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Thread: ABS triggering on light braking...

  1. #1

    ABS triggering on light braking...

    Hi all

    Just wondering if I am missing anything obvious before taking car to garage for an ABS issue (VR6). Over the years previous to now the ABS either worked or the light was on and it didn't (probably in about equal measures -- either the light didn't go off at start or would come on part way through a drive) but it has always behaved itself on MOT day.

    Now I have a new issue. Mostly the ABS light goes off after the start up check procedure and all seems well until I drive off.... then I hear rapid clicking sound and next time I touch the brake pedal the ABS kicks in (the light stays off). Sometimes though the ABS light oomes / stays on and then it doesn't happen.

    I have WD40ed the connectors on the front sensors, which are reading ~1100 ohms resistance. The sensors look clean and the sensor rings look ok although a bit rusty (does this matter?). I've not taken back wheels off as I can't see that I can do a lot there, but maybe I will when it stops raining. Unfortunately, I don't have a fault code reader...

    I have solved the issue for now by disabling the ABS (unplugged a front sensor), but I'll need to sort it out at some point. I've probably reached the limit of what I know (ie what I have found on the internet!) and just wondered if there was anything else I could try before biting the bullet?

    Sorry if I have missed anything out, I'm not that mechanically minded, so any help appreciated!

    Thanks, David

  2. #2
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    Quote Originally Posted by DavidMeredith View Post



    Now I have a new issue. Mostly the ABS light goes off after the start up check procedure and all seems well until I drive off.... then I hear rapid clicking sound and next time I touch the brake pedal the ABS kicks in (the light stays off). Sometimes though the ABS light oomes / stays on and then it doesn't happen.





    Thanks, David
    It sounds like the air gap between one of the sensors and its reluctor ring is varying triggering the ABS into thinking a wheel is locked (each sensor counts the segments of the reluctor and the ecu compares them to determine if a wheel is locked).

    I'd check the condition of the reluctor baskets on the back of the rear disks first

  3. #3
    Thanks Dox -- will I be able to check those by taking the wheel off? I had a quick look under the car and it all seemed much more enclosed at the back...

    Cheers, David

  4. #4
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    Quote Originally Posted by DavidMeredith View Post
    Thanks Dox -- will I be able to check those by taking the wheel off? I had a quick look under the car and it all seemed much more enclosed at the back...

    Cheers, David
    No, you will need to dismantle the brakes and remove the disk to inspect the sensor / reluctor.

  5. #5
    Ah, ok, I think that it a bit outside my spannering comfort zone -- I suspect I could take them apart, but whether I could get them back together again and working is another question! I think I'll pop it in to the garage...

    Thanks again, David

  6. #6
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    Well the other end is the ECU, its known for the windscreen seal to fail and water gets inside causing corrosion. Its located by the passengers left ankle behind a plastic panel, check the connector pins for corrosion, if OK open it up and check the PCB for corrosion also.

    You'll only need a screwdriver to check this, but its recommended to remove the earth connection from the battery first.

  7. #7
    Cheers, I'll have a look at the ECU before committing it to a garage... A quick search on the bay suggests that the ECU is not that pricey (~£30) so is it worth getting one and swapping it out if there is any evidence of water damage / corrosion? If it solves the issue great (and it will be cheaper than a garage diagnosis), if not I'll have a spare in case it does fail in the future.

    David

  8. #8
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    You need the correct part number to match yours, so remove yours first.

    Put a wanted advert up here, many of us have known good spares rather than hit and miss eBay stuff and it all helps to keep this community going

  9. #9
    Quote Originally Posted by DavidMeredith View Post
    Cheers, I'll have a look at the ECU before committing it to a garage... A quick search on the bay suggests that the ECU is not that pricey (~£30) so is it worth getting one and swapping it out if there is any evidence of water damage / corrosion? If it solves the issue great (and it will be cheaper than a garage diagnosis), if not I'll have a spare in case it does fail in the future.

    David
    I'd be a bit surprised if it was the ECU. They are pretty robust. I've heard of owners getting them submerged in water and they still work once dried out. It can happen though. My guess is that a wheel sensor is touching a speed ring somewhere. I think I have this going on as well at my right front wheel. The ABS will kick in, the brake pedal stays down briefly and I hear clicking. My ABS light doesn't go on and no codes are thrown, but I took a look when this was happening a few weeks ago and the ABS sensor was being ground down from making contact with the ring. I replaced the sensor and bent the ring a bit to avoid contact, and it stopped for a while, but now has started again. I think I might have an issue with that wheel hub or bearing, as I'm not sure why that would occur, but if you can spin the wheel and watch with a flashlight to see if its making contact that might be a good place to start. I think there are two different wheel sensor codes, one is the sensor has failed, likely getting an out of range voltage, and there is a mechanical failure code, which I think is the one that occurs when the sensor makes contact. If you can scan your system you might see one of those codes.

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