Carrera 4 low speed abs pulse

[SlowM]

New member here saying hello
I'm the proud owner of an immaculate 1999 60k miles c4 cabrio tiptronic
The car itself drives perfectly apart from a niggling slow speed (10mph and less) gentle abs pulse? (I'm guessing this as the brake pedal pulses along with a twanging sound from the brake pedal area)
The car has been fitted with new abs sensors and the reluctor rings seem fine so a question to the 911uk members ....what's next to check, and who to visit for repairs I'm in north east London so any recommendations would be greatly received!

 

[HSC911]

:welcome:

 

[SlowM]

Thank you hsc911



Update! Just pulled abs fuse and the pulsing stopped definitely an abs issue

 

[Alex]

Hello & welcome. Mines recently started doing the exact same at around 18mph. I'm guessing its an incorrect reading from one of the sensors. My plan when I get chance was to put the car on axle stands, remove the wheels and let the the car run in gear. Yes I know this will instantly chuck up a psm fault as the axles will run in opposite directions due to the diffs, but I plan to independently take a reading from each sensor and check for any discrepancy.

 

[deckster]

Bear in mind that everything I know about the internals of my car can be written on the back of a postage stamp in 14 point text - but my C4 did this from when I bought it; Northway traced it to a split collar (? - my recollection, not necessarily their words) on the driveshaft and recommended I replace the whole thing. With a new driveshaft on the pulsing went away and hasn't come back since.

Potentially a sledgehammer to crack a nut? But it worked and I was happy

 

[kurlykris]

This was covered recently here :thumb:

http://911uk.com/viewtopic.php?t=131931&postdays=0&postorder=asc&highlight=abs&start=0

 

[SlowM]

Thanks for all the help and advice, much appreciated.
I shall dig my screw driver and handsaw out and take a look when I have dome spare time :thumb:

 

[deMort]

I've revised my opinion of the cause since that post i'm afraid .. 996 and the rear driveshaft reluctor rings get corroded and the gaps will fill in with rust basically .. at slow speeds the car struggles to see these gaps and so activates the abs .

Sensor is possible but a very good look at all 4 reluctor rings is advisable ...

Lets just say i've seen 4 in the last 5 months all doing this .

 

[SlowM]

Again thanks for the all the replys,
So are the reluctor rings visable without taking the abs sensors out?? I.e. underneath the brake discs/hub etc ...Ta!

 

[JS1500]

I had this same issue. I bought 4x reluctor rings but only replaced the rears. I still have the other two if you want them - no charge.

 

[deMort]

Again thanks for the all the replys,
So are the reluctor rings visable without taking the abs sensors out?? I.e. underneath the brake discs/hub etc ...Ta!

you can just about see them .. you are looking for a decent gap between each ring on each wheel .. rust joining the gaps together will make the car think the wheel isn't moving as it can see the other wheels moving and so decides that wheel is locked up .

Basically .. a gap then a solid then a gap tells the car that wheel is moving .. if it sees something different on a wheel ( due to corrosion ) then it assumes that wheel has locked up and so applies the abs ... a lack of gaps .

Front or rear can cause this with the last few i have seen all being rear .

Edit .. there should or at least might be a fault code .. if not an actual value a check on a tester will indicate which one is at fault .. for me .. i tend to spot the problem whilst its on a ramp .. perhaps a good inspection via an indy with this knowledge might help .

 

[SlowM]

Update!
So wheels off today, rear, reluctor rings look new, like the after market ones along with new abs sensors, fronts mmm hour to get the sensor out.......passenger side reluctor ring rotted away/broken and same on drivers side, so off to the garage it goes!
Thanks for all the help..

 

[UncleP]

I'd been getting a little kick through the pedal. The garage I bought the car from actually experienced the issue and replaced all 4 ABS sensors and the ABS pump (not cheap, thankfully they paid).

On my winter tear down I noticed the front driver side reluctor ring looks like it's made of weetabix while the other 3 look like new. I'm going to guess that was the issue all along.

 

[deMort]

To both the above .. yup .. thats what ive seen recently .. corroded reluctor rings .. it's not senors .. its rusty rings basically .

Cars are getting older i'm afraid and new faults are appearing .. this is just the latest .

To understand it .. it's an on off switch .. 1010101010101 .. with a corroded ring you get 1110101010111 etc .. the car thinks a wheel has locked up .

At faster speeds and it kinda misses the 111 .. it looks at the other wheels and sees this is not accurate .

 

[e92]

Does anyone know if you can just buy the rings by themselves instead of buying the whole axle? I have a 996 turbo if that makes a difference :thumbs: