Xantia ABS Fright

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NiSk
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Post by NiSk »

I have had exactly the same experience with the ABS on my '93 XM TD12. The "no brakes at all" feeling when approaching a junction at low speed. Note - this never happens at speeds above 10 km/h. The fault in my case is a front wheel sensor that has lost contact with the cable (occasionally!). Until I can afford the time and money to put it right, I have removed fuse 10, which is the one feeding the ABS system. So now I have standard non-ABS brakes, which never hesitate at any speed and since I have removed the "silly spring" they operate rock hard directly. In fact, in emergency conditions, non-ABS brakes will stop you faster than ABS brakes (as long as you have road grip) since ABS doesn't allow you to lock all four wheels at the same time (which is how you get maximum braking - mind you without any steering!).
//NiSk
philhoward

Post by philhoward »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Dave Burns</i>

Sticky sensor, no not at all possible, they don't work like that.
They are Hall Effect Devices
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Ah, my mistake...i thought they might be inductive...hence my explination!
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Post by mark_sp »

Okay chaps this seems to be getting a bit heated. My theory is that it was the combination of slow speed and light braking that meant I was only getting braking from the front disks. The warning light lit during the braking (accompanied by a twitch from the brake pedal). Maybe the system then thought it had to release pressure at the wheel, okay it doesn't make sense. The main thing is that it recovered, braking was restored even though the ABS remained in error.
Mark
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noz
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Post by noz »

For the record I'm with Anders nearly all the way on this one. Having said that, I can offer no explanation to those who have experienced 'brake failure' coincident with 'faulty sensors'. The circuitry inside the ECU is riddled with safety circuits and the net result of any of them returning a less than healthy signal is to render the ABS fail safe ie normal brakes.
Can you imagine the lawsuits if what is being suggested here is correct? Mr Bosch will be interested !!
Just to cover all the angles, Jeremy rightly asked if the accumulator sphere could be flat. Al though the answer came back that it was replaced the day before, it doesn't actually answer the question. It is possible, however remote, to have a brand new but nonetheless, empty sphere. A leak in the diaphragm or the filler plug whilst on the shelf will allow the pressure to escape. I think Jeremy's option is valid until accurately discounted.
To verify the accumulator sphere pressure you have 3 choices:
1. Count the time gaps between clicks of the Pressure Regulator
2. Remove the sphere and put it on a rig to test for the pressure charge
3. Replace the sphere with one of a known pressure charge.
If the accumulator proves fine then the problem lies elsewhere and requires further investigation.
Next step for me would be to have the ABS plugged into the computer and the fault history extracted. The history should highlight any failures of any component which would cause the light to come on.
Unplugging or disabling the ABS would certainly work by returning the car to a non-ABS equipped state. However, I find this a strange suggestion from this forum since the entire ethos here would seem to be that everything can be fixed if only you would ask for help. Disabling the ABS seems a bit defeatist to me.
In any case, you would need to reconnect it every year to pass your MOT. If the car is fitted with ABS it must pass the MOT test ie the light must come on with the ignition and go out when the engine is running. Anything else is a fail.
cheers
noz
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Post by NiSk »

The ABS does not work at speeds less than (I think) 8 km/h. This has been confirmed to me by both a Citroën mechanic (who I actually trust!) and a car tester at the Swedish state car testing facility. As I said I think the speed is 8 km/h, but it could be up to 13 km/h - I'm not sure. In anycase, this is the reason why you can test the braking on a rolling road (as the Swedish car test prescribes) and actually lock the brakes - i.e. at speeds less than this level (8-13 km/h) the ABS is non-effective.
//NiSk
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Post by alan s »

