Technical C Class W203

Mercedes C Class (W203) Alarm Problems

If your Mercedes C Class alarm starts to behave strangely with random false alarms sounding, indicators flashing without any alarm sound or even chirping whilst you are driving, then the number one cause for these issues is the siren module.  The siren module is a round black plastic canister about the size of a small bean can, containing an alarm sounder, control electronics and back up alarm system batteries.  It is these internal NiMH batteries that will start to age and deteriorate, giving rise to the conditions stated above.

Mercedes C Class Siren Sounder Battery Replacement 1

The problem with the batteries is not quite as simple as it sounds.  What happens is they start to leak and spread over the PCB inside the sealed alarm unit.  The electrolyte from these cells is conductive, as it spreads, it bridges circuit tracks and components making the alarm system unstable and causing it to do peculiar things.  A replacement sounder is around £100 from a dealership and once fitted will in the majority of cases, cure any problems you may have.

Mercedes C Class Siren Sounder Battery Replacement 2

The first thing to do is locate and remove the faulty sounder.  On the C Class (W203) it is located behind the plastic protection panel under the left hand (near side UK) front wheel arch.

Simply remove the road wheel, remove the five plastic 10mm nuts and remaining fixings that secure the inner protection panel. Remove it from under the wheel arch.  You should now be able to see the siren, fitted to a steel bracket that is fixed to the bodywork.  The torx self-tapping fixings that hold the siren bracket to the vehicle are security types with the inner peg.  If you don’t have the correct security tool, just tap them round slightly on their outer circumference to loosen with a small chisel or punch, once slack, they will usually remove with firm finger pressure.  The rearmost fixing is a strange 8mm crimped nut affair, that quickly removes with pliers. Once the support bracket is free, the siren can be unplugged and totally removed from the car.

Mercedes C Class Siren Sounder Battery Replacement 3

To replace the siren simply plug in the new unit with the car unlocked and alarm de-activated, re-fix the bracket and build up the inner arch panel. Fitting the wheel and lowering to the ground to complete the job.  If you are doing a straight swap-out with a new component and you have good elevated access, it is possible to replace the siren unit without removing the road wheel – by just undoing the trailing edge fixings of the plastic arch liner and pulling them out of the way while you remove/replace the siren unit.

Replacing the batteries in the siren module / sounder unit

If you have been following this site for a while, you will have probably realised that there is a little more on the subject to come –  there is an option that could save you some money and get your alarm working correctly again, for around a £15 chance investment with some DIY time and a soldering iron.  This style of siren module/unit is fitted to many models of Mercedes vehicles spanning many years and this repair technique is applicable to all instances where this type/style of sounder is used.  If you are interested then read on…

Mercedes C Class Siren Sounder Battery Replacement 4

The first thing to be aware of is that the batteries, as they leak, do start to corrode the PCB within the siren unit, if you do catch this in time (and it really has to be quite bad to be unsuccessful) then you can repair the unit very economically.  If however the PCB has deteriorated ‘just too far’ or there is another fault in the unit, you will have probably wasted your money and time, needing to buy a replacement. If its worth the risk of chancing your luck – that is up to you!  Interestingly, if you can live without any audible alarm at all, then you can just leave the siren unit disconnected and tape up the connector. This has no detrimental effect on the rest of the alarm system or its linked components.  My logic in all this is:  If I can fix it for a few pounds then all well and good, if not then leave it disconnected as £100 dealer charge for new or buying a used unit that is probably almost as bad inside as the one I am removing, albeit at the moment still working, are not in this case acceptable options. Equally a customer may decide that the cost to repair a sounder on an older vehicle is just not worth it – so you can just leave it disconnected!

