ABS operation

BackintheFold · 11-05-19, 05:02 PM

My brakes are clearly playing up.
Everything is fine if I switch the ABS system off.
With it on, as soon as I brake (when moving) I get a weird valve-fluttering noise from the brake area (ABS unit and/or the" bomb")
It keeps making a noise like its trying to pressurise or a relay is fluttering until I switch the ABS off.

Any ideas? Is it likely to be the sensor gaps needing adjustment, or the sensor "ring" needing cleaning somehow.
Is it possible to check the sensors somehow? I don't have an oscilloscope so it would be by measuring voltage or resistance while spinning each wheel
Off the ground...
Thanks in advance for any/all pointers.

mikes2 · 11-05-19, 05:15 PM

In general, if you're only getting problems on the move, it's more likely to be a sensor.

It does a self test when you start up. Your abs light will come on with ignition and once you've started it up, it will go off again after the self test - typically around 5-10 seconds.

You're probably hearing the abs pump cutting in and out at low speeds - typically up to 5mph.

You can measure the resistance of your 4 sensors - they should all be roughly the same.

You can also measure a voltage output (mvolts) from each one by spinning a wheel. This is quite tricky as the it will vary with the speed you spin at. At least if you see output from all of them, you know they work.

What state are the reluctor rings in ? Pretty sure the ur uses the small teeth 96 I think. You may need to extract each driveshaft to check them. You can have a look at the sensor position while you're at it

urquattr085 · 11-05-19, 05:24 PM

Quote:

Originally Posted by BackintheFold

My brakes are clearly playing up.
Everything is fine if I switch the ABS system off.
With it on, as soon as I brake (when moving) I get a weird valve-fluttering noise from the brake area (ABS unit and/or the" bomb")
It keeps making a noise like its trying to pressurise or a relay is fluttering until I switch the ABS off.

Any ideas? Is it likely to be the sensor gaps needing adjustment, or the sensor "ring" needing cleaning somehow.
Is it possible to check the sensors somehow? I don't have an oscilloscope so it would be by measuring voltage or resistance while spinning each wheel
Off the ground...
Thanks in advance for any/all pointers.

It sounds like an issue I experienced with mine. Turned out to be the front o/s sensor out of adjustment. I assume after the fluttering, you get the ABS light on the dash?

Firstly, check you have continuity by unplugging the sensors at the strut tops/either side of the fuel tank in the boot and testing across the 2 pins.

If that's ok, it's likely the sensors are out of adjustment. If not, and like mine recently, the cable is probably toast, crumbling and corrosion is to be expected.

I tried trial and error adjustments to no avail, they are incredibly sensitive to changes in distance to the driveshaft ring. I only solved mine with an oscilloscope, I believe the output is around 50 millivolts, not readable with an everyday voltmeter.

It doesn't have to be a specific output, but the 4 corners do need to be more or less the same, it is this difference the system measures as a self-check and throws the system out if it doesn't get 4 readings the same.

Make sure the rings are clean though as well. It won't help if they are clogged.

I now also find I don't need to buy the "disposable" caps for the sensors, I didn't find setting them with these alone to be accurate enough. I do have to revisit the procedure on occasion too. My rear sensor outputs are stable (it's only the fronts that seem to change) so I use these for a reference reading and set the fronts to match.

Sorry it's a bit wordy, but hope it helps.
Geoff

BackintheFold · 11-05-19, 06:01 PM

Thanks guys. Some excellent pointers there.
The self check works, so that suggests the sensors are "whole" and the problem is a sensor gap/rings full of crap problem.
Given that all my other nuts and bolts have been easy to remove, I expect the abs sensors will be the same.
( �� )
Well here's hoping anyway.
I can't recall if the light comes on when the problem arises. It wasn't high on my list to do so I haven't paid that much attention.
I'll take a better look next time I test it.

