Components and Assembly
Brake Servo, Foot Brake Handbrake.& ESP/BAS

Warning Hydraulic fluid should not be allowed to come into direct contact with skin, eye protection must be worn at all times when using hydraulic fluid. If exposure does take place immediately flood the contaminated area of skin with copious supplies of cold water, remove any contaminated clothing.
If eyes are contaminated with hydraulic fluid use copious supplies of clean cold water to flood the affected eye's, seek medical advise.
Hydraulic (Brake, Clutch ) fluid coming into contact with your car will severally damage the paintwork, should such contact be made, immediately wash off with cold soapy water.

ESP/ABS icon is showing on instrument cluster when engine is running.
If for any reason you have had the battery disconnected on this car then you will need to re-set the ESP/ABS and if you have electrically operated windows re-set them also.
Re-set ESP/ABS following battery disconnection by:- slowly driving/moving the vehicle,(This avoids damage to the tyres & your tarmac drive-way) turn the steering wheel fully clockwise then fully anti -clockwise and which point the icon in the display should go out.
where the icon displays without the battery having been disconnected, see details below.

To re-set electrically operated windows see mypage 35.

The Brake Servo B and Master Cylinder A are located on the O/S of the engine bay attached to the bulkhead the master cylinder as it name implies has a built in reservoir of hydraulic fluid which it provided on demand to the braking system wheel cylinders of which there is one on each wheel .In addition to supplying the braking system the master cylinder on the 'A' Class also provides hydraulic fluid to the clutch, the thrust bearing being hydraulically operated by the displacement of fluid.

It should be noted that although the hydraulic fluid is utilised in this way it does not result in any loss of fluid. However over a long period of time as brake pads, shoes, and clutch plate wear the fluid level in the master cylinder reservoir will drop slightly.
Where a rapid loss of fluid is noticed the cause must be investigated without delay, failure to do so could result in finding you have no brakes due to the fluid loss.
Fluid low level indicator connection

2 Fluid reservoir

3 Master cylinder
4 Vacuum tube to head light level system

5 Vacuum connection from engine to Servo

6 Hydraulic pipes to ESP/ABS unit .

For this reason it is vital the the fluid be kept at the correct level and being hygroscopic, (Absorbs moisture from the atmosphere) is changed at the prescribed intervals laid down by Mercedes-Benz, although the hydraulic fluid is quite expensive, money is saved on replacement wheel cylinders which last longer if not exposed to the water moisture in the fluid.
Because of the above reason containers or part containers of hydraulic fluid should be kept air tight, if it is intended they be used at a later date.
Hydraulic fluid should never be re-used.

This is a mock-up of the brake Servo and master cylinder, as fitted on the A class, makes you think when you see what is behind the brake pedal?
The 'A' Class Servo may not be internally identical to this one but it will be very similar and does the same job.
To see a demo of the servo working go to the link bottom of this page.

it is interesting to see what comes to light when an 'A' Cass is broken for parts having been either written off or reached the end of it is life for one reason or another.
This is the brake servo assembly complete with the fluid reservoir and all pedal controls.
This unit obviously came from a manual car as the clutch pedal is present.
Remember that on the this car the single reservoir serves both the hydraulic clutch and hydraulic brakes.

Only use Mercedes-Benz hydraulic fluid, DOT4 Plus, although a little more expensive it will be the same as installed when the car was built and it is always unwise to mix different manufacturers fluids of any sort.

So what is the purpose of the Servo what does it do?
The brake servo intensifies the force or pressure when the driver applies pressure to the brake pedal

using negative pressures (Vacuum, pressure below that of atmosphere) on one side if the internal diaphragm, 2 which is achieved by the connection of a tube to the induction side of the engine 3 and atmospheric pressure (pressure above that of atmosphere (14.5 bars 14.7 psi) at sea level on the other side.
With the result that when even limited pressure is applied to the pedal, fluid is indirectly supplied to the wheel cylinders which are activated by the displacement of the fluid, which in turn brings the pads and or brake shoes into contact with the braking surface on each wheel.
Hence the brakes are effectively applied; when the pedal is released the return springs ease the pads, shoes away from the braking surface but only a very small amount, just sufficient to clear the braking surface.
Where the component parts of the braking system are not in a serviceable condition the pads and or shoes may slightly rub on the braking surface when released this can produce what is generally termed brake squeal, this is more common on brake callipers (Pads)than drum brakes.
It will be noted that their is a further connection attached to the vacuum side of the servo 1, this is the vacuum pipe going to the head light height adjusters.

