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Old 6th January 2011, 21:00   #31
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by DerAlte View Post
......
What we are missing in all the discussion is that failure of ABS, intended or not intended, is not going to impede braking. That, I think, is what @Sutripta started questioning.
......
So you are saying that no matter how an ABS component fails, the braking system will continue to work as if the ABS system was not there?

Anyway, lets say as of now we agree to differ. Strongly. Each person stating his PoV Ad Nauseum is not going to convince the other.


@SS. Thanks for providing the reference to the book.
The supposed advantage of posing a question on an open forum is crowdsourcing. Someone somewhere knows more than you do. e.g. I do believe that this forum has people who code the ABS control units. However, they, and their ilk are keeping away. I'd suggest we start a private conversation, and let this thread die.

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Sutripta
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Old 7th January 2011, 01:17   #32
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Default Re: ABS - How Failsafe

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Originally Posted by Sutripta View Post
So you are saying that no matter how an ABS component fails, the braking system will continue to work as if the ABS system was not there?
Absolutely. No question about it. Excellent explanation is given by DA below explains how and why.

ABS failure is a mild failure, but complete brake failure is severe. Even when brakes fail, there are two circuits (diagonal split) and the other should continue to work. FMVSS xxx also prescribes that the car should stop in a specified distance with one circuit working.

Coming to software, the #lines of code will be only like 1/5 of what it is now if you did not care about diagnostics and error proofing. DA can add more to this.

FMEA also describes how progressive and non-catastrophic the failure should be. Meaning you design the system to moan and groan and deteriorate progressively instead of letting go in one shot as it fails.

Suspension, steering and brakes are usually designed with 2-3 times the factor of safety compared to other systems of a car.

Quote:
Originally Posted by DerAlte View Post
ABS valves are energized to bleed, not energized to supply / block Master pressure to the brake. If it cannot be energized (blown solenoid, for example), it does not impede braking and does not participate in the braking system. ABS systems are designed not to act if ANY of its components are not functioning as designed, and most of the time an ABS ECU keeps performing diagnostics.
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Old 7th January 2011, 02:11   #33
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Default Re: ABS - How Failsafe

I once test drove an Accord, In Chicago, (while looking for my car) which had its ABS working all the time, Like the pedal would jerk even at low braking pressures- even when there was no wheel lock.

I supsect the sensor was always detecting that a wheel ws locked and relieving pressure on the brakes.

The effect of this malfunction was significantly reduced braking distances.

EDIT:- Just read the previous page, The car I was test driving ahd the so called false modulation- the effect was the braking distances were larger than usual. I can say so because i was test driving quite a few vehicles at that time and this specific car took quite more distance to stop than other 1999-2002 accords, which i test drove

I was in the market for that specific model.

Last edited by Jomz : 7th January 2011 at 02:18.
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Old 7th January 2011, 14:01   #34
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by Jomz View Post
...Like the pedal would jerk even at low braking pressures- even when there was no wheel lock.
The effect of this malfunction was significantly reduced braking distances.

EDIT:- ...this specific car took quite more distance to stop than other 1999-2002 accords, which i test drove
@Jomz: Which was it? More distance required to stop, or less distance?

Edit: Yours does seem to be a first-hand experience with false modulation on this forum, apart from what I've personally seen.

Last edited by SS-Traveller : 7th January 2011 at 14:03.
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Old 7th January 2011, 14:26   #35
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Default Re: ABS - How Failsafe

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Originally Posted by SS-Traveller View Post
... The specific component that triggers false modulation is a faulty wheel speed sensor ...
The ABS ECU knows that something is wrong, and turns on the ABS light to warn us of failure, when false modulation DOES occur. ...
Nope - false modulation happens to all channels despite the fact that only one sensor is triggering it. That's why the car feels like it's trying to brake on loose sand
1. Couldn't find, in your quoted text, any mention wheel speed sensor as being the cause for FM!
2. WSS is a simple passive reluctance sensor, and one of the ABS components that fail the least, if ever
3. If one WSS fails (wire open, no signal, etc.) the ABS ECU marks it failed and no longer uses it for any processing. The average of the other plausible signals is then used for that wheel. No, 1 sensor failure does not affect processing for other wheels, ECU continues to control based on their signals
4. The ABS light indicates sensor or other processing failures - NOT 'false modulation' (arey baba, 'false modulation' is a concept understood only by humans. For the ECU all plausible signals lead to a result and all results, in this case ABS valve modulation, are plausible)
5. And no, 1 sensor failure cannot make the car feel it is braking on loose sand (that would be an exaggeration). 2 sensor failure may, e.g. both front , both rear or both on one side WSS fail

Quote:
Originally Posted by SS-Traveller View Post
... I'm going to curse the ABS system as a whole, not be forgiving to the ECU for being handicapped!
The ABS ECUs are more than adequate, just that you are imagining too much capability to be present!

Quote:
Originally Posted by SS-Traveller View Post
... Incidentally, actually driving on sand without ABS allows the wheels to dig in and get some braking - with ABS the feeling of being out of control is rather remarkable. ...
On sand, braking is usually straight (and not because the wheel digs in). Fishtailing happens when any locked wheel is slipping on the road and the other is not.