Mercedes C Class Siren Sounder Battery Replacement 5

If you decide to have a go at changing the batteries you will need to remove the sounder from its bracket, again a single security torx fixing is the order of the day. Once this is removed the sounder can be cut open.  It is ‘weather-sealed’ and the only way to get it open is to cut carefully around the seam with a junior hacksaw, rotating the sounder as you go.  Do not allow the blade to penetrate deeper than half its width, as if you cut too far inside you may damage the PCB or sounder wiring.  Once the cut is continuous around the siren unit, pull apart the two halves.  The front part will contain all the electronics with sounder, the rear is just a shell cover with an aperture for the electrical connector pins.

Mercedes C Class Siren Sounder Battery Replacement 5

Unclip the sounder plug, pink and white wires inside and unclip the PCB pulling it off its two locating pegs.  It should now be out of the plastic body and you will be able to get a full visual idea of how bad the PCB has been damaged by the battery leakage.  The one pictured is quite bad and to be honest if it was any worse than this I would not go further, so in this instance it was a borderline repair, reducing my odds of it all working out successfully due to the poor condition.

Mercedes C Class Siren Sounder Battery Replacement 7

Note how the batteries are polarised, take a photo so you can be sure you are fitting the new NiCd’s with their correct polarity as once you get cleaning, often the markings on the PCB also get removed.  Using side cutters clip off the old batteries flush with the soldered holes and throw them away.  Using a soldering iron heat the remaining part of the battery pin until the solder melts fully, then quickly bang the PCB onto a cardboard box.  This shocks the molten solder and pin from the board and should leave you with clean holes for the new battery contact pins.  Repeat on all eight holes.

Mercedes C Class Siren Sounder Battery Replacement 8

Now you have the board clear, use PCB flux cleaner spray or other electronic solvent to scrub away the electrolyte that has leaked from the batteries, use an old tooth brush and lollypop stick to scrape the most stubborn material away. You can now get an idea if the battery juices have eaten into the copper track, if it passes inspection, continue cleaning the circuit board with an abrasive pad, I used one similar to those used to clean model railway tracks.  Eventually it should look like the photograph.

Mercedes C Class Siren Sounder Battery Replacement 9

You will need to purchase a pair of replacement batteries from a Maplin store or other electronic component outlet, they are quite a common 3.6v 140 to 160 mAh NiMH cell pack, measuring approximately 24mm x17mm x 15mm  The only issue the replacement batteries have is that the component legs on one side will need slight modification to fit the PCB but apart from that they are drop in replacements. The batteries cost in the order of £7.50 each. Part Number BN-24B

Mercedes C Class Siren Sounder Battery Replacement 10

Once you are satisfied with your cleaning, bend up the single mounting leg/tab of each battery into  a ‘U’ shape as you can see in the photographs.  Fit and solder the twin-tab side of the battery to the PCB, observing the correct polarity.  Once you have this side soldered in, cut a short length of thin stripped wire or tinned copper wire if you have it.  Using a small piece of cardboard packing under the edge of the battery solder one end of the stripped wire into the PCB.  Now loop it over the ‘U’ shape you have formed on the battery tag, threading it back into the board. Pull the wire tightly and solder the remaining end to the board.  Now heat and solder bond the wire to the bent tag as it passes through its formed ‘U’ slot. Clip off any protruding wire from the back of the PCB

Mercedes C Class Siren Sounder Battery Replacement 11

Fit both batteries and check your work for solder bridges etc.  Once you are happy, mask off the siren sounder plug and three prong external connector pins.  Spray the complete electronic board with clear lacquer, both sides and allow to dry thoroughly.  Once dry, refit the electronics to the front section of the siren unit. Connect the red and pink wires to the sounder unit then run a continuious bead of silicone sealer or epoxy resin around the plastic seam of the housing.  Offer up the rear plastic housing section and press it into position.

Mercedes C Class Siren Sounder Battery Replacement 12

Clamp the assembled sounder/siren unit in a vice gently until the silicone/epoxy as hardened. Then for good measure, although really not necessary, tape around the seam joint with insulation tape making a tight secondary water resistant seal.

