W202 Brakes pedal stuck up then brake Zinging when it releases!

stuboy

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When i put my foot on the brake sometimes about one in 5ish the brake pedal sticks then as i apply more pressure it eventually gives and the brake makes a zinging noise, like the ABS working.
Here's the background info:- I had to do some work on the brakes to get it through the MOT all was well except the ABS light was on (sensor to far away on front right wheel) some idiot (err that would be me) forgot to do up the ABS wheel sensor bolts tightly so had to pull it all apart again to do them up fully. After completing the job I now get the zinging, I have subsequently taken the caliper apart and all seems to be freely moving, I have taken out the sensor cleaned the castlated part of the hub and repositioned and cleaned the sensor. Just taken it out and its exactly the same. What could be the difference from before and after???????? I'm puzzled :confused:
Anyone had a similar problem?
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Stuart
 

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Never heard od this Stuart, the fault must be it the servo or on the pedal I would have though,,not easy
 
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stuboy

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Zinging Brake more info

OK I had a brain wave and disconnected the wheel sensor for the front right and hey presto all is well again with the brakes however the ABS/BAS light is now on, so it seems a faulty wheel sensor is possibly to blame. I am going to stick an oscilloscope on it spin it and I'll report back :D At least I'm getting somewhere.....If only i knew where....lol:mrgreen:
 

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If a sensor is faulty a zinging sound may be heard as the system re sets. I use very sharp probes that pierce the cable to make contact with the scope
 
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stuboy

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Beware idiots at work!

Checked signal on scope, mmm bit low i thought, it should be about 4 volts peak to peak on a manual spin, then all becomes clear.
Some idiot (sadly me again!) put the disk deflectors on first and then the sensors on top, both sides! The moral of the story take pics and make sure you reassemble the same way as they came off!
The deflector plates are about 1mm thick not a lot you might think but the sensors are literely about 0.5mm from the castleated part of the hub so that 1mm is 200% to far away, reassembled and suprise suprise all is now back to normal.
Let that be a lesson to you ...., no hang on I mean ME :mrgreen:

By the way its very easy to test the sensors without peircing the cable:-
Jack car up till wheel just comes off ground that is enough access to get pointy pliers in to release the metal clip from where the connector enters the top of the wheel arch. Then you will need what electronics engineers call a 3 way 0.1" female header.
Its just a socket strip with 2.54mm spacing and pins on the end for attaching to a PCB like this:-
http://www.harwin.com/M20-982/M20-9820306-product.html

A 3 way one I found plugs in nicely to the connector making measurments easy. hope all that helps someone else in the future :rolleyes:
Regards
Stuart
 

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I have an ABS fault on my V70R you go over a bump and you hear this twang and the lamp comes on,,it the little control box dry joined.


Surely when you spin your wheel, the voltage stays the same and only the frequency changes
 
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stuboy

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Reply to above

No on the Merc its a passive sensor, I would think its a Hall effect device, so no movment equals no volts!
Stu
 

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But the output from a hall sensor does not rise by much, as a pulse is a pulse and no more than that
 

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On mine (124) they are a coil around a magnet, this will give an output voltage which is proportional to the rate of change of flux, so more speed = more volts as well as higher frequency. Id have to check, but somewhere at the back of my mind its about 0.5 - 1v on a DVM for a good 1 rev/sec spin of the wheel.
 

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On mine (124) they are a coil around a magnet, this will give an output voltage which is proportional to the rate of change of flux, so more speed = more volts as well as higher frequency. Id have to check, but somewhere at the back of my mind its about 0.5 - 1v on a DVM for a good 1 rev/sec spin of the wheel.

Even then its accumulative, the voltage will only rise by the effect of the time it takes for the last pulse to decay, what is left of the previous pulse will be added onto the next pulse giving the effect of a higher voltage that is not important as it is the pulse that is counted
 

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Sorry Malcolm I dont agree with the possibility of any accumalative effect. The pulses from these probes are bi directional one pos, one neg adinfinitum as the magnetic field rises and falls, that should mean they cancel out any standing voltage, I have not studied the input circuit of the ABS controler but it is normal to apply these probes signals to a comparator to clean up and condition them for the subsequent stages of processing.
 

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I did say in post 8 that any rise in output would be minimal but yes a slight increase with speed. anyway it is not the voltage here that is measured or needs to be, The output from these sensors is almost a sine wave. The ABS pump contains a comparator reference oscillator clock that all 4 wheel pulses are compared against.
Inductive ABS sensors are supplied with a reference voltage of 5volt
Digital ABS systems supply the sensor with reference voltage of 11.2 volts, and the out put from these is a square wave

There are many different types in use
 
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stuboy

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Getting off the subject...lol

I gave the info on voltage as it is that which will let you know if the sensor is ok or not, low volts or no volts probably means its dead or dying.

The W202 does not have 4 sensors either it has 3! The rear wheel sensor is in the top of the Diff, it can sometimes get oil ingress and fail, its a complete bugger to get out as invariably it will have got stuck in. :rolleyes:

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Stu
 

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