Quote:
Originally Posted by SS-Traveller View Post
... In such a situation, IF there was a switch to turn ABS OFF, one could brake normally.

Try the handbrake?? Downshift? Maybe turn off ignition to tell the ABS ECU to shut up? I dunno...
Better 'safe driving' practice is to concentrate (while braking and skidding) on steering the car to a safe situation:
1. Keep enough distance to the car in front at all times that you feel you can tackle while braking
2. Steer away if there is an obstacle in front (including releasing brake and accelerating)
3. Turning the steering wheel in the direction of the skid if there is no obstacle in front

Quote:
Originally Posted by Sutripta View Post
... I do believe that this forum has people who code the ABS control units. ...
There are, and they did talk! Pity you were not in a listening state of mind. All the best. Peace!
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Old 7th January 2011, 14:53   #36
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by DerAlte View Post
1. Couldn't find, in your quoted text, any mention wheel speed sensor as being the cause for FM!
2. WSS is a simple passive reluctance sensor, and one of the ABS components that fail the least, if ever
I do not like to use Wiki as an authoritative reference, but here's a quick shortcut... Answers.com - What causes the ABS light to turn on

Quote:
Another thing to remember about ABS is that it's easy to damage those delicate wheel speed sensors while doing brake work, or to cause metal particles to attach themselves to the magnet. Either is likely to put the system into default, or make it go permanently into anti-lock mode (called "false modulation"), and turn on the warning lamp. On ABS, speed sensors are at the top of the list of failures. As far as the ABS problems are concerned, the first is speed sensors with metallic particles sticking to the sensor nose. This may cause system default, or make it constantly go into anti-lock mode called 'false modulation.'
FM on radio sounds better than FM on ABS... Listen Red FM Online 93.5 Bajate Raho!

I echo the Old Man...
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Peace!
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Old 7th January 2011, 15:58   #37
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Default Re: ABS - How Failsafe

I wouldn't know how fail-safe ABS is, but it sure did save us a couple of years ago one rainy night near Mysore. We were driving back from Kutta. The road from Hunsur to Mysore was being widened, and unmarked diversions were more the rule than an exception. I was driving along at a fair clip on a recently asphalted/concretised section with a crazy Santro tailgating me.
At one point he overtook me from the left around a curve - only for us to hit an unannounced diversion. I hit the brakes hard, cut right to the safer part of the right; ABS/EBD/BA kicked in and I was safe. Santro did the same, tramlined and ended up in a ditch. So, thank heavens for the ABS
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Old 7th January 2011, 16:13   #38
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by Jomz View Post
I once test drove an Accord, In Chicago, (while looking for my car) which had its ABS working all the time, Like the pedal would jerk even at low braking pressures- even when there was no wheel lock.

I supsect the sensor was always detecting that a wheel ws locked and relieving pressure on the brakes.

The effect of this malfunction was significantly reduced braking distances.

EDIT:- Just read the previous page, The car I was test driving ahd the so called false modulation- the effect was the braking distances were larger than usual. I can say so because i was test driving quite a few vehicles at that time and this specific car took quite more distance to stop than other 1999-2002 accords, which i test drove

I was in the market for that specific model.
Firstly, the ABS light should have been ON on that car. Some used car dealers are known to disable the light to get the car sold.

Secondly you are looking at 10 year old cars with 2nd or 3rd owner. More that likely the car has worn out/glazed pads and/or stickly/leaky calipers and/or rotors worn beyond limit....all of which can be attributed to the long stopping distance. Also if the ABS was kicking in all the time you will feel the vibration at the pedal. THere should be no jerk.
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Old 7th January 2011, 17:22   #39
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Default Re: ABS - How Failsafe

I could feel the vibrations, By Jerk I meant the typical ABS vibrations on the pedal. The car had that vibration on low/ moderate braking on dry roads.

Even I asked the dealer about the jerk and the dealer told it was ABS kicking in.
The light might have been disabled by the dealer- after all it is just a bulb in the instrument console. The bulb have been burned out tooo, I did not check the intial key on test.

For clarification, This car (ABS kicking in car) took longer to stop than other cars.

Edit :- Did some googling and from
http://www.hondacarforum.com/honda-3...ss-brakes.html

" have a 93 Honda Accord. I just got front rotors, front pads and
>right front calipers installed. I drove out of the mechanics garage,
>and the ABS light came on. Also, everytime that I press the brakes,
>the ABS seems to be kicking in( I can feel the brake pedal kick back,
>the noise of ABS, etc).

Think about what ABS does. It stutters the brakes when they are
applied and the syatem senses that one wheel is not turning as fast
as the others. Now your wheels are turning at the same speed, but the
system thinks otherwise.

Gee, would a misplaced wheel speed sensor cause that?
And you just had some work done on the brakes on those wheels?"

My problem on an older car...