One can best quantify the amount of assistance provided by the Servo when being towed, i.e. broken down or tow started. Without the engine running the amount of effort on the part of the driver to effectively stop the car is quite alarming If you have not experienced this then take great care or ensure that a rigid bar is used between the towing and towed vehicles.

The servo developed in the 60s appeared when bigger and faster cars made a switch from drum to disc brakes initially on the front axle, and more recently the rear axle as well, when the required pedal pressure to operate the brakes was considered to be too high, hence the introduction of the servo, which are now fitted to most if not all factory produced vehicles.

If you have problems with stop/brake lights the switch is in the cabin, located by your feet. If the switch shorts out the lights will remain on all the time the ignition is ON.
The Switch A is activated by the brake pedal action, it being located to the rear of the pedal. There is no doubt that this is a complex switch with a number of moving parts, irrespectively of the way this switch works it is the cause of many problems associated with the ESP light showing and well as causing automatic transmission vehicles to go into 'limp home mode' if you have problems then a replacement switch purchased from Mercedes Parts could cure the problem and at £10.00 is well worth a try .

Defects on this switch include:-
Damaged /broken switch

Sticky action which leaves the white plunger B pressed/jammed in the switch casing, which is what causes the stop lights to fail. The sticky action of the switch can be cured by cleaning the plunger with a suitable cleaning solution. However should this not cure the problem then the switch is relativly cheap at £10.00 and therefore it may be wiser to replace.

In the case of failed stop lights
If both lamps fail to operate Check the fuse.

Remove the switch contact block clean electrical contacts.

Check bulbs, although normally two elements in different bulbs would not blow simultaneously

Check earthing points

Last resort, Replace switch, Part Number MA001 545 20 09 4304 Brake switch £8.78 inclusive of VAT 12/01/2010
Defective Stop light switches will also cause other engine performance problems As it is a relatively inexpensive item under £10 or 13€ it is worth checking the switch before looking for other causes.

Replacing brake light switch procedure
1. Remove the wiring connector from the end of the switch ( press in small tab to release.)
2. Remove the existing switch from it location by pressing the black locking lever B At the same time rotating the complete switch clockwise (to the right) a quarter of one turn to remove.
3. Having purchased your new switch, pull the white spindle A until it will come no futher. You will hear a series of clicks until is is fully extended.(if it is not fully extended when purchased).
4. Depress the brake pedal
Fit the new switch, rotate to the left (anti-clockwise)
5. Release the brake pedal, the switch is now automatcally set.
6. Reconnect the electrical connector.
7. Test( brake lights work without the ignition being turned on.)
For refernce , The plunger when fully extended is 15.75 mm, when fitted and set by the pedal it's reduced to 13mm you can also hear it clicking as the pedal pushes the plunger back into the switch body as the brake pedal is released. (measurements take using a digital caliper.)
it does appear from the comments from one reader that this switch has been modified by Mercedes-Benz at least once.
If you are having problems with the ESP light showing on your instrument cluster, you are advised to replace the brake light switch, or at least check the length of the plunger which should be 15.75mm when fully withdrawen from the switch body, if the plunger on your switch does not meet that length then you are advised to purchase a new switch from Mercedes Parts, (Check length of pluger supplied on new switch 15.75 min.) to replace your existing switch.

These are example of satisfied readers:-

Hi Lofty First of all thanks for your A class website. it is been very useful. A bit of info you may be interested in regarding the brake light switch. My wife has a 2001 A160 full auto. The ABS ESP lights etc came up, and as in your case, the brake light switch was faulty. The other problem with a full auto is the not only will it only rev to two thousand RPM, it won't change up out of second gear whilst in "limp Home mode". Therefore you get 30 mph max! The other thing that may be of interest is that GSF (German Swedish and French Parts )do not stock the switch, so it is a Mercedes, Parts dept trip. However at £8.75 all in, it is a cheap fix., thanks Pete

Hello, I just want to say a big thank you. Based on the info on your site I was able to resolve the problem of ABS light permanently on!
I went to a local dealer here in Portugal and bought the inexpensive switch. Replacing it was very easy and, surprise, when starting the engine again the lights were off! The switch cost about 13?.Best Regards,Gonçalo, Portugal

I am currently suffering with an intermittent problem, in that my ESP/BAS light occasionally come on at present I'm unable to find the precise cause but as a precaution and as it is a cheap item to replace going to replace the stop light switch , it takes minutes to fit and costs under ten ££ so it 's worth a try I will let you know how I get on. 24/06/07

The diagram is looking at the contacts housed within the switch body, the wires are obviously located in the plug connector going to the loom. The only thing I have so far determined is that the stop lights do not work if the main contact plate is removed from the switch housing, So they do play a part in the stop light operation.