Last point-when I drive with the ABS "on", the abs light doesn't come on for sure-does the system only check the status when you brake?
Many thanks. I'm in the "research phase" of this problem, learning what I can before I start poking around...

urquattr085 · 11-05-19, 06:24 PM

From what I recall, the system checks itself once you pull away, over the first few hundred metres.

My sequence of events after determining it was an adjustment issue was :

Ignition on, abs light comes on
Start engine, abs light goes off
First few yards you hear various whirs and minor noises, light still off
Occasional solid brake pedal - followed by "holy cr*p" or similar effin' & blindin' or brake pedal flutter
More usually, normal braking, abs light comes on within half a mile

Switch abs off and back on, light goes out but returned again shortly after.

Hope you can get sorted.

Mikey-S · 12-05-19, 12:00 AM

If you need 96 teeth reluctor rings they are dirt cheap on ebay.

Sent from my SM-G960F using Tapatalk

Hanuman · 12-05-19, 01:58 AM

The ABS is remarkably tolerant.

In the case of my ‘85 WR, the toothed ring on one drive shaft was so corroded, you’d have been hard pressed to see any teeth for about 1/8th of the ring, yet the ABS would still work normally most of the time.
One sensor has actually been damaged by being inserted without the ablative cap, and that still detects and I get normal operation.

If the system is not detecting any permanent error once moving, then I’d suggest that one or two sensors need adjustment, or the rings need cleaning or replacing. I’ve cleaned the rings before by pulling the sensor out, and gently feeling the ring teeth with a small terminal screwdriver. The teeth get clogged with general grime, and as they corrode, the gaps fill with corrode material deteriorating the signal.

The rings are about £10 from eBay. I managed to do one in situ on the car without removing the entire shaft.

The system has a ‘combination relay’, which is the item that ultimately results in the ABS light being turned off.

It will only turn the ABS light off if a number of conditions are met. My fading memory suggests seven required conditions to allow the light to go out at start-up, including-

System voltage normal (a dying battery and failed charging system can drop to 10v before this gives an error)
ABS on/off switch not pressed
System ‘healthy’ signal from the controller

The signal from the sensors isn’t checked until the car is moving. I think this is just a check that some signal is measured at each sensor, and the system simply compares the 4 signals as being reasonable. The bit about being very tolerant of poor condition rings!
There’s a distance over which this occurs. I think it equates to 60meters (x number of rotations).
If the light comes on, it can be reset by turning the ABS off then on, and it will again compare signals.

Once you get to the occasional solid pedal, sound of a rapid buzzing from the other side of the bulkhead without any heavy braking to induce lock-up, it’s likely that the system is now getting one bad signal and is ‘just’ enough to keep functioning.

In my case, once the rings were cleaned (not diligently, either) it was several years before it finally failed.

Damp/wet conditions will exacerbate a grubby ring (ooh-er). I found the ‘flutter’ and eventually ABS dropping out happened mostly on wet days, resulting in the ABS light coming on, until the dodgy ring was sufficiently eroded as to be useless.

Groundhog · 12-05-19, 08:17 AM

As above ( and very eloquently put) this is the classic symptom of one of the sensors being out of adjustment but not out enough to put the warning light on.
In my experience of dealing with, literally, hundreds of these it is a front sensor more often than not.

BackintheFold · 18-05-19, 02:17 PM

Thanks for the tips.
I had a go at this today (having failed to remove my clutch slave cylinder! Grr)

I cleaned the teeth on the rings. There's a small gap at the bottom of the hub where you can get a scraper in-and also a toothbrush to remove dusty rust.

So I cleaned both front rings and took it out for a test.

Stationary engine off. Ignition on. Abs light on
Start car. Light goes off-AND DOES NOT COME BACK ON AT ALL
But... noise still happens. Even when just driving in a straight line
without brakes on. It sounds more like an electric pump now I'm paying attention.

If I had to guess, I'd say my brake bomb wasn't holding pressure, and that sometimes it is "topped up" with an electric pump. Is that possible?

