Xantia ABS gremlin
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  1. #1
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    Default Xantia ABS gremlin

    My vehicle is a 1996 Xantia 2.0i 16v. Glad to hear of any ideas on the following brake problem.
    Intermittently at low speed under gentle braking, the ABS seems to cut in unexpectedly and the car rolls to a stop with, seemingly, no braking action and firm pedal. Replacement of ABS control unit has reduced occurences, but the problem has recently recurred.
    Anyone else had anything like this?
    Cheers,
    Michael

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  2. #2
    Moderator Alan S's Avatar
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    Personally, I think that's more of a hydraulic problem than an ABS or electronic one.
    Check for a binding caliper or air in the system as there has to be something activating the ABS; it's there to stop brakes locking up when you have activated the braking system and I really can't see how it can activate the brakes in the first place; it flies in the face of logic that it could.
    I might be wrong, but personally I think you're searching for an answer in the complicated side of the system and it's more likely to be something far more simple.


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  3. #3
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    Quote Originally Posted by Alan S
    Personally, I think that's more of a hydraulic problem than an ABS or electronic one.
    Check for a binding caliper or air in the system as there has to be something activating the ABS; it's there to stop brakes locking up when you have activated the braking system and I really can't see how it can activate the brakes in the first place; it flies in the face of logic that it could.
    I might be wrong, but personally I think you're searching for an answer in the complicated side of the system and it's more likely to be something far more simple.


    Alan S
    Thanks Alan, for quick and thoughtful response. Seems to make sense and I'll investigate accordingly. One query though: if ABS systems work by "venting" system pressure (as rapid pulses) when wheel locking is identified, couldn't this venting also occur under low speed/low pressure conditions, if an appropriate fault condition developed in the ABS?
    Regards.

  4. #4
    Moderator Alan S's Avatar
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    My thoughts are that the way ABS works is on magnetic pulses. They have a magnet that sits about 1mm away from a serrated wheel (that looks like a circular saw blade) and if the teeth on any one wheel start to go slower than the other 3, it lifts the pressure to that wheel. Now unless there's a build up of rust or grime/grease on the serrated wheel that is causing it to give the sensor a false reading, I think it has to be pressure being applied via the hydraulics.
    ABS seems to scare the crap out of a lot of repairers and they always go looking for deep and meaningful reasons for any faults.
    My 16V had just been serviced prior to me buying it and had been looked at by a well known repairer down there at least twice to my knowledge from what I saw of the service records. One time was for $565 which was all for testing and when I collected the car, it came with the notation marked on the last invoice saying it couldn't be found and assuring the owner it was safe to drive.
    I had a look at it when I got it home, found a broken bracket to a front strut, tested the sensor attached to the bracket and found it was open circuited and replaced the sensor. All up around hallf an hour ro find and as long to fix, so watch they don't get carried away looking for something complicated when something as simple as a spray with a bit of degreaser or a thorough bleeding of the brakes may be all that is needed.


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

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    Thanks again Alan. The thing you're describing is called a shaft encoder, and they may use magnetic, optical or physical sensing. Very common in industrial applications, and increasingly used in almost every appliance and vehicle on the planet. I have no idea what sensing system Citroen use, but let's assume it's magnetic. Ideally, the encoder disc and sensor should be in a completely sealed unit, but perhaps in auto environments (or just Xantias) this is not so. Magnetic would be more dirt/grease tolerant than optical, but anyway, a good lead for me to follow up.
    Next point is to work out what the pressure-dumping system consists of. The fluid can't be vented to the outside, so there must be a return line and a valve on each wheel, with the valve actuated by a solenoid receiving rapid pulses from the ABS controller. The controller would be using a fancy array of sensors and one or more algorithms to determine when and for how long it should fire the pulses. Lots to go wrong there, but as my controller has been replaced and is "diagnostically healthy", I reckon your mechanical/hydraulic theory is looking pretty good!
    The above is all just engineering speculation on my part, as I have no idea how Citroen actually implement the design principles on Xantias. I'll let you know what happens anyway.
    Cheers/Michael

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    Moderator Alan S's Avatar
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    Slowly my brain is clicking into gear and this may/may not be helpful, but the ABS cuts out below 10 MPH on a BX and I'm fairly certain the Xantia does also.
    This being the case, that speed (16 kph) may be worth remembering when troubleshooting.

