ABS cutting in at low speeds

[mryesno]

I'll just ask them for a refund and go from there.

[Arctic]

Hi Les.
Yes rust on the ring will show up the fault, I found this to be the case with mine not long back, I keep meaning to get round to the how to I did but real life gets in the way.

I also found the back plate to be in a corroded condition and as I did not have ant to hand at the time I cleaned the area changed the sensor and the hub, job done I still need to change the back plate when I have time now that I have sourced them, because I am sure if you do not change the corroded back plates the problem will raise it's ugly head once again in the near future

[HarryM1BYT]

Originally Posted by Sector-9

Secondly, if your speedo and cruise control still work then you can forget the front sensors.

I agree with the rest of your post, but the above part is not correct....

It is possible for those two to work at reasonable speeds, yet for the sensor's output to fail at lower speeds. T4 will define which corner has a problem, as will Frenchmike's multimeter test method and my own adaptation of Frenchmike's method - listening to clicks on a piezo earphone.

His problem is irregular / unreliable pulse output at low speed. Usually that traces back to a sensor being slightly too far away from the disk. So his first problem is to determine which sensor is misbehaving, then file the lug to allow it to go in to its socket deeper.

His garage should fix the issue for what he has already paid, but many garages know little of the techniques of testing ABS.

The sensor should not only produce those voltages, but produce the voltage changes with absolute regularity. The only way to prove that is to mark the tyre wall with chalk, at every pulse and roll the wheel round a few times, to make certain the pulses always show up at the marked points.

ABS is triggered by a pulse(s) failing to appear, when compared to the output of the other wheels.

[Mike Noc]

Yes I found exactly what you have mentioned Harry. When one of my rear sensors was playing up I connected a home made extension lead and plugged it into the OSF ABS plug.

The speedometer worked perfectly - the sensor was only missing the odd segment at very slow revolutions of the wheel, when the speedometer would hardly be registering anything anyway.

In my case it was a slightly too big air gap between sensor and the magnetic reluctor in the bearing - both aftermarket items. Easily found using French Mike's voltage test.

[ElmerT]

I recently had a similar issue to those described here. Front, offside brake emitted a gravelly crunching noise and noticeable pulsing through brake pedal, no ABS light coming on and no other problems. Mechanic replaces both rear discs and pads, which he said were quite rusty and said that the fronts were ok but that a couple of bolts were slightly loose on lower wishbone bush brackets. Anyway, noise stopped and I’m happy, for a while. A few weeks later, exactly the same symptoms when travelling up North. Took to friendly mechanic who changed both front discs and pads but, because he hadn’t cured the issue, only charged for parts, (trade cost £60). He did say he thought it must be an ABS problem. Having read the stuff on here, I removed the ABS fuse and took the car for a spin. Braking smooth and no problems or dodgy noises. Does this indicate a front O/S sensor problem or are there more options?

[HarryM1BYT]

Originally Posted by ElmerT

Braking smooth and no problems or dodgy noises. Does this indicate a front O/S sensor problem or are there more options?

It only suggests the obvious, a possible issue with your ABS, which shows up at low speed, which could be as a result of many things. Just changing parts randomly and hoping, rarely works and is never cost effective.

Best advice is to jack each wheel up in turn and check for missing pulses from the sensor, because a missed pulse is what the ABS detects to trigger itself.

You do that with a test meter, T4 or etc., as posted and explained several times on here.

[ElmerT]

Thanks Harry. As you may have guessed, I know nothing at all about ABS and recognise that mechanical logic often does not work where electronics are involved. I removed the ABS fuse and braking is definite and smooth and I appear to have lost no other functions i.e. the speedo and fuel gauge are still working so am happy to drive in central London without the embarrassment of the shuddering crunching noises. I am still tempted to try a replacement of the cable as it is relatively cheap, I have had a lot of front suspension work done prior to the issue and the sensor fitting appears un-rusted but probably would not bother if there is the possibility of one of the other wheel sensors causing the crunching on the front N/S wheel. What do you (or anybody) think?

[HarryM1BYT]

Originally Posted by ElmerT

I am still tempted to try a replacement of the cable as it is relatively cheap,

What do you (or anybody) think?

You can go on forever throwing replacement parts at it, until you accidentally replace the faulty item. Problem is, it is an expensive and time consuming way to fix it, plus you might introduce more faults to the one you already have.

Please read up on how to test the output from a sensor or find someone with T4 to diagnose it for you.

It is illegal to drive a car fitted with ABS, with the functionality ABS disabled.

[ElmerT]

Thanks Harry, point taken, I’ll read up on suitable meters, I could do with a new one, my multi-meter is a bit ancient, it has a needle and Roman numerals.