Chasing ABS fault
 

Hello all...

I've read a few threads on this now including one with replies today! Similar fault I think but not conclusive yet so any advice welcome.

I've got a slight pulsing through the brake pedal at low speed. Higher speed ok. Speedo ok. No fault codes to read with TOAF. I did the multimeter test on all four wheels and got 96 changes of voltage in total per wheel (I chalked every switch from 0.7 to 1.7 volts and vice versa).

However, nsr (particularly) and osf gave some of their readings reluctantly. At first pass there seemed to be an obvious gap. If I went over the gaps again they would give a reading. So I seemed to end up with a full set of chalk marks eventually.

Could this reluctance itself be a sign that all is not well with one or more bearings/sensors? I'm loathed to tear it down and replace parts that are fine but if even a slight whiff of a fault as above is enough to prove it then that's fine. How picky are these sensors? Has anyone else had similarly borderline results?

I have been in touch with Jamie Welch as he has a T4 and is local, but I get the impression it might not tell us very much, or is perhaps not the ideal tool for diagnosing ABS faults that don't log codes. Still worth a shot though I guess.

Thank you all.

Jim

Had this on my tourer few months ago turned out it was the rear back plates rusty causing the abs fault, I would strip the rear brakes down remove the bearing hubs and then clean all the muck and debris out, I fitted new back plates because mine were very corroded car has been fine since. :}

I should have mentioned... backplates were replaced, along with handbrake mechanisms and cables, plus osr bearing (stuck on) 7 months ago as part of a rear subframe refurb. All parts from DMGRS. Been fine til a few weeks ago.

[QUOTE]I have been in touch with Jamie Welch as he has a T4 and is local, but I get the impression it might not tell us very much, or is perhaps not the ideal tool for diagnosing ABS faults that don't log codes. Still worth a shot though I guess.

Thank you all.

Jim]

The T4 should tell you something.
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May need a new bearing front or rear, from what you say it could be the OSR bearing sticking again, or the NS bearing playing up, if and when Jamie finds the problem i doubt he will carry a bearing and do the job for you.

He could point you to out next Nano meet though, if its a rear bearing.

Yes a T4 diagnostic will help - one very useful feature is that if you look at the real time ABS data screen and drive the car slowly at a constant speed - idling in second gear say, then T4 will show you the real time speed readings for each wheel, and you will see a blip in the readings on the problem wheel.

The problem with some faults on the ABS system is that a missing magnetic reluctor segment or two give exactly the same signal back to the ABS ECU as a wheel losing grip and locking up - all the ECU sees is a gap between the signals.

That is why you can get a plausibility fault - the ECU can see one wheel out of four skidding time and again when none of the others do and that simply isn't plausible.

Jim on your car I would check the gap between the sensor and the magnetic reluctor on the bearing. A slightly larger gap will cause the problem you have. I have come across a sensor lifted away by a flake of rust on the back of the backplate (difficult to spot), and also an aftermarket rear sensor that I had to file away the fixing bracket to move the sensor a bit nearer the reluctor so that it would work correctly.

Seeing as you have already fitted new back plates, check the sensors are fully home - an extra coat of paint may be enough to increase the air gap and cause problems. Failing that try a new sensor or take 1 mm or so off the back of the bracket of the existing sensor and see if that works.

Abs
 

I had a very similar issue on my CDT.
Took everything apart and cleaned it all, washed everything in wd40 and reassembled it all. This was back in 2017 and the issue has never returned. I did note at the time that there were bits of rust stuck in the vicinity of the hubs, I think cleaning this away was what sorted the problem out.

Thanks everyone for your input.

Live data shows no fault from any sensor. Next up will be re-seating the sensors I guess.

Did you use a T4 for the live data test Jim, and did you get the pulsing pedal and at the same time no blip on the speed reading from any wheel?



One other question, when you get the pulsing pedal at low speeds, is it random or does it feel like it is happening at the same time on each rotation of the road wheels?

Not T4 but an Autel 808... smooth graphs for all 4 sensors but the pulsing is not so pronounced at the moment, so it's verging on intermittent.

It's definitely cyclical, not random. So now I'm thinking a bearing is breaking down. Brake discs? Possibly but they're either warped or not and would always be bad and never better at times surely?

The last time I saw this T4 didn’t pick up the change in speed when at low speed. However, using my iPhone I managed to record the data in ‘slo mo’ mode and when played back it was more obvious.;)
Incidentally my car has just started doing this too but as soon as I drive T4 stops communicating. I don’t really want to fire the parts canon at it so I’ve parked I up for another day and I’m using one of our other cars :D