    Also, I've found some detailed (factory) info on the ABS in a BX 16V and whilst I don't know how close to being identical, it should at least help point you in the right direction.

    http://www.rwbsmith.plus.com/citroen2/Brakes/abs.pdf


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  7. #7
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    Yep, I guessed that may be significant.
    A New Zealand guy replying on the andyspares.com forum has an interesting theory, that a faulty sensor on one wheel momentarily tricks the ABS unit into thinking speed of the "faulty" wheel is very high and the other three wheels are locked. I think, though, that if that were the case I would occasionally get the effect at higher speeds, which I don't - just at walking pace.
    Thanks heaps for the service manual link - great material.
    Michael

  8. #8
    Moderator Alan S's Avatar
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    Michael,

    I keep coming back to it being something mechanical. I can go with the sensor theory up to a point, but when it's all boiled down, every sensor I've seen faulty has usually been that the outer shielding has fractured and simply turned off the ABS due to a break in continuity which is why I'm a bit relictant to go too far down that track as I feel it could be a red herring.
    What I am wondering might be the two most likely scenarios are the serrated wheels possibly covered in either metal particles or more accurately, conductive particles possibly from either metal based pads, worn down pads or fine particles from the rotors as they've worn or a caliper tending to bind.
    Reverting to BXs and CXs, it is common to find these with seized rear calipers after a few years which usually means the calipers have to be removed and the pistons worked out, washed and lubed and refitted. In this case, it could also be that they may not be totally seized, so as a result if the (hypothetically) rear caliper is partially seized and the pad making contact on the rotor, is this sending strange messages to the operating system or more likely, the theory about the dodgy sensor is put into perspective in it being created by the serrations on the sensor wheel being fatter in size than normal due to the particle build up, or the wheel to the sensor seems to be going faster than it really is, then this coupled with the binding caliper could give a set of circumstances that could create this sensation.
    My guess is that to test this, do a full wash off the 4 serrated wheels and also do a proper bleed out of the system via all the brakes. I recently did one for a friend with a BX that was supposedly serviced including LHM change 400 klms earlier and had LHM as black as coal coming from each of the 4 wheel cylinders.
    The brakes n that car are now performing better than they have ever done, and as these brakes were also previously binding, to me says this may be good starting points.


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  9. #9
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    Hi Alan,

    Very thoughtful, and plausible. I'll keep an open mind as I plan (with my workshop) what to do next.

    I think you'd be aware of the Andyspares forum based in UK. On the Citroen forum, a thread called "Xantia ABS fright" started in March 2004. It was still going a year later. You will see there my symptoms described in uncanny detail - it's almost as if the users posting have driven my car and relayed the experience (except that I have never been caught short and had to use the handbrake).

    The Andyspares thread contains lots of sincere but rather speculative and off-topic posts. My summation of the sound posts is that there may be some flaws in the ABS algorithms, but unlike modern electronic equipment you can't get firmware upgrades to fix them. For example, I may have missed a post to the contrary, but it seems the ABS probably gains its "current vehicle speed" value from the wheel sensors (applying some algorithm) rather than an independent source (eg the speedomoter, or a separate shaft encoder on a drive shaft).

    Cheers/Michael

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    Moderator Alan S's Avatar
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    Quote Originally Posted by mkelly
    Hi Alan,

    The Andyspares thread contains lots of sincere but rather speculative and off-topic posts. My summation of the sound posts is that there may be some flaws in the ABS algorithms, but unlike modern electronic equipment you can't get firmware upgrades to fix them. For example, I may have missed a post to the contrary, but it seems the ABS probably gains its "current vehicle speed" value from the wheel sensors (applying some algorithm) rather than an independent source (eg the speedomoter, or a separate shaft encoder on a drive shaft).

    Cheers/Michael
    Yes Mike, know Andyspares real well as I was the original moderator with Jon Wood when it was originally formed.

    I also believe that the sensors are the determining factor in the current vehicle speed and that this info is relayed to the ABS ECU which is why I keep harking back to this region all the time.

    It has to be remembered though, that UK conditions and cars have one major variable over ours; the snow and the salt they mix with it that causes corrosion problems way beyond anything we ever see in some of the underbody mechanicals and electronics and this in itself can cause similar problems from different sources. By the same token, there are some very knowledgeable people with Citroens over there but temper their responses with that side effect in mind.


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  11. #11
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    Quite right about the corrosion environment being quite different in UK & Aus.

    If I determine the ABS does, in fact, calculate "current vehicle speed" indirectly by polling the individual wheel "speeds", I'll happily pull the ABS fuse and have nothing further to do with it. Hard to believe Citroen (or competent brake specialist) would design it this way, so for the moment I'll assume otherwise.

    Mike

  12. #12
    Moderator Alan S's Avatar
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    Why not pop the fuse anyway and see if it still persists or alternatively if symptoms then show that don't when the ABS is in circuit?

    Just a thought.

    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  13. #13
    Fellow Frogger! Clogzz's Avatar
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    Quote Originally Posted by mkelly
    Glad to hear of any ideas on the following brake problem.
    No braking action and firm pedal.
    Anyone else had anything like this?
    Hoping not to look ridiculous after all that technical talk, but I've had that problem.
    I had a piece of stiff carpet on the floor instead of a proper car mat, and it got caught in the brake pedal, restricting normal movement.

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