ABS Activation at Low Speed

[rg]

Folks,

I've had a problem with the "out of use" light staying on for some time, but yesterday it went off (as it occasionally does for a short period), and I experienced the ABS cutting in at low speed, when coming to a halt.

I've read about this in the past, have done a thorough search of the forum, but not come up with an answer. I seem to recall it either being an issue with air in the circuit(s), or some strange resistance issue with the sensors. One guess is that the signal gets weaker as the ABS ring spins slowly, and the processor thinks that the wheel has stopped.

In the absence of a code-reader, I'm going to check if the wheel sensors are clean, and the resistance of the front ones as a first step. Then probably bleed each wheel, as I'm suspicious that the LHM feed to the pump is pulling air due to the age of the hose, as after an overnight, the light takes around ten seconds to go off, which differs from the usual two to three seconds of past years.

Can anyone add anything else?

r

This post has been edited by rg on March 17, 2013 10:42 am

[robertmnorton]

Hi rg, this link http://www.picoauto.com/tutorials/diagnosing-abs_2.html gives a comprehensive guide to fault isolation, but to cut to the chase, sensor/reluctor ring distance and position.
robertm

[CitroJim]

That link seems to have died Robert. Shame as it looks like a very valuable one..

Hopefully the site is just down for maintenance,...

I agree about sensor to reluctor ring distance. The problem arises because the amplitude of the signal from the sensor falls too low to be recognised and interpreted by the ABS ECU and it thinks a wheel has become locked and hence the release of brakes...

It's scary when it happens...

[xmexclusive]

Link is fine for me.
Was reading it while you posted Jim.
Just logged out and back in again and link still ok.

John

[CitroJim]

It's come good for me now John...

It (or I) must have been having a funny five minutes... More likely I...

That is an excellent guide to ABS fault-finding. I'm pleased it does actually work!

[rg]

Thanks, chaps!

I suspect some kind of metallic contamination of one of the sensors, as this was a problem on one of the rear ones in recent times, and removal of a lump of stuff cleared a fault.

Thankfully, all four have been freed off, a satisfying hour or so's work in the case of the rears, so I should be able to get them out and take a look.

r

[citroenxm]

If rear sensors won't come out for a check over there is another way in...

Remove the wheel caliper and disk and then undo the 7 or so hub securing bolts and pop the hub off complete.. You can get to the end of the sensor that way too....