was to roll back the top lip of the gaiter all round the top (turn it inside out around) then offer up the gaiter. While pushing it up against the mounting lip, roll over the top lip of the gaiter onto the suspension mount – worked a treat!

A straight forward job that can be accomplished with the minimum of tools in a short time.

Waterproofing Mercedes SL Mohair Soft Top – Fabsil

This has to be one of the most simple and worthwhile jobs you can do.  Since I purchased the SL over Christmas I have run around with the hard top on.  About three weeks ago when we had that first little burst of nice weather I stored the hardtop in the back of one of the Storage Units and have run with the rag top ever since.


I kind of imagined that in the 20 years since the car was manufactured, the roof has never seen any attention for waterproofing.  This was based on the fact that water just soaked into the material and left it looking like a drenched wash leather!  Even though it remained water tight throughout, I decided it was about time to proof the mohair fabric.


I researched a few products and what came out top was Grangers Fabsil in a can.  Other stuff looked to be a little messy and in many cases was an aerosol spray (I didn’t fancy masking off and dealing with overspray on the paintwork) so I opted for a brush-on product.  Fabsil is a silicone based ‘clear liquid’ that is free flowing and dries very quickly without any residue.  In fact, even in instances where I was a little careless with the brush it wiped off the paint without issue and left no marking what so ever.  Even the brush you used will dry out overnight and not harden – this was the product for me!  eBay Link to the Fabsil product in 2.5L Can – Here.


There is a little in the way of preparation to do if you want to do a good job, but I suppose the proofer would work equally well if you just slapped it on – it really is that easy!

I dry brushed the whole roof down to remove any debris and rain ‘wash-down dirt’ that collects in the lip and edge rolls, for this I used a stiff nylon bristle nail brush and worked with the grain.  Once this was completed and all other marks removed and brushed out, vacuuming the complete roof area proved to be fantastic at removing and collecting up all the loosened debris.


Obviously pick a dry day and one that looks like being so for at least three to four hours. Blanket off the windscreen (don’t fancy silicone smears!!) and the boot lid area, tucking the protective cloth under the rear roof lip.


Use an old sandwich box to decant a decent amount of Fabsil and obtain a new or ‘ultra-clean’ 2 inch brush.  As far a quantities used goes, I gave my roof a generous coating, twice, and I used a little under half a gallon of Fabsil.  I have seen this sold in half gallon cans, but my shop only had the larger gallon can, that was in my case, far cheaper than buying its equivalent volume in one litre tins! – Plus once in stock, in the future I can proof anytime I care too !

I read several painful reviews of people brushing on the proofer once in one direction of weave then the second coat the other way.  I personally went for the ‘flood and soak’ approach, to the point where the material was soddened with Fabsil – making any weave brushing just a total waste of time!  It worked perfectly.  There advantage of this method is that by keeping a wet edge to your work with the brush, you don’t get any brush marks whatsoever, and the agent ‘wicks’ into the most hard to get places.  Painting around the windows is a cinch!  Just get to within a couple of millimetres from the seam edge with a loaded brush and let capillary action do the rest. Easy…


After the first coat you will notice how the colour of your your roof darkens to a beautiful ‘as-new’ black.  As Mohair ages and gets an ingrained surface dirt coating from the elements of our poor weather, it tends to go a dark charcoal grey and it really does look fantastic once the proofer has dried and it returns to its ‘as-new’ black finish.  The second coat goes on much the same as the first, taking about four hours to dry totally.


Some of the reviews I have read, beef about the smell involved in using Fabsil. I have found this really, to be honest, unfounded and if you ventilate the car well and let it dry properly, I must say I can’t understand why it is worthy of comment.  It smells slightly like ‘turpentine’, which I suppose is the products base solvent.

Great Job and well worth the effort (not that much effort required) – Its child’s play!


I shall report back once I get chance to take a few photographs in rain, so far we have a couple of ‘dewey’ nights and the water droplets sit on top of the fabric – we have not had that before!

Added pictures of waterproofing after first downpour and hard frost a couple of weeks later…