ABS kicks in occasionally

[spartacus 68]

I've noticed this the last couple of weeks on my A4. Driving in to work or home (50 mile round trip) the ABS kicks in when braking. This isn't soft braking, but it's not extreme emergency stop braking either. Weather has been cold but not frosty, roads are relatively dry and there's no snow yet.

Brake pedal feels like there's a little more travel in it recently. Discs and pads are still good (stripped the whole lot down in October), re-greased rear caliper springs and fitted new handbrake cable. I also replaced a rear piston seal and bled it afterwards (used hose clamp) so didn't think it was necessary to do a complete wheel by wheel bleed.

Had to slam on the anchors tonight after some numpty indicated late. ABS didn't kick in and car came to a complete halt without any issues - which is just as well!

Front ABS sensors were not removed when I did the front suspension and CV boots. The CV joints were extracted from the strut bearing and the ABS sensors were cleaned with a rag.

I've also checked the recess under the battery - so no water or blockage, as initially I thought it was the brake servo. Any ideas - or do I need to get fault codes read? Incidentally - no ABS lights on the dash.

I'm going to bleed brakes tomorrow (and clean alloys while Im there). I'll post back. Any suggestions - just holler!

[adamss24]

What tyres do you have fitted on the front ?

[spartacus 68]

Dunlop Sport Fast Response (205/55VR16), probably 5mm tread remaining. Rears are older Dunlop Sport 2 and down to 3mm.

Bled brakes all-round today - but haven't had a chance to check to see if it's made any difference. Pedal feels a little harder. Cleaned alloys too and topped up with carnuba wax before the first snow fall and dreaded 'road salt'!

[spartacus 68]

Update: Bleeding brakes has made a difference. Ran 1.5 litres of DOT 4 through the system, so running nice and clear. Brake pedal feels firmer and more responsive. No ABS for the last couple of days. Fingers crossed it's sorted. Only other thing it could be is a faulty sensor, although I'll need to get codes read to identify.

[spartacus 68]

This fault is still ongoing. When braking the pedal depresses to the point where the ABS eventually kicks in. This happens on long-braking such as approaching a roundabout on an A-class road from around 60mph. I've done a few emergency stops and the car brakes properly but ABS doesn't engage unless the tyres lose grip.

I've no ABS errors on dash - but haven't had it checked for codes.

I cleaned the plenum chamber the other day and one rubber grommet was blocked with a little water in it. The other drain hole was clear so water was able to escape. I don't think the servo was damaged.

If brake servo was at fault - wouldn't it fail completely so brakes were rock-hard without pump assistance? Could also be leak in master cylinder? The car doesn't lose brake fluid. Any suggestions on what to do next?

[adamss24]

Is this on the quattro b5 ? If it is then the reluctor ring on the rear cv joints are known to split which will do the same as the ABS system thinks that the wheels are locking. Try the driver side rear cv joint first, last 10 quattro i've done were split on that side for some reason. The reluctor rings are cheap off ebay but if you're feeling flush- you can replace the whole cv joint !

[spartacus 68]

Is this on the quattro b5 ? If it is then the reluctor ring on the rear cv joints are known to split which will do the same as the ABS system thinks that the wheels are locking. Try the driver side rear cv joint first, last 10 quattro i've done were split on that side for some reason. The reluctor rings are cheap off ebay but if you're feeling flush- you can replace the whole cv joint !

Cheers adamss24? Will get up early and check out both sides tomorrow. I'll post back. Thank you.

[spartacus 68]

Is this on the quattro b5 ? If it is then the reluctor ring on the rear cv joints are known to split which will do the same as the ABS system thinks that the wheels are locking. Try the driver side rear cv joint first, last 10 quattro i've done were split on that side for some reason. The reluctor rings are cheap off ebay but if you're feeling flush- you can replace the whole cv joint !

Had a quick look today, but it's blowing a gale and then it started to snow!

How would you normally access the reluctor ABS ring? If I detach the driveshaft at the rear diff will it drop down? Or do I need to remove the bolt (nearest the wheel) on the wishbone in order to allow it to be removed?

I did look at the Bosch ABS sensor, but the allen bolt is worn and can't be removed. I thought I might be able to remove the sensor and look into the hub that way.

[delambo]

You say there is no abs light at all - does the light come on when you start the car then go out after a few seconds? If not then there is a big problem. When the ABS kicks in does the light come on? If not then it it probably not the abs coming on. Could it warped brake discs? This is commonly confused with the ABS kicking in. Just a couple of suggestions.

[spartacus 68]

You say there is no abs light at all - does the light come on when you start the car then go out after a few seconds? If not then there is a big problem. When the ABS kicks in does the light come on? If not then it it probably not the abs coming on. Could it warped brake discs? This is commonly confused with the ABS kicking in. Just a couple of suggestions.

There's no ABS light on the dash. It illuminates at ignition, then cancels. I'm going with adamss24 suggestion about a split rear reluctor ring. Only trouble is - you need to pull the rear axle drive-shaft to check. I did check earlier in the week but weather has taken a turn for the worse so had to abandon.

I fitted new pads and rotors a couple of years ago, and then completely stripped them down in the autumn, and rebuilt, etc. I've experienced brake judder due to pad-transfer (sticky piston) on my old 80 Avant and it's definitely not that - but cheers for suggestion.