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Anyone had this ? Feels like 'cadence braking' is happening... happens at low speeds too

Suspected the ABS (eg. a sensor) but no faults showing via diagnostics !

('95/TRW)

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Sounds as though the ABS is kicking in even if it isn't needed, does the ABS light flicker on when it is doing it? That would suggest an ABS fault, probably one of the sensors but usually you would also get ABS Fault on the dash and the ABS and TC lights on all the time.

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Thanks Richard, Yes it's definitely an odd one, nothing happens on the dash either !

EDIT: Just read elsewhere brake pads may be binding (unevenly) - but that should still show up as a fault...
Oddly the MSV2 states "all four wheels are rotating evenly"....

Will try some 'drive rapidly in a straight line and brake hard' cycles as per the other info I read: Not exactly ideal if the brakes are 'iffy' of course; Will report back ....!

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I've known issues when the battery is a bit on the flat side with that, though its usually a sign of an iffy sensor. Likely it will get worse and throw an error to give you something to go on if it keeps doing it.

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pulsing peddle is most likely warped rear discs , need machining or replacing

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I was also thinking warped discs, but how do you know that the rears are more likely than the fronts?

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ABS and warped disks feel quite different...

Warped disks is a much lower frequency and is generally a "smoother" pulse or judder. It also varies with speed.

ABS is a very high frequency "vibration" and often the pedal will feel very odd as the pump takes control.

I've had both on the P38. ABS engagement due to a front sensor with too large an air-gap, but it was very intermittent. It also felt bloody scary, as it would effectively turn the brakes off trying to get the wheel to unlock, which ofcourse never happened as it wasnt locked in the first place. Eventually it started producing a code for the front right sensor, and a small tap fixed the issue.

If its warped disks, you can sometimes tell by looking on the disk surface for a pad "imprint". Looks like the outline of a brake pad left on the surface of the disk. Warping is almost always caused by "user error". If the disks are hot (from lots of braking) and the car is then stopped with the brake pedal held on, the heat causes pad material to transfer to the disk surface. Sometimes this material is then simply worn off again, but depending on the exact thermal cycles etc, sometimes it doesnt go away and interacts with the steel to form an extra hard deposit. Over time the disk around the deposit wears away, but the deposit doesnt, creating a high spot (and we're talking fractions of a mm here) which is what then causes the vibration.

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Thanks for all your input chaps, much appreciated !

For clarification the effect is fairly slow - about 2 pulses per second but does not vary very much with speed. That said I took it out for some speed/brake cycles and it changed slightly... I'm not sure if that means it could be corrosion/dust on the pads/discs/etc (?) Has to be something mechanical though, as my P38 probably just thinks I am on ice or a slippery surface so does not report a 'fault' as such (?)

My nano is on loan at the moment but over the weekend it will be returned so I can then monitor the (individual) wheel speeds with it to see if there are any discrepancies. Either way it is probably time for a strip-down/clean up of brake components if the pulsing persists/worsens .. Might even do a 'run-out' check on the discs too and see what that shows.

-As an aside if anyone who has not experienced it wants to feel what ABS feels like 'in action', ie. without all the tricky stuff (ice etc) - just put different sized/profiled wheels on the front and back (an inch diameter overall is usually enough) !

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if the front discs are warped it vibrates or shudders thou the steering wheel . the rear one's pulse thou the peddle , usually noticed when rolling up to the lights with your foot lightly on the brakes . other than that you have to spin the disc and use a dial gauge manually .

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That makes sense.

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Update: First of all the nano "wheel rotation speed" facility is actually very limited on pre-99 Wabco-C units (as it stops communicating once the wheels exceed 1.6mph.... ): https://www.nanocom-diagnostics.com/downloads/preview/wabco-c-type-p38

-However when I first read those speed 'inputs' with the nano (for some reason) it gave Front Right/Left & Rear Right/Left values of 4/1 & 2/2 (kph) respectively which may/may not have been significant.. !? Anyone else done/tried this ?

Some more "fast/straight hard braking" cycles later therefore the symptoms improved further....
I have thus decided - rather that to just 'clean up the brake components' - to replace discs/pads,
starting with the rears...

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Because warping is usually pad deposits, heavy braking can clear it up, sometimes. it may return depending on many factors.