To me it isn't any reason to get too excited about, after all it did it the once, and in both Marks & my case it happened whilst we were 'having a fiddle' with a known faulty sensor and if we were experts in the field of automotive electronics I think we'd just shrug it off as experience & say "well next time we'll know better."
In layman's language (all I know & simple at that) the ECU for the ABS I understand does a check on start up this is why lights come on & after a few seconds go off again. In that time I have been told it simply does a reading of values of the entire electronics of the ABS system and upon those readings being within certain limitations, it then switches the warning light off indicating that the ABS system is activated; if the light stays on, it's not. Simple.
Now, in the case of an internal break that is still making contact/circuit of sorts or a sensor with an internal fault of similar nature, the reading on that sensor is down in comparison to the others. There would have to be a tolerance of readings built in there or you'd be forever recording faults, and the ECU will compensate for that diferential in readings within reason. Upon the differential changing as it would with a dodgy conection be it with the wire or the connector to possibly show the wheel is about to lock due to the number of serrations passing over it to appear to be less than actually are, the ECU will need to reset the other 3 to compensate, after all, that's what the ECU is there for to keep it all even (see Dave Burns' explanation) & I feel it is this "compensation phase" that both Mark & I have experienced.
The moral of the story is don't stuff around trying to squeeze a few more miles out of a known suspect sensor because if a sensor is known to be sus it is logical to believe that it won't be sending the pulses as it would when working properly & being computer controlled, it will compensate accordingly which is exactly what it did. I think most of us with ABS cars are aware of the 'no ABS under 10kph' and I know in my case I was well over that speed & I would suggest reading Mark's report, so was he.
You could replace the computer at the same time as the sensor & yes it will work but no better than it will work if the current ECU is left in place because in reality, it did what it was designed to do & you can't expect any more than that.[:D][:D]
I think at times using logic, common sense and experience to diagnose is more successful & is a far cry from getting too technical & trying to diagnose on theory. If we based all problems on theoretic outcomes, we'd all give up driving.
Alan S [;)]
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Post by AndersDK »

Phil :
You're in fact on same track as DaveBurns on the sensor being inductive. Inductive sensors are widely called HallEffect sensors - either being a coiled inductive sensor (pick-up) - or a later solid state sensor with an electronic (inductive sensitive) device.
Noz :
I'm 100% with you in not just "cutting" the ABS. The idea solely applies to a shortterm solution untill practical repair/replacement re-gains a corrrect & 100% reliable ABS function.
Alan :
Mate [;)] - you're trying to excuse the fatal behaviour of the ABS ECU - out of your experiences with a dodgy sensor. But that does'nt count !
A dodgy sensor of course upsets the NORMAL function of the ABS. But never, ever, in ANY combination of possible drive conditions, should a dodgy sensor cause the ABS ECU to behave like this. Not even for a split second.
The ECU CONSTANTLY - all the time - checks the continuity of sensors - and checks their signals for sensible values. Likewise the continuity to the valveblock electrovalve coils are checked - CONSTANTLY.
If ANY abnormality detected - the ABS ECU instantly (within MICRO seconds) switches into non-functioning mode - exactly to prevent dangerous ABS behaviour.
If the ECU then still thinks it's allowed to do frightening & dangerous things - then it's got a mind of it's own - and over right shoulder it must go then [}:)]
There is only 1 person that should have the terminal control over the brake pedal : the driver [8)]
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Post by alan s »

[Quote/]
"Alan :
Mate - you're trying to excuse the fatal behaviour of the ABS ECU - out of your experiences with a dodgy sensor. But that does'nt count !"
Anders, this seems to be the problem; <b>this was the one & only time it ever did it</b> so it HAS to count as I feel the exact same thing happened to Mark.
"A dodgy sensor of course upsets the NORMAL function of the ABS. But never, ever, in ANY combination of possible drive conditions, should a dodgy sensor cause the ABS ECU to behave like this. Not even for a split second."
I agree, but we are talking an intermittent problem & unfortunately, this gets back to what I said; in theory possibly no; in reality, it did happen.
To me this "throw the ECU away" is akin to having a faulty plug on a TV that causes the TV to not operate properly & so throw the entire TV away in case the tube blows in the future.
Based on my own personal experience, I'll lay odds that once the faulty sensor is replaced, there'll be no further problems unless a similar set of circumstances are allowed to recur and after the experience, I can't imagine this happening.
Alan S [:D]
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Post by acrowot »

Mark
In your OP you say you were braking gently from a slow speed, what speed would you say that was?, in my cases it was always under 10km when my fault accured, in all other respects your symptoms seem identical, even to the slight pulse from the pedal and the light coming on. As I said if the light had been on all the time I had no problem. Do you think the garage that said on of the front sensors was faulty had actually tested them, or run a diagnostic on the ABS?. I think the most common fault is normally one of the front sensors due to their working conditions ie wheels turning and consequently the ABS wires also turning, I have not got a lot of faith in what garages tell me. Please let us know the eventual outcome, my money still definately on the ECU.
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Post by uhn113x »