I found that it is sometimes possible to drive at 25, slow almost to a standstill without the noise, then just as I'm about to stop the noise comes on, maybe for a fraction of a second, maybe for 2 seconds.
When the noise happens, the pedal does not pulsate. That suggests to me it is not the ABS removing brake pressure, and I am inclined to think it is the brake bomb-but I don't get a red warning light for that.

Does this help clear anything up? I think, as the brakes work fine, and no ABS light comes on, it will pass the mot, but I want to fix it of course!

BackintheFold · 18-05-19, 08:35 PM

Quote:

Originally Posted by Groundhog

As above ( and very eloquently put) this is the classic symptom of one of the sensors being out of adjustment but not out enough to put the warning light on.
In my experience of dealing with, literally, hundreds of these it is a front sensor more often than not.

Id rather not try and adjust the sensors, I worry that if I break one I'm in a world of hurt, and I don't have an oscilloscope to test the outputs either.

A Question comes to mind-Would the system make a noise-under any circumstances-when you are simply driving and not braking? If so, why?
Thanks!
S

11-05-19, 05:15 PM	#2
mikes2 4 ring whore! Classic Audi Club Member Join Date: Nov 2007 Location: Polmont Posts: 2,617	In general, if you're only getting problems on the move, it's more likely to be a sensor. It does a self test when you start up. Your abs light will come on with ignition and once you've started it up, it will go off again after the self test - typically around 5-10 seconds. You're probably hearing the abs pump cutting in and out at low speeds - typically up to 5mph. You can measure the resistance of your 4 sensors - they should all be roughly the same. You can also measure a voltage output (mvolts) from each one by spinning a wheel. This is quite tricky as the it will vary with the speed you spin at. At least if you see output from all of them, you know they work. What state are the reluctor rings in ? Pretty sure the ur uses the small teeth 96 I think. You may need to extract each driveshaft to check them. You can have a look at the sensor position while you're at it __________________ 1991 90 quattro 20v sport 1991 90 quattro

11-05-19, 06:24 PM	#5
urquattr085 Senior Member Classic Audi Club Member Join Date: May 2016 Location: Displaced Northerner in Essex Posts: 1,528	From what I recall, the system checks itself once you pull away, over the first few hundred metres. My sequence of events after determining it was an adjustment issue was : Ignition on, abs light comes on Start engine, abs light goes off First few yards you hear various whirs and minor noises, light still off Occasional solid brake pedal - followed by "holy crp" or similar effin' & blindin' or brake pedal flutter More usually, normal braking, abs light comes on within half a mile Switch abs off and back on, light goes out but returned again shortly after. Hope you can get sorted. __________________ 1985 WR quattro 2010 A6 3.0Tdi Allroad 2010 A3 1.4 tfsi Sport - Sold but still in the family 1984 80 Quattro - Sold many moons ago 1981 Coupe GT - Sold to buy above Last edited by urquattr085; 07-11-20 at 05:18 PM. Reason: Spelling*