Mercedes C Class Siren Sounder Battery Replacement 14

You can now refit the rebuilt unit to the car and test your work, either it will work perfectly or your time has been wasted!  If successful then you have saved yourself  a wedge of money, if it does not work then you have wasted £15 and can just leave it disconnected or go out and buy a new one – the choice is yours!

Hopefully all will be well and your Mercedes will once again have a fully functional alarm.


Broken Front Spring Replacement – C Class Mercedes (W203)


Broken Front Spring – C Class Mercedes – W203 and others

It was MOT time again and I confidently took the wife’s C200 to the local Testing Station.  The tester and I were flippantly discussing the benefits of strut suspension over the corroding W210 E class spring and perch arrangement when he detected a problem with my own off side front spring. The spring had recently broken on the final bottom turn of the coil and needed replacement, so off I went fail certificate in hand, to get the job done!

EuroCar Parts were the best and most convenient source of a new spring, I opted for the OEM spec Sachs spring at a very reasonable £35.00 although I could have opted for a far cheaper economy spring should I have wished.

Mercedes C200 Strut

Here is the cast aluminium spring seat, strut platform that the lower part of the spring locates into.

The task of removing the front strut on a C Class Mercedes is in itself far simpler than most strut suspension vehicles around, however the removal and replacement of the spring from the strut is in many cases beyond the scope of the home mechanic (Unless you make a special tool).  Do not think that you can just buy a cheap set of spring compressors from the auto-store and complete this task, as you will most probably fail and at worst end up damaging yourself or other object of value in your working surroundings!  If you are in any doubt of your competencies in carrying out this task I advise entrusting this one to an independent repair shop.  But if you are ‘gung-ho’ read on.

For those who are interested in tackling this job, here we go….

Slacken the road wheel bolts and jack up and support the vehicle on the working side, the forward jacking point is the perfect position to lift for this job as it leaves the suspension in free air which is ideal.  Remove the road wheel and lift the bonnet/hood.

Unclip the brake pad sensor wire and ABS cable from the black plastic holder on the suspension leg, note that both wires run beneath the hub carrier bolt to their sensors, this will be important on reassembly.  Use side cutters and snip the single tie-wrap that secures this bracket to the bottom of the leg.  Push in the upper and lower latch fingers of the bracket and rotate it around the suspension leg, once free move it out of the vicinity upwards of the flexible brake hose.

Using a 15mm spanner and 21mm socket almost remove the upper hub carrier to leg bolt. Prize off the black plastic cap on the suspension strut top in the engine bay then using a 21mm ring spanner and a 7mm Hex key wrench undo fully the top nut and remove the retaining bracket.

Mercedes C200 suspension strut

Here you can see the new spring installed, note how compressed it is. Note the new white nylon tie-wrap holding the cable retaining bracket on the lower section of the strut.

Place a support under the brake rotor/disc (a wooden block is perfect for this) and then lower the vehicle to take the weight on the support.  Lower the vehicle enough to allow easy removal of the upper roll bar link ball joint from the suspension strut. This is a 17mm spanner job with a 7mm socket to hold the central threaded portion from rotating.  Once removed, free the drop link from the strut and pull it to one side.  Raise the vehicle again to working height.

Undo and remove the bottom strut to hub carrier bolts with a breaker bar and socket (2x 19mm), now completely remove the upper carrier bolt and pull the hub outward from the vehicle. The top lug on the carrier should now disengage and allow you to apply downward pressure on the hub and release the complete strut from under the car.  Be careful not to damage the sensor cables when manoeuvring out the strut.  It will easily come downward and out if you direct the lower end toward the passenger foot well whilst holding down the hub assembly and lower arm.  You should now have the complete strut out from the vehicle.  Have a drink of tea… you will need it.

If you open the large blue box that holds the new Sachs spring you will wonder if they have supplied the correct part, as it is just soooooo.. long!

Broken Mercedes C200 Spring

The C200 Spring is a seriously long spring uncompressed and takes a great deal of careful compression to fit. You can see here the broken section.

