Xantia ABS Fright

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David W
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Post by David W »

Interesting thread. I see there have been many references to ABS "not working" at low speeds (under 5mph or similar) but I haven't quite seen the emphasis on just why this could be at the route cause of the low speed incidents.
No-one has actually spelt it out that the default condition of the ABS system in working condition is not to allow the wheels to lock...at all. It is only the software/electronics kicking in below 5mph that are programmed to disconnect the ABS effect because otherwise the system would try and prevent the wheels locking...i.e. It would try and prevent the car from ever stopping.
Thinking of it in that way you can see that all you need to get this heart stopping feeling of no brakes in slow traffic situations is for that one circuit to "glitch" and indicate that the car is above the 5mph cut out speed. Until it comes to its senses all brake effort will be removed from the system, however hard you stamp on the pedal.
So yes Anders it should never happen for all the reasons you mention...but also it is quite likely if you accept a computer can ever be less than 100% perfect!
I have experienced this in a mild form several times, and heard of others reporting it. You may just be aware of a buzz/groan when the ABS pump activates for a second as you roll to a halt in town, almost stopped. The pedal may pulse a little and you think that's odd because the ABS wasn't needed. It only lasts a second though and you think no more about it if it doesn't happen again ever or for ages.
Our ZX was doing it perhaps once or twice a month and I resolved it by cleaning up the dirt/corrosion from the front wheel rotors when I had the driveshafts off to do the clutch.
You'll find the nice sharp profile of the teeth is usually well spoilt by the debris over 100K and cleaning them with a sharp small scraper seems to do the trick.
I'm no expert on ABS electronics software but I think these dirty rotors give a less than perfect signal to the ECU which confuses it into thinking the vehicle speed is higher than reality.
It isn't 110% conclusive proof but this has worked for me with every car I've looked at suffering this fault.
As a matter of interest others report poor wheel bearings can lead to the same effect but I have no direct experience of this.
David
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Post by acrowot »

Quote from Dave
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I have experienced this in a mild form several times, and heard of others reporting it. You may just be aware of a buzz/groan when the ABS pump activates for a second as you roll to a halt in town, almost stopped. The pedal may pulse a little and you think that's odd because the ABS wasn't needed. It only lasts a second though and you think no more about it if it doesn't happen again ever or for ages
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This is what happened in all the cases that I experienced the groaning noise was apparent every time seeming to come from the direction of the front nearside wheel (left side as sat in car),I thought the brake pads had worn and were contacting the disk but I had only recently replaced them. In all cases it was also accompanied by the ABS light illuminating, I did not clean any of the sensors or the castalations, I did change the ECU and that did cure my problem, for the last 6 months anyway. It would appear that from the posts on this subject that this fault occuring at very slow speeds has different causes. Very disturbing, I feel a little less confident in my Xantia now.
alexx
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Post by alexx »

Anyway, if you activate ABS above 10 km/h, it won't be disabled when the speed drops below 10, but will continue to work until full stop, if you are still pressing the pedal with enough force.
But on some cars ABS works quite bad on low speed, just before the car stops, like on Renault Laguna mk II that I tested during this summer. C5 was much better (on the same street, of course). Indeed, there was a recall concerning ABS system on Laguna shortly after that.
On Xantia, there's no ABS pump, because pressure from the system is used, but other parts of ABS are the same as in other cars. It's also interesting that, if you activate ABS on the snow, where low force is needed for it, pulsing/groaning on the pedal is proportional to applied force on the pedal, although stopping ability doesn't seem to reduce much with greater pressure, just whole thing is more noisy and unpleasant.
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Post by mark_sp »

Hello All
Sorry about the delay in responding, here is some more info.
I've noted my speed recently as I approach the same junction and I estimate at the time of the original incident I was travelling at 20 - 22 mph.
I have had a second occurrence under the same circumstances but this time at a different location and at a slower speed of approx 15 - 18 mph. Same thing, ABS light comes on as I'm gently braking, pedal twitches, braking ceases and nose of car rises then braking is restored. This time it all happened more quickly, sub 1 second I'd guess.
It's disconcerting to say the least.
On a related note can anyone tell me the correct AS part no for the front sensor, my car is a 1998 (mark 1 facelift) 1.9 TD Xantia.
Cheers
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Post by David W »

Mark,
I'd have the front wheels off and get a really good look at the front driveshaft outer ends...see if there is a build-up of muck in those rotor teeth.
David
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Post by mikeb »

