Xantia ABS woes

[Old-Guy]

The Green Lady is up and running again after a major engine strip-down revealed no obvious damage. Jim was right, the XUD9 is incredibly robust - the cambelt had jumped one (crankshaft) tooth and the horrendous noise (like a big-end gone) was nothing worse than No 1 inlet valve just touching the piston crown (the other 3 were just visibly kissing the carbon) - I guess the tolerances and clearance on that valve/piston stacked up a tiny bit tighter.

The MoT expired last month so that's the next priority, BUT one problem - when I fired her up for the first time after the re-build, the ABS light was stuck ON. Wasn't like that when she broke down in the middle of France. Working on the front end of the engine I noticed that somewhere in being transportered home, the O/S/F pad wear wire got broken (holding down strap I'd guess). It's not the usual grotty connection in the ECU plug and socket. Of course it could be a fault anywhere in the ABS circuit, but it seems a bit of a co-incidence. Could the broken wire be causing an ABS fault? Does the ABS ECU test for an earth connection on the pad sensor circuits (to check that each wire is sound)?

So near yet so far!

We (yes, 'the wife' and I!) had both been hoping to have her back on the road before the Easter.

[Hell Razor5543]

The ABS is tested as part of the starting sequences, and this includes a basic sensor continuity test. If this fails the ABS light is left on. Once you have found which sensor is faulty and replaced it, the ABS system requires a more comprehensive test. What you will need to do is to take the Green Lady out for a quick test run. Once she goes over 20KMH and the ABS system is happy with the sensors it will switch off the ABS light.

Make sure to get a good quality ABS sensor (assuming that is what the fault is). I got Gracie a cheap sensor to get her through the MoT, and it lasted less than 6 months. The replacement sensor cost me over 3 times what the cheap one did, and is still going strong.

This post has the pinouts for the ABS connector;

http://www.frenchcarforum.co.uk/forum/v ... 11&t=26012" onclick="window.open(this.href);return false;

[Old-Guy]

I've used Kev's (Red Dwarfers) comprehensive post in the past to sort the ABS (grotty contact syndrome mis-diagnosed as a faulty sensor), but it only gives the pin-out for the sensors but there are another 10 unidentified contacts. The BoL contains no useful info on the subject, let alone an ABS circuit diagram. I wondered if the ABS start-up diagnostic test checked the pad wear sensor earths for continuity?

I do hate 'swap parts until the fault goes away'. It's expensive, time-consuming and unscientific; it's all too easy to be fooled by an intermittent fault that goes away for while. Then you've got to do the job all over again. That's why garages too often use this method of 'fault-finding'.

Does anyone have a circuit diagram for S1 ABS please?

[Mandrake]

I've used Kev's (Red Dwarfers) comprehensive post in the past to sort the ABS (grotty contact syndrome mis-diagnosed as a faulty sensor), but it only gives the pin-out for the sensors but there are another 10 unidentified contacts. The BoL contains no useful info on the subject, let alone an ABS circuit diagram. I wondered if the ABS start-up diagnostic test checked the pad wear sensor earths for continuity?

No, the ABS system does not monitor the brake pad wear wires - they're only for lighting a warning light on the dashboard.

I do hate 'swap parts until the fault goes away'. It's expensive, time-consuming and unscientific; it's all too easy to be fooled by an intermittent fault that goes away for while. Then you've got to do the job all over again. That's why garages too often use this method of 'fault-finding'.

Does anyone have a circuit diagram for S1 ABS please?

You mention at the top that you have the pin-outs for the sensors (at the ABS connector I'm assuming) - have you ohm checked all four sensors yet ? The most likely cause of the light staying on before you've even driven the car is a short or open circuit to one of the wheel sensors, this can easily be checked at the ABS ECU connector. They should all be around 1100 ohms +/- 100 ohms from memory.

Given that one of your pad warning wires was ripped out probably by the wheel clamp straps, chances are high that one of the ABS sensor cables has been damaged in the same way. That's where I'd start if I didn't have a Lexia. Failing that see if someone near you with a Lexia can scan for codes - even if the fault is not currently present there should still be a stored fault code identifying which wheel is at fault.

[Hell Razor5543]

It may also be possible to live monitor the wheel speeds (based on my experience at CitroJims, when we replaced the reluctor rings). If so, then using a Lexia you could either take her for a drive or rotate each wheel individually and see which one doesn't show a true speed.

[Mandrake]

It may also be possible to live monitor the wheel speeds (based on my experience at CitroJims, when we replaced the reluctor rings). If so, then using a Lexia you could either take her for a drive or rotate each wheel individually and see which one doesn't show a true speed.

You can definitely do that on a S2, not sure about a S1.

Testing the wheel speed signal while the wheel is turning is really a later step after basic continuity has been tested though - as some faults like a broken reluctor ring will not cause the light to come on and stay on before you drive - that kind of fault can only be detected once the car is in motion by a coherence test with the other three sensors, where the light will come back on a few seconds after you start driving.

In this case the light doesn't go out which means a more basic problem like a continuity problem to the sensors (which the ECU can test without wheel motion - it's basically doing its own ohms check) or maybe a lack of power to the ECU. (Remove the fuse to the ECU and the light will stay on as the warning light circuit is arranged so that the ECU has to manually turn the warning light OFF rather than ON, even with the ECU connector unplugged the warning light will still stay on)

[Old-Guy]

Thanks chaps - most helpful.

I didn't really expect the ABS ECU to monitor the pad wear circuits, but without a circuit diagram with Citroen expect the unexpected!

.....or maybe a lack of power to the ECU. (Remove the fuse to the ECU and the light will stay on as the warning light circuit is arranged so that the ECU has to manually turn the warning light OFF rather than ON, even with the ECU connector unplugged the warning light will still stay on)

Ah, now that IS a vital piece of data. Another 'idiot check' to do.

IIRC last time I investigated a 'faulty' ABS sensor, when each wheel in turn was spun fairly slowly, measuring the resistance of each sensor I could actually detect pulses with a DVM as the resistance value flickered up and down rythmically.

Back under the car now - I'll post an update tonight.

[lexi]

I love your dedication to the TD. Most refreshing.
You cannot beat enthusiasm and they are great old tools those cars.

[Old-Guy]

I spent a couple of hours on her this arvo. I borrowed a spare battery to take her for a short test drive (interesting comparison with the VSX - more on that later) - all well except that the ABS ECU didn't reset itself (It's definitely the Yellow ABS light). Tested all the sensor circuits starting with O/S/F (open circuit), all the others were showing around 1070?. Why is the O/S/F sensor far more likely to fail than any of the others - after all the N/S/F is the same part!

A quick check with service.citroen.com reveals that the SX and the VSX use the same sensors, so I've taken the dead one off ready to 'borrow' one from the VSX on Sunday (working tomorrow).

There's no sign of damage to the dead sensor's cable - not even any marks. What's really odd is that a solid state device should decide to turn up it's toes while the car is sitting unused without a battery for some 7 months. When I've got some time, I'll chop the cable off the sensor head and check continuity of both cable and sensor. If it's the cable, I'll save the sensor as an emergency spare.

Incidentally, I notice the heat shield is NFP, so I'll de-rust it and give it a couple of coats of paint (I happen to have a large spray can of black heat-resistant wood-stove paint) after work tomorrow evening.

First thing Monday morning I'll get her booked in for an MoT.