W202 Problem - Help Needed

[-V-]

Hello all, we seem to have developed a rather worrying fault with our W202 C200 Elegance (2.0 automatic)

The car starts fine but as soon as you pull away (and reach a certain speed?) the ASR and BAS lights appear on the instrument cluster and the car completely bogs down.

At this point the car will not go any faster than about 20mph no matter how much you press the accelerator pedal. When you turn the car off and on again, it is fine until you again reach a certain speed where it will go into some kind of limp mode. I must point out that the car does not stall or cut out.

We've tried to press the ASR button to turn it off but it does absolutely nothing.

The car is at a mechanics and they have supposedly plugged it into a diagnostics machine but nothing more was said after that. Correct me if i'm wrong - but doesn't a diagnostics machine usually tell you exactly where the fault is? The garage can't figure out where the problem is.

Does anybody know where the fault could be or they've had something similar happen to them? Is this likely to be an electrical fault?
Everything else on the car works fine apart from this fault that seems to be overriding the normal driving conditions.

If anyone has any ideas, I would be more than grateful if you are able to share them.

Thank you guys!
V

 

[dog68]

Hi, is there any fault codes at all? could be a guessing game untill you put it on star, however this could be battery/alternator related.

 

[-V-]

Thanks for your reply!

So when it's plugged into the diagnostics, it should provide a fault code? I shall be seeing the car later so I will try and obtain this fault code. Would this code generally point in the direction of where the fault is? Is 'star' a different form of diagnostics machine? The garage is said to be a Mercedes authorised garage so i'm not sure if they already use that machine - I can certainly find out though.

If it was battery/alternator related wouldn't the car have problems starting?

V

 

[-V-]

Anyone else able to throw some light on this please?

 

[Dieselman]

The ECU thinks a wheel is spinning, check the ABS sensors and reluctor rings.

 

[-V-]

Thanks dieselman!

Just to add, the garage initially suspected a faulty ECU but after replacing it with a known good one, the problem was still there.

Is there a way of checking which ABS sensor is at fault?

Also, what are reluctor rings?

Thanks!

 

[DrNick]

Star is Mercedes own diagnotic tool.

The cars computer is able to log fault codes when it detects a problem, and these codes give clues as to what may be wrong.

Its not an exact science as some 'faults' may only be logged due to some other fault, so it still needs an experienced techician to successfully diagnose, but at least it points you in the right direction.

SOme problems do not cause faults to be logged - the MAF sensor being a common one. As you've got lights occurring onthe dashboard, I would be hopeful that this causes faults to be logged in your case.

One reason for ASR lights to come on (there are probably many) is that speed signals from the wheels are not realistic ie it thinks the wheels are going at different speeds. This can be due to eg a worn tyre on one side but not the other (so the radius is different).
Or maybe just a speed sensor - below 20mph it may be that the ASR signal is not used?
Or maybe a tooth has fallen off the sensor ring gear on a hub (if this is still the way speed is sensed these days). Star should be able to point at least to the wheel thats causing the problem. It may require a road test with Star plugged in to get live codes

DM got there first!

 

[-V-]

Thanks Dr Nick,

I'll keep the above in mind and pass them on to the garage in the hope of finding the culprit.

Thanks again guys - if anyone has any other suggestions please feel free to add them here and I will update if/when the problem is solved.

Thank you!

 

[esprit200]

Had the same on my CLK320. Brake pedal switch from Mercedes for £13 cured it. Take yours off and test it, 5 minute job and could save you a lot of money.

 

[DrNick]

Brake pedal switch doesn't usually cause a loss of drive - just lights up the dash and turns off the affected functions
(on mine anyway!)

 

[lupi]

it does affect a loss of drive on W211 270cdi.!!

 

[-V-]

Time to bring this back to life again with a little update....

Thank you for all your replies, they came in very handy - but the mechanic who was working on the car was adamant on it being something different each time and the car still doesn't work properly!

The latest part (from his advice) was a new throttle body - the car drove fine, smoother and faultless for longer but then it bogged down, lights came on and it goes into limp mode. The air mass sensor (i think) was the first to be changed and then the throttle body.

They are now looking to go with my original suggestion of it being the 'brake pedal switch'. Car is back with me now so does anyone know where it is located and how it can be changed?

Someone mentioned it being behind the panel under the steering wheel, is this correct? What does the part look like?

Very frustrating time but hoping that this will sort it out. Any help would be greatly appreciated, many thanks!

V

 

[expat]

One very simple thing to try -- disconnect the battery. Sometimes the systems get confused and as long as there is electrical power that brain cloud won't go away. Probably not the solution, certainly try the brake switch, but it has worked in the past.

 

[paris2463]

Brake switch

Its just above brake pedal locks in like light bulb push/ turn unplug connector fit new same way two minute job think you should find another inde merc dealer this one is throwing parts at car hoping to find fault

 

[-V-]

Thanks Expat and yes Paris, exactly my thoughts!

Sounds fairly easy to do so will try both of those and see (hopefully!) that is solves the issue.

Will try and get hold of the part this week and give it a go. Will keep you guys posted!
Thanks again

 

[grober]

Because the fault only appears above a certain speed it almost certainly road wheel related = where the ABS sensors are located. Fault is probably sensor or reluctor [=fine toothed wheel ] as has been said . As far as I know there is a test that can be done on each sensor by spinning each wheel individually and monitoring the waveform produced by the sensor= have they done that?
This is quite a good explanation http://www.picoauto.com/tutorials/diagnosing-abs.html