I have had this slow speed brief lack of brakes as well on my xm 2.1 td. It only happened when slowing at a junction with the steering pointing slightly to the left. It was a wee bit alarming but only lasted for a split second.
It was a partialy broken sensor wire on the right front wheel which broke contact (perhaps several times?) at the above steering wheel angle. Cured with a new sensor.
I would go with the confused ECU theory.
Mike
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Post by heath robinson »

here's a guess at this one:
logically, if the ecu can cut braking to any wheel, then surely it has the capacity, if faulty, to cut the braking to all wheels; thus no brakes? couldn't part of the fault be that it loses the capacity to operate only above 10 m.p.h?
i have the dreaded abs light on/off. i can often get it to go out by turning the ignition off and on again. however, while the abs is functioning i have experienced many huge reductions in braking efficiency when under 10 mph and gradually coming to a halt. given i can hear and feel the abs reducing braking, my guess is that the abs is releasing pressure to 1 if not both front wheels. the net effect is crap brakes under 10 mph. possibly the ecu is duff; it is a later ate black rather than alloy finish.
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Post by jeremy »

It should only work if you lock some wheels. Remember that a squealing tyre sounds much more dramatic on a dry road. I'm not sure it really is any use at 15 MPH on a dry road having nearly hit an opened door but it did stop without any drama whatsoever.
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Post by weety »

just to add my two pennyworth i was moving my 94 xantia with abs in front of my house at low speed a couple of years ago and ended up with no brakes (had to use the handbrake to stop) but i put that down to the system not being fully pressurised (hadnt reved the engine hard or had it running very long)....and a few years ago i had a carlton TD with abs and that lost brake pressure at quite a high speed (only once but serious brown trousers moment)...though with that one i never suspected the ABS and changed the brake servo and it never did it again..... but reading this thread does make me wonder.
Also i seem to remember a few years ago BMW bike riders pulling the fuses on their ABS as they found it made the bikes very unstable at low speed
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Post by xantiav6 »

I think that the car that started this discussion had the accumulator replaced the day before the behaviour began. Perhaps a wrong sphere type (e.g a damping sphere) was fitted, and that is restricting the hydraulic flow needed for operation of the brakes with ABS.
BTW, I think that you will find that ABS sensors are reluctor coils, not Hall effect.
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Post by PowerLee »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by alexx</i>
Hmm, never heard of anything like this, might it be related to the
brake problems experienced by Volkswagen in their latest Passat ?
Both C5 and Passat are equipped with the same latest generation ABS
brakes from a German company named Bosch ...... Passat is known to
loose its brakes under certain conditions.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Bosch Automotive products are normally the best on the market, Im sure if there is a problem & its the Bosch ABS system at fault, It would be corrected very quickly.
Heres a link to the Bosch website, It has some videos showing how ABS, TCS & ESP work
http://www.bosch-esperience.com/en/schleudern.html
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Post by FrenchLeave »

I'm no expert so I have nothing to add except to congratulate everyone on a fascinating and well argued topic.
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Post by fastandfurryous »

Hmm.. Personally, I really don't like the idea of ESP. A computer that has the ability to individually brake a wheel even when the drivers foot isn't on the brake pedal is dangerous in my view. ABS is dodgey enough.
Personally it strikes me that ABS TCS & ESP are all substitutes for being able to drive properly. Not just drive as per any legal requirement, or to pass any required test, but actually know what's going on as you drive. instead of ABS, Learn how to cadence brake. Instead of TCS, Learn how to back off the accelerator if you're loosing grip, and as for ESP, spend a day on a skid pan, learning how to control a car. Not only do you learn an awful lot, but it's fantastic fun. This means you are not relying on a computer that might malfunction, and you take these skills with you into ANY car you drive, not just the cars fitted with the system.
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Post by PowerLee »

Well, just about all NEW U.K. & Europe cars have to have ABS fitted by law since last june & I think thats a very good thing the EU has done for us.
I also think the law should be changed to include TSC, ESP & EBA as well.
If a child steps out in front of you, Will you remember to candance brake or will you panic? With ABS just stamp on the pedal & steer around the child if needed.
Theres lots of drivers out there who havent got the skill to control a car correctly in an emergency, Need I say any more!
I can candence brake fine, But the ABS system is so much quicker at stopping the car then any human can pump the pedal.
I know 2 people who last year bought there first ABS equipped car, Both have been saved by ABS thanks to the stupidity of other drivers around them & both have said they will never have a non ABS equipped car again.
I for one wont have a car unless it has ABS fitted, Its a very good safety device when used correctly.
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Post by fastandfurryous »

Unfortunately the effect of ABS, ESP, EBA, TCS, and in fact, airbags, good brakes etc etc. in many cases has simply been that people drive faster and use up any benefit the systems have given. All over the world it has been shown that despite massive advances in car safety technology, the number of people being killed on the roads refuses to go down.
I am very much aware that just about every member of this board does not fit into this category of driver, as we have made the effort to be interested in the cars, and no doubt how we drive, but unfortunately we are the minority.
There is no substitute for frequent re-testing of drivers.
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