This is the array of wires going to the multi-pin socket on the ESP/BAS unit , n/s of engine bay , below the screen wash reservoir. Both Red/Yellow and Red/Black wires finish up attached to the ESP/BAS unit. I am therefore even more convinced that the switch was the cause of my problems.

Having replaced the brake switch I was interested to know if it had contributed to the fault on the ESP/ABS indicator lights, I opened the switch with care and it was immediately obvious that it could have been the cause of the failure.

Double action brake light switch. The switch is double acting (two switches in one) When the brake pedal is operated the white plunger is depressed causing the sprung loaded plunger A situated in the centre of the main plunger C to make contact with the switch plate thereby completing the circuit, in my opinion these are the brake light contacts.
As the pedal is pressed harder so the main plunger B disconnects the second set of contacts.
As the pedal is released one set of contacts 'make', followed by the second set that 'break'. I am firmly of the opinion that these contacts that play a part in the operation of the ESP/ABS system or instrument cluster warning lights.

Thank you to Jan Singer, Communications Manager of Singer Instrument Co.Ltd. for enlarging on the details as to how this switch works, not just for the stop lights but its involvent of the ESP system as well.

1 Shows the switch at rest ...2 Shows the small contacts closed...3 Shows the main contacts open.
1 Small plunger 2 Large plunger A Small contacts B Large contacts.

On inspecting the contacts in my switch the main contacts were in a very poor condition, For those readers familiar with the old ignition points problems , this was an identical problem, one contact had a hole burnt into the contact the other a point, In my opinion neither would have been making good contact when the brake peddle was pressed.

Hand brake operation

Yes very basic information, and yet we can prolong the life of the rachet on the handbrake lever by simply depressing the knob on the end of the brake when we engage the brake rather than hearing the

racket operating every time we use it.

Another point worth noting is that if the hand brake is not fully released then on this car you will get an audible warning as well as the brake light remaining illuminated on the instrument cluster display.

The adjustment, ideally the handbrake needs to be set to give a maximum of five clicks on the racket, any more and you stand the chance of failing the MOT, not enough movement and the self adjustment feature on the rear brakes will not operate.

Before adjusting brake cables (Bowden Cables Flexible steel cables housed in an outer flexible protected sleeve) always check the self adjusting mechanism on the rear wheels, if these are not operating correctly there will be more movement of the handbrake than there should be. This can be checked by removing the rear wheels and manually adjusting the ratchet through an aligned wheel bolt hole, see mypage 19 for details.
Alternatively jack up the rear of the car, follow all safety rules see mypage 5, and pull the handbrake on two notches, turn the rear wheels, if you can feel resistance then the adjusters are fully functional, you should not be able to turn either rear wheel when the hand brake is fully applied, and check to see that the balance of resistance is the same on each wheel, If you can turn either wheel when the handbrake is fully on remove the drum and check the wheel cylinder for leakage and the shoes /pads for contamination from the leaking fluid.
If having done that the handbrake is still sloppy, then there are cable adjusters just forward of the petrol tank, BUT make sure you only adjust a small amount at a time and tighten both sides evenly. remember to re-tighten the adjuster locking nuts.

In extreme cases the handbrake cables may need replacing, remember that the under body floor panels have to be lowered the exhaust system removed back to the catalytic converter and the arched floor insulation panel removed before your can access the handbrake mechanism.

Safety Features

What Mercedes have to say about the following!
ABS/BAS The anti-lock Braking System(ABS) prevents the wheels from locking. This means the brakes can get a better grip, and the vehicle remains steerable, even on ice or in the wet. Brake Assist (BAS) helps you when you need to stop the vehicle as quickly as possible, detecting an emergency braking situation by monitoring the speed at which the brake pedal is depressed. It then quickly applies maximum braking force.

ESP/ASR The Electronic Stability Programme(ESP) helps to stabilise the vehicle in critical situations with precisely controlled braking of one or wheels. Sensors continuously measure speed, lateral acceleration and yaw (rotation of the vehicle about it is vertical axis. On the basis of this information the system can access whether any intervention is necessary to stabilise the vehicle. Acceleration Skid Control (ASR) prevents wheel-spin by selective intervention of the individual wheels.

