ABS sensors OK, what now?

[reblack68]

Soon after I got my car I noticed that the ABS warning light would come on if I exceeded a certain speed. It would go out the next time I started the car. Then it started to come on if I went round a left hand bend quite quickly. Sometimes it wouldn't reset until I reached about 20mph on the next journey, which tallies with what was said on another thread.

One evening the light came on, there was a sound like something falling off the car and the light has never gone out since.

I checked the resistances at the ECU plug and they were all 1040-1050 ohms, which seems about right. That suggests that the wheel sensors are OK, and that no part of the sensor circuit has fallen off.

Is there any other bit of the ABS system that could have fallen off, or did I just happen to run over a stick at the same time as the fault occurred? What else, other than a wheel sensor, would make the ABS light come on?

Any ideas?

[xantia_v6]

Check that your tires are all matched. If one is a different profile, it can upset the ABS computer with the symptoms you have described.

[reblack68]

I had a new pair of front tyres fitted last week. I think the old ones were the same profile but it's entirely possible that they weren't, they certainly weren't the same make. All the tyres I have now are definitely the same size, although the rears are a different make from each other and the fronts.

If one of the front tyres was wrong could that have upset the computer to the extent that it now needs reset?

The car in question is the 2000 Xantia HDi Estate btw.

[AndersDK]

If one of the front tyres was wrong could that have upset the computer to the extent that it now needs reset?

Nope

That would require at least some 5-10% speed difference on the wheels. Which in turn means different wheel sizes that severe that the car would suffer other symptoms as well 8)
Thats not the case here.

Removing the battery minus pole for half an hour should reset just about any computer you have on board if you want to try it.

[Mandrake]

I checked the resistances at the ECU plug and they were all 1040-1050 ohms, which seems about right. That suggests that the wheel sensors are OK, and that no part of the sensor circuit has fallen off.

Is there any other bit of the ABS system that could have fallen off, or did I just happen to run over a stick at the same time as the fault occurred? What else, other than a wheel sensor, would make the ABS light come on?

It will still be a sensor - just not an obvious fault.

Measuring an open or short circuit for a sensor at the ECU plug is a sure sign that a sensor (or cable) is faulty, but measuring the correct resistance only means that it MIGHT be ok.

There are two other things that it can be that a static ohms check like this won't reveal - an intermitant fault (affected by the cable flexing with suspension movement) or the sensor is not delivering any pulses. (Sensor gap not adjusted correctly etc)

In the first case you can try provoking it with suspension movement - probably a two person job, measure each sensor continuously, while you lower and lift the suspension from minimum to maximum height, bounce the height by hand, and in the case of the front wheels, turn them from lock to lock.

You might get lucky and find an intermitant fault like that. You could even get someone to try wriggling the ABS sensor cable in the front wheel arches. (Car supported securely etc)

The other thing it can be - particularly if the light always goes out as normal when starting, but then consistently comes on at a certain road speed (approx 20Km) is that no pulses are being sent because of a physical problem with the sensor - the ECU can only do a continuity test at startup, its not until you start driving that it can see whether road speed pulses are delivered or not.

Another forum member (richardw?) posted a clever trick to find this problem - connect a small speaker (the kind you'd find in a small transistor radio) or some earphones to one sensor at a time through the ECU plug and go for a drive - you should hear a buzz that varies with road speed, and when you find a sensor with a weak, intermitant, or non existant buzz you've got your culprit

Regards,
Simon

[jeremy]

I used the audio method to sort out the ABS on my BX. I had an old radio which could be switched to act as an amplifier - and I used an ABS lead coax on one end and a 5 pin din on the other and drove round listening for crackles.

You could use a microphone socket on a cassette recorder - or probably even your laptop - and just record the output and then listen for crackles when you play it back. Its surprising the noise that bumps make.

You can probably find the fault by simply connecting your meter to the sensor and flexing the lead. Its unlikely to have broken on a straight run or where its clipped down - its probably where the cable bends on its way to the hub.

