My A4 has developed an abs issue (I think) that manifests as a pedal kick to the foot along with grabby brakes when under braking and below 20-15 km/hr. Pedal becomes normal again once car stops. This happens without fail everytime.

Occassionally when on a dusty road, the abs light comes on, but never lasts long enough to get the laptop hooked up. No codes remain.

I've posted this before and got suggestions on what to replace, but am more interested in root cause for this phenomenon...

Anyone who has lived through it, vicariously or otherwise please shout out.

 

Check your axles for excessively dirty or cracked tone rings. Additionally, if you have an oscilloscope, you can look at each wheel speed sensor for hiccups in the signal wave.

 

Thanks tweaked for the response.
I've since read on the forum here that its difficult or impossible to remove the abs sensors without destroying them. Based on the corrosion mine are showing on the outside I concur. How does one check tone rings?

Can compressed air remove particulates that cause this issue?

 

do you have any warning lights on? also check your brake pads.

 

Lights may come on per original post, but never stay on, nor do I see any residual codes with VAG. I have not tried the abs sensor watch trick, but will try it to isolate what sensor is not playing nice.
Pads are all fresh. Problem spanned pad change.

 

I replaced both front axles as one blew her marbles.
I replaced both outer rear cvs and turns out both had cracked tone rings.
Replacing one made no difference as they were done on different dates.

VAG ABS outputs did not expose this, I watched. Carefully.
I graphed too, no hint of the issue.

ABS light seemed to come on when on dusty roads, which defeated the abs and allowed regular braking. This was always internment.

Now to get used to not easing up when it kicked in....
:-)

 

not even graphing? wow that's crazy because the sensor is magnetic and when it's disturbed there should've been a crazy spike or flat spot on the graph. well glad your problem is solved lol

 

VAG graphs - not via scope.
And no, the cracks didn't register a difference in the data.
Tried rolling slow where it seemed to cause abs, but nothing.

This is probably more prevalent then many with an abs light on/off/intermittent, know.

I'm sure there are all kinds of failure modes:
1) my symptoms
2) Ring shreds sensor face leaving magnetic debris
3) Ring pops off and grinds area, punctures bearing inner seal

I should note that I replaced a wheel bearing on one side and it failed again in under 30k.
This time bearings on both sides while I was in there doing rear stubs.