ABS kicks in at low speed

[courty]

Hi,
I have a 75 Club CDT from 1999. A recent fault where the ABS light was on and didn't work, fuel gauge and speedo also not working, was fixed by replacing a faulty front o/s sensor.

After this repair, I noticed that the ABS was kicking in when braking at low speed, completely unnecessarily. It works ok apart from this.

I understand from other threads that this may be because of a hairline fracture on some type of ring (I'm not very technical) that the ABS only detects at low speed, hence it kicks in thinking that one of the wheels is locked.

The mechanic I use states that the only way to check which ring is fractured is by checking manually, as the machine doesn't specify where the fault is. This is apparently difficult to do as the ring is covered, so it is labour intensive to check each wheel, hence if it's the fourth ring he checks, it'll cost a lot more than if it's the first one he checks!

Is there a short cut to finding which ring is fractured? The starting point must be the front o/s as that is where the sensor was changed.
Anyone have any experience of fixing this fault please? Could it be anything other than a crack in the ring?

As always, your help is much appreciated.:D

Courty

[newcybrown]

I recently changed my o/s front sensor and had exactly the same issue at low speed braking.
I removed both front abs sensors and carefully cleaned the bearing rings thru' the sensor hole then I refitted the sensors but I swapped them over ie o/s to n/s and visa versa. Problem solved no more annoying low speed abs activation. Worth a try Courty

[Dragrad]

Ummm,, What do you consider to be "low speed" ?:shrug:

My ABS kicks in at low speed when I brake hard...... Does 55mph sound low? ;)

As a matter of interest.... Is there a limit at which point the ABS will not cut in, ie. allowing "normal" braking? (Does that make sense?:confused:)

[newcybrown]

Ummm,, What do you consider to be "low speed" ?:shrug:

My ABS kicks in at low speed when I brake hard...... Does 55mph sound low? ;)

As a matter of interest.... Is there a limit at which point the ABS will not cut in, ie. allowing "normal" braking? (Does that make sense?:confused:)

When talking of "low" speed Andrew I mean as you are slowing down for traffic lights, junctions, roundabouts, etc right at the last kinda 5 - 10 mph as the car is slowing to a stop the abs was annoyingly kicking in.

Under really hard braking, at any speed, then my system is working perfectly but at these very low speeds there is no reason for the abs to activate.

It's as if the abs is seeing, as suggested by the OP, the wheel locking at these low speeds under normal braking.

I will ad that both front rings were quite corroded and and had rust built up, on them after a good clean my problem was resolved.

[dustybin]

the abs will record a fault in the rings even a small crack because it will be out of spec with the others and its built in set point...it has to know how many spaces to be able to know when its completed one full turn. just out of interest is it just the peddle pumping or can you hear the abs pump running at the same time ?,

[tigerchubs]

Most common cause of low speed ABS operation is a faulty sensor. Are you sure it's not just distorted brake disks.
Does you garage have access to an MOT Brake tester this will indicate which wheel has the fault because it should give fluctuating readings.

[courty]

Thanks for the replies. By low speed, I mean less than five miles an hour, eg in stopping and starting town traffic, the brake peddle pumps and the car doesn't stop as quickly as it should, or when setting off to overcome inertia then having to take some speed off, it will do it.

It basically kicks in on the final two yards of braking before coming to a rest, when it hasn't engaged during the initial braking at higher speeds, eg 50-20 mph. It doesn't do this all the time, maybe 40% of touching the brakes at the lowest speeds.

The front brake discs and pads were changed at the last service and this didn't make a difference. It passed its MOT with this fault...just iritating.

[dustybin]

what about the rear discs and is the handbrake binding ? sounds like a brake system fault and not so much abs, but check everything at least once just to make sure..