Strange Abs Fault

[Lewis]

Abs self test at 4 mph is good no fault lights show for the braking system at this point. Fluid level is also correct.

Its only once a speed of 70mph is achieved that the ABS warning light comes on and stays on even as speed is reduced, if I turn the car off and back on again the light is off and again won't come on untill 70mph is reached again.

Also several times when stopping from slow speeds say crawling forwards at a max of 10 mph in traffic and stopping again, when the brake pedal is pressed sometimes it feels stiff and the brakes don't appear to work very well (similar to no servo assistance feeling) then the pump can be heard running up obviously trying to build up pressure in the accumulator this is accompanied by juddering on the brake pedal and a pulsing noise.

The target rings on the front( haven't checked the rear targets yet) don't look the best but as the abs light doesn't come on at lower speeds maybe they are good enough?

Any ideas to the fault and what to do please?

 

[Lee_R21Turbo]

Dirty ABS targets mate - give them all a good clean with a wire brush usually sorts the job out!!!

 

[Andyquadra]

Its the taggets m8, if it doesn't come on till 70 you should get away with cleaning them with a wire brush, and the pedal going hard and clicking is the abs ecu thinking the wheels have locked up because it can't see the targets properly.

Andy

 

[DaveL485]

I Concur.

The sensors look at a ring made up of 96 teeth. It sees tooth-gap-tooth-gap-tooth-gap. It monitors and compares all 4 wheels and when it see's a rapid deceleration in tooth-gap sequences on a wheel it thinks 'oooh ****, locked wheel, release the brake' and does so.

Whats happening on your car is dirt or corrosion has blurred the tooth-gap profiles and when you touch the brake the system thinks that the dirty or damaged part of the ring is a wheel-lock situation and releases the brake (thats the pulsations you can feel through the pedal). At over a certain speed depending on severity of the damage the computer sees repeated wheel lock events without the brake applied even and realises there is a problem and disarms the whole ABS system-and hence your warning light. It'll stay off until you turn the ign off then on again, it reboots and starts all over again.
Basically, those pedal pulses you feel are the ABS computer trying to unlock a wheel that isnt.

Damage or corrosion of the rings should be evident. the teeth need to be well defined and clean. Just one missing tooth or a crack in the target can upset the whole boat.

New front rings are available from thr robbing barsteward dealers. Rear only as part of a whole bearing assembly.

 

[Lewis]

Thanks for the help, I'll give all the target rings a good going over with the wire brush and see what results I get.