If you have an ESP/ABS light lit on your car then it can be caused by many things, sensors can go defective as well as wheels heavily curbed can be misaligned . even different tyre sizes can can cause the light to illuminate, when looking round a prospect purchase just glance at the wheels and ensure they are the same size ,
The abs ESP fault was due to an incorrect rear wheel size, that was free....x. Unquote
Different wheel sizes on the same axle/car is unforgivable, and with a car with ESP/ABS you will without doubt get a warning light telling you so. Even worn bearings on the trailing arms will cause the Warning light to show, so if you have this problem on your car be thorough and do your home work first!

This ABS/ESP unit is located beneath the screen wash reservoir, there is little the DIY enthusiast can do with this unit but if its failure is associated with sudden and accounted for loss of screen wash fluid then check to see if the container has developed a leak and as a consequence contaminated the various electrical connections or blown a fuse that protects the BOSH electrical motor the powers this unit. It is remote but it is also possible!
The ABS/ESP system is and integral part of the brakes, and includes the ABS. The fluid in the system simply uses the brake fluid which is DOT 4 plus, Mercedes part number, 000 989 08 07 10.

When filling your screen wash reservoir take care not to flood the ESP/ABS unit and motor, quite easily done as the unit is directly under the reservoir. On the back of the ESP/ABS unit there is a large connector similar to the one on the ECU. This is a multi pin connector that is particularly venerable to water damage from overfilling the screen wash reservoir, which could well cause severe problems in the long term due to the corrosive properties of the screen wash additive.
I have protected the plug connection with a polythene bag held in position by an elastic band , One again a question of prevention is better than cure.
see below.

When working on your car, within the wheel arches, take care to ensure you do not damage the wire runs to the ESP & and low pad sensors, if removing these from their clips
1 and 2 ensure they are replaced before completing the job.

It is worth noting that after taking the photograph above it was noted by looking at the picture that the wiring going to the ABS detector head was in fact dislodged from it clip, I was able to replace it without taking the wheel back off.

This feature seen on the drive shaft hub end, is part of the ABS(Active Breaking System) and is there to enable the detection head which is fitted just above it 1 the monitor the speed and motion of the wheels. In the event of wheel spin the breaking system will act on that wheel to correct the spin.
There appears to be no maintenance that the D.I.Y owner can do accept to keep the jubilee clip like feature which is of course the transmitter which is continuous around the shaft, clean. An old tooth brush is as far as I would go for fear of damaging the mechanism. Further information received from Nick enlightens us further as to the roll of the unit.

This is hard wired to the ESP control unit as are all 4 wheel speed sensors. The ESP control uses this information to control the "anti-lock brakes" and the "traction control" operations, albeit from one control unit. The control unit monitors the wheel speeds constantly The ESP control unit sends the wheel speed data (amongst others) digitally around the rest of the vehicle for the use of other control units. For example; the instrument cluster uses the speed signal to display the speed and the central locking uses the speed signal to operate drive-away locking (when enabled), the cruise control uses the speed signal to ensure the car maintains a constant speed when going up or down a hill;

Another issue I've had with my B Class which I assume may manifest itself on the A Class as well is replacing Drive shaft gaiters, Boots, bellows which ever you choose to call them!

Note that there have been a number of ESP/ABS problems caused by the detector ring item 2 above on the drive shaft coming loose as well as breaking, from the point that the ring moves or breaks the ESP/ABS lamp will show , this will cause your car to fail to MOT so check you drive shafts for rusting which can be the cause of the problem, spray with duck oil or 3 in 1 oil anything to prevent the rusting and the problems that result.

A Bellows/Gaiter. Protecting the moving parts of the UJ and roller bearings etc, should be inspected prior the MOT for splits which can cause the car to fail.
B Drive shaft. A very substantial unit, I have not heard of any problems with these units.
C Detector head, ESP/ABS Speedo/Central locking (while in motion) Traction control, A multi functional hard wired detector monitoring all of the aforementioned features on this car. Relays information back to the EMU, picking up its signal from the transmitter. E
D Drop link. Part of the vehicle stability feature. Vulnerable to wear.
E Transmitter.

This intermediate bearing in positioned approx half way along the length of the O/S drive shaft. I am unsure at this point as to whether the drive shaft splits at that point or is merely supported. The red seal that can be seen is an oil seal integral of the intermediate bearing housing, which should be free of any sign of leaks.

