O/S/F wheel intermittent braking without pedal being depressed around 70 mph

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Hi Guys im tony new to this forum.My Mercedes c class w203 220cdi 2004 has developed an alarming braking issue. Upon start up and drive the braking sytem is fine with no warning lights ,once i get speed up to around 70mph after taking a right hand curve the o/s/f brake starts to grab and releise .the best way to explain it is almost like the abs is working in slow motion through that wheel only ,to the point that you can feel as if that disc is warped.Once you stop the car and set of again the problem has cleared itself until that speed and is reached again.The brake grabbing is quite harsh enough to blue the disc in a short distance. I have run diagnostics with no faults found.Im suspecting the N/S/F front speed sensor. Any one had this before
 

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I do not believe the ABS systems of that era are able to add any additional brake pressure themselves, only remove/reapply in an ABS event. Thus, if you are not pressing the brake pedal yourself at the time this happens, I suspect this is not related to ABS.

I'd be suspecting a sticking brake caliper that only drags enough for you to notice with speed and heat. Suggest jacking that wheel off the ground, and checking for dragging brakes, play in the wheel bearings, even loose wheel nuts.

If, conversely, this system *is* capable of (and indeed doing so) applying brake pressure uncommanded, then I would be looking at related sensors that the ABS unit needs to know you are braking, brake pedal light switch, brake pressure sensor - presumably in the ABS unit, etc.

Martin.
 

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Run a live data session monitoring the speed from each wheel sensor. You'll probably need an assistant. If they all read the same then it's not a wheel sensor causing ESP to kick in.
 

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I`d check the wheel bearing and caliper on that side. Take the wheel off push back the pads, they should move easily and check the wheel bearing for play.
 
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I do not believe the ABS systems of that era are able to add any additional brake pressure themselves, only remove/reapply in an ABS event. Thus, if you are not pressing the brake pedal yourself at the time this happens, I suspect this is not related to ABS.

I'd be suspecting a sticking brake caliper that only drags enough for you to notice with speed and heat. Suggest jacking that wheel off the ground, and checking for dragging brakes, play in the wheel bearings, even loose wheel nuts.

If, conversely, this system *is* capable of (and indeed doing so) applying brake pressure uncommanded, then I would be looking at related sensors that the ABS unit needs to know you are braking, brake pedal light switch, brake pressure sensor - presumably in the ABS unit, etc.

Martin.
Hi Martin thanks for the reply .this event first happened on way to MOT.wheel bearings fine,brakes caliper not sticking or dragging wheel nuts tight brake rollers identified no faults perfect braking.with no warped discs the braking sensation that occurs is definetly pressure being given to the caliper via brake fluid.brake light switch ok.will check pressure sensor and speed sensors and yaw sensor .it would be good to know the resistance or the relavant sensors so i can check with multi meter to se if they are within tolerences . Tony
 

M80

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If traction was kicking in die to a faulty n/s/r sensor the yellow triangle should be flashing.
Does the triangle illuminate and extinguish at start up?

Another way to test the loss of traction warning is to go on loose ground and spin the rear wheels.
 

EmilysDad

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not that I'd still be able to help but does this car have SBC brakes? If so, could that have something to do with it?
 
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If traction was kicking in die to a faulty n/s/r sensor the yellow triangle should be flashing.
Does the triangle illuminate and extinguish at start up?

Another way to test the loss of traction warning is to go on loose ground and spin the rear wheels.
Hi Martin the traction control is working fine ,this i have tested im begining to think its to do with the stability control kicking in as it only occurs once i hit 70mph on a right hand bend then keeps stabbing the right front brake until i stop and switch off after restart its fine again.I have also tested this on a left hand bend at all speeds and the problem does not arise. Tony
 
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LostKiwi

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It may be the yaw sensor playing up. Again a live data session will tell all.
 
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It may be the yaw sensor playing up. Again a live data session will tell all.
probably best option, think i may buy the correct reader any recomendations ?
 

onefortheroad

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Does it happen when driving with less haste ? On same right hander . My e350 can take the horrors if I swing right then hard left leaving roundabouts . In sports mode dropping to 2nd gear wearing a white helmet with blacked out visor ...
 

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Hi Martin the traction control is working fine ,this i have tested im begining to think its to do with the stability control kicking in as it only occurs once i hit 70mph on a right hand bend then keeps stabbing the right front brake until i stop and switch off after restart its fine again.I have also tested this on a left hand bend at all speeds and the problem does not arise. Tony

I have seen something very similar on a c. 2014 Ford Focus, and on that, it turned out to be the steering angle sensor that was faulty. Best guess is the ESP gets an incorrect signal for steering angle, believes the car is understeering as it is not achieving desired steering angle, and thus applies some brake input.

No happy ending alas in this case as the sensor was part of a hugely expensive EPAS steering column.

Another thing worth looking at in live data if you are able.

Martin.
 

M80

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Best guess is the ESP gets an incorrect signal for steering angle, believes the car is understeering as it is not achieving desired steering angle, and thus applies some brake input.

Wouldn't that initiate the traction warning also?
 
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I have seen something very similar on a c. 2014 Ford Focus, and on that, it turned out to be the steering angle sensor that was faulty. Best guess is the ESP gets an incorrect signal for steering angle, believes the car is understeering as it is not achieving desired steering angle, and thus applies some brake input.

No happy ending alas in this case as the sensor was part of a hugely expensive EPAS steering column.

Another thing worth looking at in live data if you are able.

Martin.
That is worth looking at
 

MJJ

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Wouldn't that initiate the traction warning also?

Logic would suggest that, yes, but it did not on the Focus I mentioned. One of the reasons why diagnosing was so hard.

Martin.
 
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Thanks for all the responses guys im looking at steering position sensor and yaw sensor will post results
 

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Any form of ESP initiation and the warning triangle would flash and the brake pedal would pulse followed by an attempted correction at straightening the car which you would feel through the steering, the car would also lose speed as the brakes were applied.

If the yaw or steering angle sensor were faulty, they would illuminate the ESP light as soon as the vehicle moved for than 50 yards. If there are no warning lights, there are no faults electrically with the system and you are looking at a mechanical fault, wheel bearing, callipers and excessive play in a suspension/steering joint would be the suspects.
 


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