Brakes: 850: Worrying ABS Issue - '97 855 T5

[PowerOfTheBlues]

Morning all,


For a few weeks now I've been having a rather worrying issue. Every now and then, when applying the brakes, at slow speed and usually gently, the ABS kicks in so viciously, that I can't actually slow the car.

It feels like there's a stone caught between the pad and the disc and I can't push the pedal down far enough. Hard to explain.

Anyway, the pedal kicks back violently and the car doesn't slow as intended, it certainly keeps one on one's toes...

I have obviously checked for any obstruction in the calipers but cannot find any. There are no score marks on the discs.



Any help gratefully received,

-Richard

[Luxobarge]

Is the ABS light on? (Before answering that, check that the light does come on when you first turn on the ignition, which it should do, this proves that the bulb isn't blown - or removed)

Either way, this sounds like you've got an ABS sensor down, a broken/cracked reluctor ring or an ABS ECU fault.

I'd get the codes read, that will point you at which wheel it is, or at least point you in the right direction.

[dooby]

Hi,

only time I had similar symptoms was after heavy usage on a track day. When inspected the brakes I did find a deep gouge on the inside face of one of the discs, so it's likely I had picked up a stone between the pad and disc.

Whether that was the only thing causing the vibration is a different question, as I also managed to get some pad deposits on the disc coming in off a hot lap.

If you think you can identify which corner it's coming from so much the better but either way I think an inspection is needed to rule out the discs/pads - *unless* you find you have ABS fault codes. As I say, I couldn't see the gouge on mine as it was the inner face. Perhaps the same has happened in your case?

Cheers, Pete

[Luxobarge]

Actually - another thought, just thinking this through, if it's a dodgy reluctor ring, then it won't put the ABS light on, because the system will just think that one wheel is skidding.

So if you have a working ABS light bulb but no ABS light, this is favourite.

[PowerOfTheBlues]

Yes, my ABS light works and isn't on after start up. Surely if it's a broken ring it will happen all the time?


- Richard

[Luxobarge]

Originally Posted by PowerOfTheBlues

Yes, my ABS light works and isn't on after start up. Surely if it's a broken ring it will happen all the time?


- Richard

Not necessarily. Usually the reluctor rings crack - the crack may be such that sometimes the ABS will not detect it, and act normally, and other times it may detect the crack, count more (or less) "pulses" and assume the wheel is skidding. This variance might be heat related, or pure chance.

Either way, it will only happen when you try and apply the brakes.

If you don't have the light on then you probably don't have codes, so I'd suspect something on or around the wheel/brake area rather than the ECU, however without codes it's hard to tell which wheel is causing the problem, so it's a laborious task checking them all individually.

[DSK]

ABS Tone Rings see here for details;

http://forums.t5d5.org/topic/13858-s..._1#entry160890

[The Hooded Claw]

Originally Posted by DSK

ABS Tone Rings see here for details;

http://forums.t5d5.org/topic/13858-s..._1#entry160890

Wot DSK said ...Broken Tooth on yer tone ring ....DSK is 100% CORRECT with this I've had myself on me old Saab ...

[ShadeTek]

Check your tyre sizes, I know they probably are all the same, and tyre pressures are the same. Also take the wheel sensors out of their holders and de-gunk them. The next thing would be wiring, there is a known problem with a specific range of 850's that had bad crimp connectors uner the fusebox which caused pulsey pedal at low speeds, I should know, which involves changing the wiring to the front sensors. I checked the resistance of the cable to one of my front sensors and it read 300ohms where as it should have been less than 2ohms!