12-05-19, 01:58 AM	#7
Hanuman Trickster Classic Audi Club Member Join Date: May 2009 Location: Heckling from the cheap seats, Phnom Penh, KoW Posts: 7,006	The ABS is remarkably tolerant. In the case of my ‘85 WR, the toothed ring on one drive shaft was so corroded, you’d have been hard pressed to see any teeth for about 1/8th of the ring, yet the ABS would still work normally most of the time. One sensor has actually been damaged by being inserted without the ablative cap, and that still detects and I get normal operation. If the system is not detecting any permanent error once moving, then I’d suggest that one or two sensors need adjustment, or the rings need cleaning or replacing. I’ve cleaned the rings before by pulling the sensor out, and gently feeling the ring teeth with a small terminal screwdriver. The teeth get clogged with general grime, and as they corrode, the gaps fill with corrode material deteriorating the signal. The rings are about £10 from eBay. I managed to do one in situ on the car without removing the entire shaft. The system has a ‘combination relay’, which is the item that ultimately results in the ABS light being turned off. It will only turn the ABS light off if a number of conditions are met. My fading memory suggests seven required conditions to allow the light to go out at start-up, including- System voltage normal (a dying battery and failed charging system can drop to 10v before this gives an error) ABS on/off switch not pressed System ‘healthy’ signal from the controller The signal from the sensors isn’t checked until the car is moving. I think this is just a check that some signal is measured at each sensor, and the system simply compares the 4 signals as being reasonable. The bit about being very tolerant of poor condition rings! There’s a distance over which this occurs. I think it equates to 60meters (x number of rotations). If the light comes on, it can be reset by turning the ABS off then on, and it will again compare signals. Once you get to the occasional solid pedal, sound of a rapid buzzing from the other side of the bulkhead without any heavy braking to induce lock-up, it’s likely that the system is now getting one bad signal and is ‘just’ enough to keep functioning. In my case, once the rings were cleaned (not diligently, either) it was several years before it finally failed. Damp/wet conditions will exacerbate a grubby ring (ooh-er). I found the ‘flutter’ and eventually ABS dropping out happened mostly on wet days, resulting in the ABS light coming on, until the dodgy ring was sufficiently eroded as to be useless. __________________ I wish they would keep the damned Chinese away now that I can go home, so that I can enjoy Fish amok and a draught Angkor

11-05-19, 06:01 PM	#4
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,171	Thanks guys. Some excellent pointers there. The self check works, so that suggests the sensors are "whole" and the problem is a sensor gap/rings full of crap problem. Given that all my other nuts and bolts have been easy to remove, I expect the abs sensors will be the same. ( �� ) Well here's hoping anyway. I can't recall if the light comes on when the problem arises. It wasn't high on my list to do so I haven't paid that much attention. I'll take a better look next time I test it. Last point-when I drive with the ABS "on", the abs light doesn't come on for sure-does the system only check the status when you brake? Many thanks. I'm in the "research phase" of this problem, learning what I can before I start poking around...

12-05-19, 12:00 AM	#6
Mikey-S Grown up member Classic Audi Club Member Join Date: Dec 2015 Posts: 68	If you need 96 teeth reluctor rings they are dirt cheap on ebay. Sent from my SM-G960F using Tapatalk

12-05-19, 08:17 AM	#8
Groundhog Grown up member Join Date: Nov 2009 Posts: 143	As above ( and very eloquently put) this is the classic symptom of one of the sensors being out of adjustment but not out enough to put the warning light on. In my experience of dealing with, literally, hundreds of these it is a front sensor more often than not.

18-05-19, 02:17 PM	#9
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,171	Thanks for the tips. I had a go at this today (having failed to remove my clutch slave cylinder! Grr) I cleaned the teeth on the rings. There's a small gap at the bottom of the hub where you can get a scraper in-and also a toothbrush to remove dusty rust. So I cleaned both front rings and took it out for a test. Stationary engine off. Ignition on. Abs light on Start car. Light goes off-AND DOES NOT COME BACK ON AT ALL But... noise still happens. Even when just driving in a straight line without brakes on. It sounds more like an electric pump now I'm paying attention. If I had to guess, I'd say my brake bomb wasn't holding pressure, and that sometimes it is "topped up" with an electric pump. Is that possible? I found that it is sometimes possible to drive at 25, slow almost to a standstill without the noise, then just as I'm about to stop the noise comes on, maybe for a fraction of a second, maybe for 2 seconds. When the noise happens, the pedal does not pulsate. That suggests to me it is not the ABS removing brake pressure, and I am inclined to think it is the brake bomb-but I don't get a red warning light for that. Does this help clear anything up? I think, as the brakes work fine, and no ABS light comes on, it will pass the mot, but I want to fix it of course!