Its also worth having a look at the sensors themselves and the toothed rings. Are the sensors tight? Are the gaps between them and the rings correct? Are the rings contaminated with metal filings or very rusty? (They don't seem that affected by a bit of dirt and I'd avoid vigorous wire brushing as it may cause more problems by removing the electroplate corrosion protection which is very thin)

[reblack68]

Several avenues to explore there.

The light never goes out now, not even at startup. Since the system is only doing the same check I did at that point, that suggests either that something else is up or that it's storing a fault. I have the battery disconnected just now to see if that'll clear it.

How do you go about "listening" to the sensors? Do you just shove the two wires from the speaker into the sensor's two terminals in the multiplug and drive with the ECU plug out?

[AndersDK]

How do you go about "listening" to the sensors? Do you just shove the two wires from the speaker into the sensor's two terminals in the multiplug and drive with the ECU plug out?

In essence thats what you are doing yes.
But I'd say you should either have a small amplifier (the cassette player microfone input) or high impedance earphones, as the ABS sensors can not 'drive' a standard low ohmic (4, 8 or 16 ohms) speaker.
If you consider a standard MP3 player earphone a 'speaker' then it is however straight away.

[Mandrake]

Several avenues to explore there.

The light never goes out now, not even at startup. Since the system is only doing the same check I did at that point, that suggests either that something else is up or that it's storing a fault. I have the battery disconnected just now to see if that'll clear it.

You don't need to disconnect the battery to reset the ABS ECU - it resets itself and re-tests the sensors for continuity every time you turn the ignition key off and on.

More likely what has happened is that the intermitant fault has now become semi-permanent due to flexing of the cables - if you unplug the ECU and recheck the sensors again without moving the car you'll probably find one measures faulty now!

(Been there, done that, when I was looking for a similar problem on mine it was a game of cat and mouse to actually catch it out with a faulty reading...)

Regards,
Simon

[DickieG]

If it helps (in my experience) the front sensors are more likely to be at fault.

If you can get access to a fault code reader then it will tell you right away what the cause of the problem is. If you're near to Slough then I'll do it for you for free.

[reblack68]

I've been busy moving house and didn't get a chance to look until yesterday. It turns out that the front driver's side toothed ring isn't there at all. That would be the "falling off" noise I heard.

Can I get a replacement ring or will it have to be a new CV joint?

[AndersDK]

The toothed ring is not considered a spareaprt listed on PR-net.
I believe its the old wellknown CX problem where you also had the pressfit toothed ring on the CV joint housing.

Here the ring suddenly splits because of rust build up under the ring - and the resulting pressure.

There are a couple of different front axle versions for the 110 (2L) HDI. Donno the difference. You must know the exact outer dia of the CV joint where the ring fits - to get one that fits yours from the scrappie.
The ring can gently be tapped on or off.

Either you try find a defective axle/CV joint and salvage the toothed ring - or you get a complete axle with the ring fitted. A friendly scrapper may salvage a ring for you.

Note that several PSA models (even vans) woud have the correct size axle/CV joint. Should be easy to source.

[Mandrake]

Note that several PSA models (even vans) woud have the correct size axle/CV joint. Should be easy to source.

You would need to be certain that there were the same number of teeth though! Otherwise the speed reading would be off compared to the other wheels and it would never work properly.

Probably safer to stick with one from the same model, unless you count the teeth...

Regards,
Simon

[jeremy]

I think you will find that if there are differences in teeth numbers they are quite extreme. I don't think they vary by the odd tooth or 2 but by 40 or 50.

[reblack68]

I have ordered and paid for a new CV joint on eBay but the seller phoned to inform me that there were two joints listed for my car. One has 34 inner splines and the other has 32. I tried to count them by peeling back the boot but the splines aren't visible.

I have no great desire to remove the joint just to count the splines, though I will if I have to. Is there any way to pin it down to one or the other?