Anders - I am being very brave here......[;)]
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Inductive sensors are widely called HallEffect sensors - either being a coiled inductive sensor (pick-up) - or a later solid state sensor with an electronic (inductive sensitive) device.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not so I am afraid - an inductive sensor is a coil that gives a pulse of current when a magnet passes it.
A Hall effect sensor, named after the bloke that discovered the effect, is a cube of semiconductor material that has a voltage applied to two opposite faces. If you apply a magnetic field across the second pair of opposite faces, a voltage will appear across the remaining two faces.
A third type of sensor, used first in the 652cc Visa to trigger the ignition, has an oscillator and electronic switch insode the sensor, and an iron block on the flywheel. When the block passes the end of the sensor where the oscillator coil is, the oscillator stops and the switch closes.
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Post by Kered »

The sensor does not appear to be Hall effect. I've had one to pieces and it is purely inductive. A is coil wound round a magnet and rotating teeth cause a fluctuation in the magnetic field and thus a pulse being generated as each tooth passes. Looking through the sensors section of the RS Comonents catalogue I note that the that the resistance of the coil is quotes as 1200 ohms which is not disimilar to the resistance quoted for the Citroen sesnsors.
Another point is that if it was Hall effect then the output would be a semiconductor and any resistance measurement made of the sensor would be polarity concious - and that does not appear to be the case.
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Post by Kowalski »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by NiSk</i>

In fact, in emergency conditions, non-ABS brakes will stop you faster than ABS brakes (as long as you have road grip) since ABS doesn't allow you to lock all four wheels at the same time (which is how you get maximum braking - mind you without any steering!).
//NiSk
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Locking wheels doesn't give you optimum braking, since the coefficient of dynamic friction is less than that of static friction, that and the fact that the surface of the tyre gets burned away (tyre smoke) when you lock a wheel. In the wet locking wheels stops the car tyre tread from being able to channel water from under the tyre and it "piles up" in front of the tyre lubricating the tyre / road junction more.
Optimum braking happens at the point where you are just about to lock wheels, without actually locking a wheel.
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Post by AndersDK »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by uhn113x</i>

Anders - I am being very brave here......[;)]
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Inductive sensors are widely called HallEffect sensors - either being a coiled inductive sensor (pick-up) - or a later solid state sensor with an electronic (inductive sensitive) device.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not so I am afraid - an inductive sensor is a coil that gives a pulse of current when a magnet passes it.
A Hall effect sensor, named after the bloke that discovered the effect, is a cube of semiconductor material that has a voltage applied to two opposite faces. If you apply a magnetic field across the second pair of opposite faces, a voltage will appear across the remaining two faces.
A third type of sensor, used first in the 652cc Visa to trigger the ignition, has an oscillator and electronic switch insode the sensor, and an iron block on the flywheel. When the block passes the end of the sensor where the oscillator coil is, the oscillator stops and the switch closes.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Well Mike - are'nt they all used to pick up a magnetic field [;)]
- can't argue against the different constructions and function methods - but my reference to these all designated as HallEffect sensors - is common by mechanics.
Logic you may say - as they all have the same basic function : to provide a sensor signal by inductive means.
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Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by AndersDK</i>

- then my first action after getting a "new" car with ABS would be to rip the ABS circuit [:(!]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Unfortunately that would not be an option (at least in the UK). People have been prosecuted for disabling their ABS system.
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Post by AndersDK »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Homer</i>

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by AndersDK</i>

- then my first action after getting a "new" car with ABS would be to rip the ABS circuit [:(!]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Unfortunately that would not be an option (at least in the UK). People have been prosecuted for disabling their ABS system.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Granted Homer - as I noticed up thread :

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by AnderDK</i>
Noz :
I'm 100% with you in not just "cutting" the ABS. The idea solely applies to a shortterm solution untill practical repair/replacement re-gains a corrrect & 100% reliable ABS function.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
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