The strut itself has aluminium casting that platforms the base of the spring and a pressed steel top cap and a rubber top bush that sockets into the suspension cup of the chassis.  For future reference it is good to familiarise yourself with the orientation of this rubber top mount with the shape of the cup inside the wing top. The broad rubber locating flute is inboard and thinner one outboard – useful to note before reassembly.

If you have access to a set of expensive twin yoke spring compressors that is great, if not read on..

The next step is to make a ¼ inch steel plate that saddles the strut underneath the alloy support spring platform and extends a couple of inches out either side, drill two holes to accept the threaded portion of your standard spring compressors.  Latch the upper hooks as high up the spring as possible on either side and use a couple of small hose clips (jubilee clips) on the spring coils, tightened to prevent the compressor hooks slipping down the coils as you compress.  You will need to compress the spring almost fully, until it is nearly coil-bound to be able to remove the bush and top cap – ensure that you are comfortable with your construction and that it is as safe as it can be, the fitted spring is and will be under extreme tension as you proceed and it needs to be intrinsic for your safety and well being.  Fully compress the spring.

Once compressed, use a small screwdriver or thin drift to tap round the locking collar on the threaded portion of the strut just as it exits through and seats just inside the Metalastic top bush. You may need to hold the strut from rotating with a 7mm hex key. The strut cartridge is biased to retract, which does not help either disassembly or reassembly, so you will need in some instances to lever the top bush slightly out of the upper spring seat to carry out these operations.  Once the locking collar is removed you can lift off the top bush from its taper seat on the strut shaft.  The spring can now be carefully uncompressed, ensuring that you balance the release of tension equally between the two spring compressor tools as you go.

Spring compressor sadle plate

The two larger holes are the ones I drilled to accept the spring compressor rods, the others existed prior on the scrap plate I used.

Remove the broken road spring and fit the new one, locating it into rubber slot on the alloy lower support platform. Re-attach the compressor tools and fully compress the spring – use the same vigilance and safety precautions as before.  Once fully compressed, locate the top cup onto the spring and refit the upper bush, screwing home the locking ring on the threaded shaft fully.  Once you are sure that all is well and the spring is seated correctly and the damper rod is secured seated fully into the top bush, begin to release the spring compressors in the same way as before.

Once fully relaxed the compressors may be removed and the assembled strut refitted to the car.  Remember to position the top bush so that the larger rubber locating flute is inboard and the thinner one outboard, this will assist greatly in relocating the strut back onto the vehicle.

Reassembly is a direct reversal of removal.  Congratulate yourself on a job well done !



Mercedes C Class W203 – Dim Multi-Function Instrument Display (MFD) – LCD Panel Replacement

A Common fault with the Mercedes C Class  instrument cluster is a dimming of the orange LCD multi-function display.  This fading often gets worse when the car is hot or has been out in the sun all day.  The usual places to show the fault are the auto gear selection and W/S icons top right and the lower and upper edge, in some cases totally obscuring any information that may be displayed.  When the car is cold, the display often works better for a time and appears of reasonable contrast only to later fail as temperatures rise.


Fading Mercedes Display

The LCD unit itself forms part of the instrument cluster and can be removed and replaced.  The negative is that the LCD unit is not available as a spare part (you have to buy the whole console!) The special LCD is manufactured by Motorola (Part number HLM7804) and is common part used in all variants of the C class dash with the arched speedometer and MFD.

Mercedes instrument pod removal

Mercedes instrument cluster removal

Mercedes instrument cluster removal tools

So what to do… Buy a new speedometer (lots of cash) or a used one (incorrect odometer reading)?  Or swap out the display from the cheapest used one you can buy !

To give an idea of the cost of a used instrument cluster from a breakers they are usually about £65 to £95 GB pounds.  The online auction sites are a great source of cheap parts and if you are prepared to scour the listings you will find a bargain. You have the advantage of being able to choose any similar instrument cluster, not bothering about, model year, petrol or diesel types, engine size, 4 or 6 cylinder etc…  This gives you the edge!  Just buy the cheapest you can find.  I managed to pick one up for £30, advertised with a photograph showing the display clearly illuminated still fitted to the car!