Having taken a much closer look at the O/S drive shaft I am now convinced that the shaft is split, the outside shaft going from the stub axle (wheel end) to the intermediated bearing & housing, the inner shaft acting as a lay shaft, from the intermediate housing to the gearbox. If I am correct the the outer shaft will have a bellows & Universal Joint (UJ) at either end, the outer end will have the stub axle attached. However with no point of reference for this type of information I could be wrong, so If you have any different views or have changed the O/S drive shaft I would appreciate an email from you and will of course credit you with any information supplied.
Thanks Nick Of MB Chichester West Sussex, for some of the technical information contained on this page, your help is appreciated.

Learn more about Braking Systems, including ESP, examples of various associated braking components with demo.(Click on the heading below the various components for demo),

Oh well none of us are exempt for the odd problem, I have had to follow my own advise today 17/06/07. Got he car out on Thursday and the spotted that my ESP/BAS light was lit while I was driving along. Electronic Stability Programme and Brake Assist System> I stopped the car switched off the ignition, re started the car, no lights so was it just a blip? No same thing happened on Friday and Saturday Stop the car re start no lights. Never the less there had to be a cause as this is not normal?

So today I have removed the multi pin plug on the back of the ESP (mypage 18) unit and sprayed the contacts with 'Electolube' (Electrical contact cleaner) and also checked the wires in the wheel arches for damage and progressed on down to clean the sender units on the drive shafts. There I think I found the problem lodged in on of the grooves in the sender unit ring was a piece of debris, I think stone chip that was blocking the groove almost completely. I removed that and cleaned the sensors both sides as well and doing a bit of other cleaning and applying duck oil and No lights on two further trips, so with everything crossed I hoping I found the problem. Thinking about it it is logical that it could have been the cause because the detectors would have been receiving different signals as the wheels turned. bearing in mind that it is the grooves that the detector acts on. I will keep you informed but hope I have cured the problem.

Having cleaned the front sender units I decided the following day to check the rears, The off side unit was completely rusty so much so that you could barely see the grooves in the casting.

However cleaned up with an electric drill fitted with a brass wire brush it came up like new I lightly coated it with duck oil before returning it to the hub, The near side unit was in sparking new condition so no cleaning was required.

The procedure for removing and checking the hubs on which the sender is cast is as for replacing rear brake shoes or Trailing arm bushes. In my opinion either of the problems located , the stone chip in the front near side or the severally rusted unit on the rear could have been responsible for the false signal given by the car, with both of these located and corrected it will be interesting to see if there is a reoccurrence of the problem. Chances of a garage finding the cause? Well Sir we will have to put it on the diagnostics and we want the car for half a day!!?? And the chance of the car being returned without the fitting without new units and £££££££ for labour NIL. So if you have problems do not write the car off take a look round it really does pay dividends.

Other components liable to cause the ESP/ABS warning light to show are worn trailing arm bushes which affect the alignment of the rear wheels and worn steering column. Both of these items need to be checked if you have a problem with these warning light remaining on when driving your car. But do not forget the brake switch which is cheap to replace and is less of a problem to inspect and fit than the other two items mentioned.
ESP/ABS light fails to go out
Other consideration, if you have checked the other less complicated a costly items then check to ensure the the tyres are the same size, If one wheel effectively turns faster than the other 3 the ESP/ABS will pick it up and alert you on the dash
The other main thing to check is tracking, bearings & bushes the later being on the trailing arms, if these bushes are worn allowing the wheels to slightly wonder from side to side as the car is driven, this will cause problems with the ESP/ABS warning lamps staying lit as it effects the tracking of the rear wheel's

This e-mail from a fellow owner clearly indicates the sophistication of the 'A' Class ESP/ABS and it appears is even more complex in the earlier models:-

Just to let you know that I have finally got my BAS/ESP light problem solved - unfortunately it was not a cheap process so if anyone else can learn from my misfortune and save themselves some cash then so much the better.
So to recap, my particular BAS/ESP problem showed itself it quite particular ways. I think there are many causes of this light and if the way it shows itself is different to mine then the solution is probably different to mine.
- It is intermittent (Ie sometimes first brake press of journey sometimes not at all) - It can happen while the car is stationary (Ie handbrake on but engine running) - It only happens when you first touch the brake pedal (Ie it might come on when you first start braking on a hill/slip road, never half way through) - if you detach the brake pedal switch it can still happen
So it went to Mercedes who put it on the computer and worked out it was the brake servo pressure switch. Unfortunately on my particular model (Y reg A160) you cannot change just the switch but have to replace the whole servo - ouch !!!

In conclusion if you see the BAS/ESP light under exactly the same circumstances as me then I am sure there are cheaper ways of changing a pressure switch than have Mercedes change the whole servo.
Please feel free to add this information to your excellent site. Thanks,

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