A B S Light

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acrowot
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A B S Light

Post by acrowot » 01 Sep 2003, 02:46

The ABS light on my Xantia is indicating a fault with the ABS. When I switch on the ignition the ABS light comes on for a few seconds and then goes out so I am assuming that when the ECU tests the wheel sensors it finds the resistance of each one to be OK. The light stays out when the engine is started and will remain out until I start to drive the car. The light will illuminate after driving a few yards is this fault still likely to be a wheel sensor fault albeit not a resistance fault.

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AndersDK
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Post by AndersDK » 01 Sep 2003, 03:31

I would NOT "assume" the wheelsensors are ok - just by the ABS warning light behaviour.
The wheelsensors are the MOST common causes for ABS problems - wellknown to either disrupt themselves - or their cables.
Have the sensors checked - should be some 1K ohms.

jeremy
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Post by jeremy » 01 Sep 2003, 03:36

The ABS ECU will monitor the circuits including the sensors for continuity at startup, and when you start to drive it it will continue to monitor and will monitor the signal received from the wheel sensors.
If the light comes on as soon as you begin to drive the car it suggests that the signal from one or more sensors is not present or is weak. I had this problem on my BX after re-wiring 3 sensors and eventually found it using an audio amplifier and listening to the sensors in turn. In my case I'd got a whisker of a wire which was partially shorting, meaning that one signal was very weak.
Other causes of weak signals could be loose sensors or it has been suggested dirt on the toothed wheels.
If you have a broken wire to a sensor it will probably break the circuit when you go over a bump rather than immediately after you move off. I have used the audio method to trace a broken sensor wire as well.
jeremy

acrowot
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Post by acrowot » 01 Sep 2003, 05:08

Driving my car into my garage just a few feet brought the light on, I removed the plug on the ECU tested the sensors from there
FOS 1080 ohms
FNS 1102 ohms
ROS 1060 ohms
RNS 1069 ohms
Also tested each sensor to earth all infinity,Reconnected drove the car out of the garage light stayed out, test drive 12 miles light stayed out. It was disconnected for about 30 mins., I had discnnected previously to reseat plug but to no avail, I am sure I have not heard the last of this fault. Did the 9 volt from my DVM "wake" up the sensor, was there a poor connection in the plug did the ECU need to be powered down for sometime?, very strange.

alan s
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Post by alan s » 01 Sep 2003, 05:28

I had a similar fault in my BX 16V.
Start up, light on, then off & stay that way. Drive off all OK turn left all OK, turn right light back on.
Found an internal break in a front wheel sensor lead just where it leaves the sensor.
If it's a dirty connection, then removing & refitting will clean it up enough for it to work, but as you say.....it's out there somewhere waiting to come back & bite you!!
I'd suggest cleaning the serrated wheels & sensor heads, clean up connections & do a wheel at a time followed by a test to try to narrow it down to a particular wheel. The reason I say this is that my car did the rounds of the Citroen dealers & service agents and literally hundreds if not thousands were spent on trying to trace the fault, yet by doing a systematic troubleshoot, we solved the problem within an hour or two.
Alan S

jeremy
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Post by jeremy » 01 Sep 2003, 14:04

I have heard it said that some of the earlier Xantia ABS ECU's are prone to failure. I dont know much more except a conversation with my friendly mechanic but whereas BX ones seem to last well the xantia one may be under suspicion.
The strange thing I keep finding about electronic components is how often their faults are nothing to do with the complicated bits but simple joint problems, ranging from my central heating boiler control board to my BX clock. Its well worth having a look at any suspect board with a strong magnifying glass and look for dirt and cracks in all the joints.
I have seen somewhere that some repairers will not touch ABS unitsthat have been opened as this is a safety device.
have a look at this link
http://www.bba-reman.com/index.htm#
Jeremy

acrowot
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Post by acrowot » 01 Sep 2003, 17:16

What do you mean by early ECU's Jeremy my car is 1996 TDSX pre face lift, is this considered early.

jeremy
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Post by jeremy » 01 Sep 2003, 19:10

I think your car may well be one of those in question.
How reliable this source is - probably a distillation of mechanics impressions and so there could be something in it.
another link:
http://www.carelect.demon.co.uk/
Jeremy

timmsd
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Post by timmsd » 01 Sep 2003, 19:15

If you want a s/h abs module I can help.
£40. wheel sensors are a tenner.
email me davidtimms339@yahoo.co.uk
Dave

acrowot
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Post by acrowot » 01 Sep 2003, 19:30

Jeremy, the link that you gave says that the ECU should be suspected if the light never goes out, whereas if the light goes out and then comes back on when the car moves ( as mine did ) then the sensors are to be suspected, having said all this there must be circuitry in the ECU that responds to the sensors, so the ECU could still be at fault in my case. My light is still working correctly after a further 20 miles

jeremy
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Post by jeremy » 01 Sep 2003, 21:21

If you are lucky your investigation has cured the problem. You are talking of a very small current being generated by the sensor and then processed by the ECU. Co-ax type plugs are very often very difficult to get reliable connections with and often I have resorted to pliers to distort some part of my hi-fi plugs to get reliable connections. With luck you have cleaned or re-aligned a connection and all will be well.
Jeremy

acrowot
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Post by acrowot » 04 Sep 2003, 18:31

This problem will not go away, yesterday the light stayed on when the ignition was switched on and stayed on until I stopped at my first call, switched off,came back later light now working OK. Went out today light came on while driving, got to my first call switched off, came back later light now working OK, but and this is worrying when braking just before coming to a stop (going very slowly at this point and braking gently)the brake pedal seemed to go soft and grating noise from both front wheels and braking very poor, it did this the next two or three times when almost stopped. The light then came on and braking was restored to normal very effective and smooth, I changed the pads and discs 2000miles ago, I also changed the LHM for Hydroflush 200miles ago, cleaned the filters which were very dirty. Does this fault seem like the ABS is operating just before I stop?,Iwas under the impression that ABS did not work under 10MPH.

jeremy
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Post by jeremy » 04 Sep 2003, 18:48

When its working properly it does not work below 5 or 10 mph (can't remember which)
It shouldn't be operating anyway should it. Its only supposed to do anything when a wheel locks and then it operates the control valve and releases the pressure to that wheel.
So if there is any difference with the device on and presumably off when the light is showing there is a problem in the system which suggests ECU to me.
I suppose you could test it by pulling the fuse to the system to ensure nothing is happening to the valve.
certainly on my BX there is a considerable amount of ABS piping which when I think about it doesn't feature in any bleeding process. I suppose there could be some air in these pipes which would cause the mushy pedal. Thats not the answer though as the valve shouldn't be operating in the first place.
I'm sure someone will put me right on the ABS valve/pipework bleeding point.
Jeremy

acrowot
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Post by acrowot » 04 Sep 2003, 20:13

Hello Jeremy, the brakes are fine when the light is showing ie. ABS diasabled, it is when the light is not showing that the brakes malfunction and then just before the car stops, but it does not do this all the time.

jeremy
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Post by jeremy » 04 Sep 2003, 21:25

yes - thats exactly what I mean - think your ECU's bust!
Unless you lock a wheel whilst applying the brakes the ABS should never do anything beyond put its warning light on and off as it checks its circuits.
That the brakes feel different with it working and not working means that its doing something it should not do. In fact its operationg its operating valve and for a reason outlined in my previous post but unconnected with this problem the pedal feels spongy and the brakes don't work well.
So I think the fault is in the ECU - either replace or repair. If considering a secondhand one you must ask'is it working and will it work tomorrow.'
Jeremy