UPDATE – It appears there is a company who specialises in the supply of display ribbon cable kits for various MB models here.  Also there is another company, probably in China who can supply the complete W203 display  with bonded cable here (Also other MB and car models it seems)  I have not used either company but I thought the links may prove useful.

Now to replace the display, this is a simple task and can be done with only a length of welding rod, a small screwdriver and some care.

Mercedes instrument cluster removal - welding rod

How the instrument pod latch works

First remove the instrument cluster from the dash, slide down a length of welding rod into the apertures each side of the binnacle, the rod will push onto a moulded latch mechanism, as it is inserted (about 10 cm in) it releases a retaining clasp. Once this is done, hook into the top of the slot a small hexagon key and pull the cluster gently out from the dash, each side at a time.  Set the steering wheel position adjustment – fully out and down to give you maximum working room.

Mercedes C200 Instrument cluster connector

Mercedes C200 Instrument cluster connector

Mercedes C200 Instrument cluster latching connector

Mercedes C200 Instrument cluster latching connector

Draw the pod forward enough to disconnect the multi-pole connector from the rear.  This has a grey lever that has to be unlatched and arced down ward to allow the ‘lever-lock’ removal of the connector. Once this is removed the pod can be slid out of either side of the gap between the steering wheel and instrument cowl.

Mercedes C200 console removed

Mercedes C200 console removed

Make yourself some space and use a soft cloth to put the instrument cluster on.

Carefully release the 2 clips from bottom of the rear of the cluster, pull the cover slightly to just release it from its latches, now do the same with the 3 top clips. The central clip at the top has a warranty label over it that will have to be cut/torn to allow the two halves of the housing to be parted.

Instrument cluster control PCB

Instrument cluster control PCB

Once removed, the back panel can be placed out of the way. Revealed is the main control PCB and orange multi-function ribbon flex connector to the LCD display. This orange flexi-strip is removed from the connector by carefully pushing down the 2 black ears either side, about 1mm, down and away from the connector, this releases an internal clamping mechanism allowing you to withdraw the ribbon flex. Once the flex is released, turn over the instrument pod and remove the instrument chassis from the lens unit.

Remove the LCD flexible connector strip

Remove the LCD flexible connector strip

Grasp each side of the ‘D’ shaped frame that surrounds the display and un-clip it, each side in turn from the front face of the cluster. This will reveal the thin glass LCD display.

Removing the MFD Bezel

Removing the MFD Bezel

Now push up from the bottom edge the LCD glass towards the centre of the dial face, push it just enough to clear the white plastic retaining clips at the bottom, then lever the LCD up and out of the back light holder.

Push back the Motorola LCD unit, then lever up to remove

Push back the Motorola LCD unit, then lever up to remove

The orange LED  illumination area behind the LCD display

The orange LED illumination area behind the LCD display

Strip your ‘donor’ instrument cluster and remove your new LCD unit as described above.  Fit it to your vehicle’s cluster and rebuild it in the exact same manner as you took it apart (again as outlined above) Refit to the vehicle and test.  You should now be able to enjoy a nice bright, high contrast MFD unit !

A nice bright, high contrast MFD display

A nice bright, high contrast MFD display

Just a couple of points to note during this task:  Always make sure that the white loom connector to the back of the cluster is as fully home as possible when re-connecting –  this plug will often engage half way and give the misleading impression that it is fully inserted and all is OK, the lever even moves down to its locked position!  However If it is not fully home the instrument cluster will appear totally ‘dead’ when you turn on the ignition.  Dont panic! just correctly insert the plug, latching it fully home and try again.  The slight downside is, what will have happened when you turned on the ignition with the cluster disconnected, the SRS system will have flagged a fault on its self test (SRS Indicator fault) and will have raised an SRS fault code.  This will have to be reset with a Mercedes OBD scan tool or with the dealer Mercedes Star diagnostic tool. Not the end of the world, but avoided if you make double sure the console plug is correctly inserted before turning on the ignition.

