ABS Sensors.

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tomsheppard
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ABS Sensors.

Post by tomsheppard »

are there equivalents available for the BX? I was told that all four units on my BX are duff but I haven't checked for myself yet. Being quite expensive, I'll do that first but has anybody any experience suggesting that it might just be the standard Citroen counter conductivity due to corroded connections?
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Post by RichardW »

Tom,
Do you want the ABS to work, or it just to pass an MOT? It seems unlikley that all 4 sensors are duff, but I could be wrong! If you want it to work, then I think you will have no option but to replace the duff sensor(s). I believe that there are connectors under the front wings somewhere and the rear seats that can cause 'duff' sensors.
I can't recommend this, of course, but.....if you just want to get it through an MOT, find out which sensors are duff and just bridge a 1kohm resistor across the terminals at the ECU. Provided all is well with the ECU this will put the light out on the initial test (it will come back on as soon as the car gets above 5mph) and it will pass the MOT (only test is correct operation of the warning light at startup - there is nothing about whether the ABS actually WORKS or not! - Of course if the tester horses it around the yard the light might come back on and he might fail it, but......)
David Woollard has a good desctiption of BX ABS fault finding on his site ( http://www.woollard.com.btinternet.co.u ... ad.htm#ABS )
tomsheppard
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Post by tomsheppard »

Thanks Richard. I could do something inventive with some electronics and was thinking of using resistors to check the ecu's operation but I would like to have it working like it should. It saved my bacon once on a Rover I was using and the BX is a good one. I might rig up a sensor interface myself and save some money that way.
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Post by David Goddard »

I think you will find the 1,000 Ohm resistor fix will not work. After a fault is found, the ABS ECU runs a diagnostic check. It may well look for the correct resistance (1,050Ohms norminally), but on Xantias it also requires you to drive about 200m to check that the ac signals from all 4 wheel are within tolerance. If not, the ABS remains inoperative and the warning light stays on. I suspect the same applies to BX's (it does to XM's).
It won't get you through an MOT, but to turn the ABS warning light out, disconnect the ABS multi-connector and the green ABS relay.
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Post by crooser »

this worked for me tom.take out the sensors and clean them.then get some very fine wet and dry and polish the ends just enough to remove the rust so that you leave the poles shining.a common cause for failure is a build up of rust bridging the 2 poles and causing a short.
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Post by Russell »

I have an ABS light that stays on on an XM 2.1 TD saloon. Can someone describe in as much detail as possible how the sensor circuits work. I beginning to get the hang of it from the descriptions so far but could use a step by step guide to how it works. [?]
Many thanks,
Russell
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Post by tomsheppard »

Russel: call up Vanny's profile and check out his site for chapter and versefor the BX abs system. This may help.
Crooser: Thanks! I have read that corrosion makes the sensors virtually impossible to remove from rear arms. I suppose I'll find out at the weekend!
alan s
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Post by alan s »

I don't envy any of you guys who drive on salt & pepper roads the job of removing sensors and I reckon this is why you've been told all 4 are caput. I doubt they are; that's on par with getting struck by lightning a few times whilst standing in the same spot.
The basic principle of the ABS is so simple as to be almost a joke.
Take a serrated wheel, place a magnet (sensor) near it & a gadget (ECU) to count the serrations as they pass the magnet. When one set of serrations starts going slower than the other 3, ease the pressure on the restricting force (the brakes) enough so it goes at the same speed as the others....end of story!!
If the serrations get coated with rust and the magnet gets covered in road grime, the little brain gets all confused & spits the dummy. If a wire from one of the sensors gets an internal break or shorts; same thing happens. As soon as one wheel ceases to function, they all go out on strike in sympathy which is a wise move, otherwise we have one wheel doing its thing & the three others operating on this even stevens caper which would be catastrophic.
Where the problem will arise is when you try to remove the sensors; they are mounted in a cast iron mount which will be full of rust. Being rusted, it will also have grown & will be jamming the sensor in, so much so that the black plastic base could even be cracked.
If you have the time, you can plaster grease on them & allow the rust to soften at which time you *may* get them out. You will find two connectors under the hood that have fittings similar to a car radio aerial. Under the rear seat there are also two. I would start by testing these. It's usually fairly simple to see which one is stuffed; it will read radically different to the other three. If it is obvious, then you can bust it out & replace it if necessary. I would also suggest cleaning the faces of all the sensors before you start & while you're on the job, clean the serrated wheels so that you aren't competing with a coat of rust or grease.
This is a BX ABS sensor & a suggestion to help prevent future corrosion when refitting.
Image
Hope that is some help for you.
Alan S [;)]
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Post by tomsheppard »

Thanks Alan. I was looking for a picture and this helps enormously!
alan s
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Post by alan s »

Tom,
The bit my finger is touching in that pic is the part that passes through the cast iron part that rusts.
The silvery looking hole is where the fixing bolt is located & again can also be a problem as far as seizing in goes.
We don't have the salt & in the past 10 years very little rain (says he as a Cyclone bears down on him and torrential rain belts off the windows) and I ven had problems removing these things.
So it's a case of steady as she goes; patience hath virtue as they say.
Alan S
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Post by tomsheppard »

Thanks, chum; I think that this is a job for the fine brass wire brush, gallons of loos'nin' juice and wait a week before proceeding!
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