Diagnosing a Pulsating Brake Pedal Fault
 

Whilst sophisticated diagnostic equipment can be invaluable for diagnosing some ABS faults, if you have a pulsating pedal at slow speeds and no ABS alarm, then French Mike's method of using a voltmeter to test the sensor output works a treat.

Had this fault on my car, and this was the T4 screen:


https://i.imgur.com/7PduYcS.jpg?1


So thought I'd start by checking the front offside wheel. A test of the sensor outputs showed 48 high signals, which is exactly as it should be:


https://i.imgur.com/ThzZDGV.jpg?1


Next up was the offside rear wheel (the most common to fail) and this told a very different story:


https://i.imgur.com/jQiCD6J.jpg?1

The outputs were a bit hit and miss and going round a couple of times improved things, which is why a roadtest in real time didn't show up any speed variations between all four wheels.


Didn't have the chance to try a good tip from Mark (Sworks) of taking a high speed video and playing it back in slow motion of the sensor speed readouts, which should have shown up the culprit wheel.


Whipping the hub bearing off revealed the cause, rust build up had damaged the magnetic reluctor strip:


https://i.imgur.com/kA7P8YC.jpg?1


Rubbed down the offending area and painted it (note to self; new back plates to be ordered). Fitted a new bearing (luckily had one in stock) and all was well. :}

Interestingly T4 did highlight an incorrect tooth gear fitted to one of the wheels. It must do a sum calculation over time on all the inputs from the ABS sensors and compare them. The difficulty it has is that the ABS system activates when it detects longer time gaps between the signals as that is exactly what happens when a wheel starts to lock up. The very same thing also happens when the reluctor segments are damaged or missing and not picked up by the sensor, and maybe that will cause the incorrect tooth gear fault to flag up.


Thanks again French Mike for posting up the test. :bowdown:








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Hi Mike.
My MGZT had the very same corrosion of the OS rear back plate which in turn eventually damages the bearing & or the sensor, its very funny though 9 time out of 10 its always the OS :shrug:

I cleaned mine up and treated it, the back plate lasted a further 15 months before it started to get worse, in the end the sills had rotted out, belts needed doing, compressor started to leak, several other items needed to be sorted so I let it go for parts, as I was looking for a tourer anyways.

I still have the back plates in the shed which i hope i will not need for a few more years yet on the NUK as the back plate when i had the drum off last week to clean out the dust looked in good order so they were painted. ;)

Hope you are keeping well and staying safe

Is there an advantage to getting the hubs inspected, and maintained BEFORE the problem appears, and the problem becoming more expensive ?

Hang onto the wheel bearing if it's quiet Mike, as all that need be replaced is the GACO seal with a good one from a clapped out bearing ;)

That is exactly the same fault as the ZT a few weeks back, and interestingly enough, if you clear the faults, then drive for some distance with cruise enabled, it will give you which wheel is giving the plausibility error.

The same fault is also incurred if the car is driven with a space saver spare fitted, or a single tyre of the wrong profile ;)

If I have an issue like this, I find a quiet straight stretch of road, or a car park, then driving slowly in a straight line watch the live data from the ABS sensor overview.

It is almost always possible to detect which wheel is causing the problem, as the readout from one wheel will momentarily drop :cool:

Brian :D

Hi Steve, yes strange the offside suffers more often - I'd have thought the nearside would get more road salt splashed up in winter.

Keeping well thanks as I hope you and yours are. Working this week as I'm a food packaging engineer.



It is an easy enough job Mark; once the wheel is off then undo two bolts and remove the brake caliper, undo the Torx bolt and remove the brake disc, unstake and undo the hub nut and with the hub off you can inspect and repair as required.

If you are doing both sides then swap the hub nuts over and you should find a fresh bit of rim to stake.

You will need a torque wrench that will go up to 210Nm for the hub nuts though. I bought one years ago on Ebay from a lorry fitter who had retired. Cheap enough but I did have to recalibrate it.

Thanks Bri - I'll try those tips the next time it happens.

Funnily enough I saved a good magnetic reluctor ring a year or two ago and had it on the shelf in the garage. Thought I'd use it as the bearing that came off was still good but I had a tidy up a couple of months ago and now I can't find it. :getmecoat:

It will turn up sooner or later (probably when I'm looking for something else) and then I'll have a good spare for future use. :}

It’s worth noting that in some cases when you remove the hub part of the bearing can seize to the hub spigot. I personally wouldn’t strip to check just for the sake of it and only if there’s an issue but I’m maybe just unlucky :D

Yes good point. I have always been lucky with our cars and that particular bearing, but often have to remove them at work.

Having said that if you are refitting the same bearings then it gives you an opportunity to grease them up. :}

I don't see this as a service check, but maybe worth doing it the once if they have never been apart or you are unsure as to the state of them.

I had similar pulsating brake pedal and no handbrake on O/S rear wheel, the bearing had leaked grease and had covered everything including the handbrake shoes. :duh:

A friend of mine who worked as a Service engineer at Girling many years ago explained that the rear OS ( RH) disc got all the muck as it was thrown up off the LH front wheel , in a bow wave V formation onto it, when running through puddles etc which were most often lying in the gutter.