This is a simple, easy fix for a fading MFD and one that can save you a great deal of cash.

Mercedes front wheel bearing noise – or is it? – Tyre Choice

Here is a little gem of information that as Mercedes Benz owners you may or may not be aware of regarding a common occurrence that has presented itself many times in my personal experience maintaining and servicing MB vehicles.

C200 tyre noise

Correct tyre tread pattern choice will often improve your vehicle rolling noise and in some cases – A switch to the correct type will often ‘cure’ mechanical worn out bearing sounds!

If you are like me and are ‘in tune’ with the noises your car produces in the normal course of driving, you may be aware that tyre choice is fundamental to the quiet running of a vehicle.  None more so than in the Mercedes Benz range.

I recently test drove a 75ooo mile young, C200 2004 model, in excellent condition and I would have been convinced that the noise from the front off side was a worn wheel bearing in the latter stages of its life.  As usual it turned out to be a poor choice of tyre on that corner of the car.  So many times have new owners of used Mercedes vehicles described an annoying ‘drone’ and are convinced it is a worn out rotational component such as a wheel bearing, when actually it turns out to be nothing more than a poor choice of tyre fitted to the car by someone who does not know any better (or the used car dealer fitted any ‘odd pattern’ of tyre to the vehicle to make its tread depth attractive to the would be purchaser).

The truth is that MB make quiet running vehicles and resonances are often heard that would normally be drowned out by road and engine noise in lesser vehicles – that’s why new owners regularly pick up on the odd noises that play above the serene quiet inside their passenger compartment.

The outline details below are from my own findings and experiences, I am neither promoting or slating any manufacturer of tyre, nor am I suggesting that every tyre of certain types and tread pattern all produce the same noise issues, they may not – however, quietly ask a well informed reputable tyre dealer and the chances are that he will impart to you the exact same recommendations.

The worst offending tyre to generate ‘wheel bearing type noise’ on the front of a Mercedes is the aggressive ‘V’ tread pattern that is often seen on modern tyres.  As a rule the cheaper the tyre – the worse the noise!  Their tread pattern looks very similar to this:

V Tread Pattern

V Tread Pattern


The next high ‘noise’ offender is the Winter or All Weather tyre, with its much smaller tread features moulded into larger segmented tread blocks placed again in a ‘V’ pattern, rather like the one shown below:

Winter  All Season Tyre

Winter All Season Tyre – Block Pattern

Both of the tread patterns above will more often than not give increased rolling road noise and in some cases a very pronounced ‘humming’ noise on all road surfaces, this often gets progressively lower in frequency as you brake to a halt or roll to a stop line.  A far quieter, better choice of tyre would be ones with a more ‘radial grooved’ tread pattern that rings the complete circumference of the tyre as opposed to the noisy ‘V’ or ‘Block’ sections above. See below for good choice tread patterns.


Maxxis Economy Tyre


Pirelli Premium Tyre












The tyres above represent tread patterns that are far more favorable to the quiet smooth running of your Mercedes Benz vehicle, it does not have to cost a lot to get the best from your tyres, just an informed choice of what style of tread pattern you should be selecting at purchase. Past experience with both the ‘E Class’ ( W210, and  W211 Chassis ) and ‘C’ Class model ranges has proven this time and time again. Often if I suspect that there is a tyre problem or the possibility of a worn wheel bearing I will quickly switch the tyres front to back if they are a different more forgiving tread pattern, to see if the noise changes or disappears.  It often does, and by changing the rubber set up on the car will frequently cure the suggestion of any other mechanical defect.

Try it out for yourself if you are in any doubt, whats to lose? – it could save you some money in unnecessary garage bills !