Over active ABS.... SORTED...

[citroenxm]

Just in the process of gettin a Xantia SX all together and got the ABS sorted.

The rear nearside sensor needed changing... all fitted and the ABS light went out, so moved the car a bit and discovered the ABS was over activating at 10mph.. I could feel and hear the pedal working..

Ive checked the CV joint teeth and the rear teeth and they are all in good order so not really confusing the Sensors.

So the Q is would a sensor or a dodgy ABS ECU cause the over active activation ....??

Thanks

[DickieG]

Wrong/too large an air gap is more than likely the problem Paul, if its a front sensor causing the problem the heat shield should be above the sensor. As you have a Lexia go into the parameters menu for ABS displaying the four sensors at the same time, now drive the car slowly then brake and you'll see the sensor causing the problem suddenly drop road speed as the cars speed drops below 10 MPH.

[Mandrake]

Was it doing this before the sensor first failed ?

I presume the failure was open circuit or short circuit that was causing the ABS light to stay on when first starting the engine ?

Now that you've replaced the sensor and there is electrical continuity the ABS light will go out as normal when the car is stationary, however if the sensor is not reliably picking up pulses from the teeth on the driveshaft joint then it will either bring the light back on again above 10mph, or it will cause a weird pulsing while braking that you notice. ABS deactivates below 10mph, which is why you would only notice the symptoms above 10mph.

Most likely the sensor gap isn't adjusted properly or the teeth are badly rusted and disintegrating...

I know its easy for me to say as a Lexia owner... but the problem would be easy to diagnose with a Lexia as it will report the measured wheel speed for each wheel during a test drive allowing you to easily identify the problem sensor as one wheel will have a speed reading that is a lot lower than the rest, or is erratic.

Without reading live data it could be a bit of trial and error to identify which wheel it is let alone what the cause is. I'd start by checking the adjustment of the sensor gap on the replaced sensor though.

[CitroJim]

You know, I reckon the sensor gap might be the problem Paul

[lexi]

I know its easy for me to say as a Lexia owner

Ooooooooh! very posh now Simon

[citroenxm]

Was it doing this before the sensor first failed ?

I presume the failure was open circuit or short circuit that was causing the ABS light to stay on when first starting the engine ?

Now that you've replaced the sensor and there is electrical continuity the ABS light will go out as normal when the car is stationary, however if the sensor is not reliably picking up pulses from the teeth on the driveshaft joint then it will either bring the light back on again above 10mph, or it will cause a weird pulsing while braking that you notice. ABS deactivates below 10mph, which is why you would only notice the symptoms above 10mph.

Most likely the sensor gap isn't adjusted properly or the teeth are badly rusted and disintegrating...

I know its easy for me to say as a Lexia owner... but the problem would be easy to diagnose with a Lexia as it will report the measured wheel speed for each wheel during a test drive allowing you to easily identify the problem sensor as one wheel will have a speed reading that is a lot lower than the rest, or is erratic.

Without reading live data it could be a bit of trial and error to identify which wheel it is let alone what the cause is. I'd start by checking the adjustment of the sensor gap on the replaced sensor though.

Hi.. as said in my first post ALL the teeth on the 4 rings are all in good order..

Ive no idea what it was like before, it came to me with No operational ABS, so had to get that sorted..

Will have a closer look at air gaps, the rear one I fitted yesterday is ok. Ill check the fronts, need to fit a set of disks and pads at the front anyway...

I DO have a lexia, but my laptop screen is broken at the moment, new screen on the way though, so will be able to once that sorted...

Thankyou for the sugestions...

[markflip]

Don't know the history of the car, but if the components with the exciter rings attached have been changed in the past, check that the rings have the correct number of teeth - there are sometimes different teeth numbers on variants of 'the same' part that causes the ABS ECU to 'see' one wheel turning at a different speed and kick in the blocking system on the 'slower' wheel - as said above though, the Lexia should show this on a test drive.

[citroenxm]

Well, seems like I got this sorted tonight, had a look at the near side front ABS sensor, as I had fitted one, and had found that the gap between the Rotor and the sensor tip was a little too big.. so I tapped the sensor in a little bit more and tried again.

Result.. Nice steady breaking, with NO releasing of the brakes etc....

Rust in the C cup that holds the sensor allowed the sensor to sit at an angle and not square at the rotor..

Thanks for the tips DickieG, and Jim.. you were both correct....