But this one is more dangerous and This May not be addressed in An FMEA.
From http://www.toyota-4runner.org/3rd-ge...lled-me-5.html

"I posted on another thread concerning the ABS. Just wanted to chime in here. I have driven 4 or 5 different vehicles with ABS systems, and I agree that they are well designed, and for a good purpose. They do optimize stopping ability when they function correctly. The issue is not whether or not ABS is an efficient system. Normally it is. That doesnt mean you cant have a flawed system that needs some kinks worked out. What we're talking about here is the particular ABS system on the early third gen 4runners (apparently 96-98). I have the 98 sr5, and can say that it does not function the way the ABS does on the other vehicles Iv felt ABS on (honda accord, toyota tundra, pontiac grand am, ford taurus). For some reason Toyota had some design issues with it those first three years. Honesly, as Brian and others have said, its terrible, to the point of being dangerous. A few weeks ago I rolled out into an intersection going about 10 mph, because ONE of my tires hit a patch of ice. The other three were on solid concrete. But that one tire engaged the ABS, which, frankly just doesnt do what it is intended to. Rather than assisting stopping power, it entirely removes it. The 99-02 runners (even though they are 3rd gen) dont seem to have the same ABS system. I imagine the good people at Toyota worked out the kinks. But I think one would need to drive mine or a similar runner for themself, to really understand it. I'll stop rambling now."

Last edited by Jomz : 7th January 2011 at 17:31.
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Old 7th January 2011, 21:31   #40
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Default Re: ABS - How Failsafe

Wow! suddenly a lot of posts. Just when I am short of time (enthu actually. Dog tired. Guess will be till Monday. )

Interesting also to note the unanimity amongst the moderators.

Crux of my question
Quote:
Originally Posted by Sutripta View Post
So you are saying that no matter how an ABS component fails, the braking system will continue to work as if the ABS system was not there?
And the crux of the reply
Quote:
Originally Posted by Mpower View Post
Absolutely. No question about it.
I'll lay out my chain of thought later. It'll be fun ripping it apart. Actually it is no different from what I've stated earlier, but I'll have to collate it better. Not on the fly, of the top of my head. Till then let some more posts come in.

Regards
Sutripta
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Old 10th January 2011, 20:59   #41
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Default Re: ABS - How Failsafe

Quote:
Originally Posted by Sutripta View Post
I'll lay out my chain of thought later.
I believe the addition of ABS to the original braking system affects (reduces, however slightly) the reliability of the system.
Whereas the house believes
Addition of the ABS does not in any way (N.B. unequivocal statement, no qualifiers) affect the original system, which will always continue to function as before in case of ABS/ ABS component failure.

Treat the following as an example/ counterexample/ thought experiment. Once this is resolved, we can move forward to ore interesting questions.

The ABS is essentially an add-on to the original braking system, except at two points where it 'cuts into' the original system. These two points are the pressure release valves which release pressure at the wheel cylinders, essentially unbraking the wheel, and the pump which repressurises the line. These two components (and whatever controls these components) directly interacts with the the hydraulics of the original system. These components would not be there if the ABS was not there.

Take a very simple (easy to visualize) example: the failure of a release valve*. If it fails in open mode, braking is lost at that wheel. So failure of an ABS component has degraded the performance of the original system. If so, what the house believes does not hold.

Would like to find out the flaw in my reasoning.

* Failure modes. No matter how well designed, how well made, any component can fail. It essentially comes down to a numbers game - how many failures per million operations etc. In this case one might think that by designing the valve to seat with pressure, rather than against it, one makes it failsafe. I would say not so. One has reduced the chance of that failure, but it is still a numbers game. Even if we put two such valves in series, it is still a numbers game. Very small, very close to zero. But that is not the same as zero. The issue I wanted discussed is more subtle than this, but would like to clear my basics first.

Regards
Sutripta
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Old 4th March 2011, 19:20   #42
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Default Re: ABS - How Failsafe

Just saw this thread and adding a few points to it.

Issue1: ABS failure
The brake system will continue to work (with diminished capacity) in case of total or partial ABS failure. The ABS controller/ECU is programmed with many different scenario's for single or multiple component failure.

Some scenario's that I have personally tested
  • ABS off (ABS controller plugged out) and all four wheels braking
  • ABS off (ABS controller plugged out) and only primary circuit braking (brake hose for secondary circuit manually clamped)
  • ABS off (ABS controller plugged out) and only secondary circuit braking (brake hose for primary circuit manually clamped)
All the above tests (done in both GVW & Unladen+2 conditions and on both dry & wet surfaces) are a necessary part of homologation. A vehicle is required to clear these tests with certain parameters to be deemed road worthy.

Issue2: ABS+EBD
Sorry to bring in an additional component into the picture but AFAIK quite a high percentage of vehicles with ABS today also have EBD.
In the case where an ABS system has EBD the vehicle does not have a proportioning valve (as EBD does the work of the proportioning valve). And, without a proportioning valve the front and rear brake pressures are not compensated for weight transfer...so for such a vehicle the tendency for rear wheel lock will be much higher in an ABS fail condition than if the vehicle did not have ABS (in which case it would have a proportioning valve).

peace,
~kg
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Old 4th March 2011, 22:13   #43
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Default Re: ABS - How Failsafe

^^^
Hi,
No quibbles with what you are saying. What I wanted to say laid out in post # 41.

Regards
Sutripta
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