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How do it Know?

Discussion in '80-Series Tech' started by KliersLC, Mar 5, 2007.

  1. KliersLC

    KliersLC

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    So I was screwing around on some snowy roads today, and I started to wonder how the vehicle knows that it is still moving when all of the tires are locked up...

    I mean, the VSS is gets the signal from the tranny, and if you lock up the wheels, the tranny stops too.

    So I thought it could be inertia driven, but then why wouldn't it unlock your brakes for you on a steep decline?


    So, for the brains out there, why doesn't the ABS unlock the brakes when you are not moving? How does it know that the vehicle is moving when the Wheels, axles, and tranny are all stationary?

    Just curious, and wondering if whatever part makes it not unlock at a stop could potentially fail, causing all sorts of grief and mayhem.

    Thanks,
    Dan:cheers:
     
  2. chibo

    chibo

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    The ABS system deactivates at a certain speed depending on the car, 10-12mph is common. This is accomplished with the vehicle speed sensor (I don't know where this is drawn from on the 80, I assume Tcase/tranny) and an accelerometer.

    edit: If you are really interested you can check out SAE paper 2002-01-2229
    http://www.sae.org/technical/papers/2002-01-2229
     
    Last edited: Mar 5, 2007
  3. -Spike-

    -Spike-

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    The ABS senses wheel speed differential. If one wheel locks while the other three are moving, the ABS will activate. If you locked up all four tires simultaneously, the ABS would not activate. Fortunately, it's nearly impossible to hit the brakes hard and fast enough to accomplish this, so when you hit the brakes hard one or two wheels will lose traction and lock up, activating the ABS and freeing the brakes on the locked wheel(s). Another wheel may lock up, and the ABS will sense the speed differential and free that wheel, until all the wheels are doing the same speed. Once all 4 wheels stop moving, there is no speed differential between them and the ABS will not activate.

    -Spike
     
  4. KliersLC

    KliersLC

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    My ABS works on ice and snow at speeds well below 10-12 mph

    This makes more sense I suppose, but my abs functions on sheet ice just fine (well as fine as a 13 year old ABS system works) but it seems like sheet ice would give an instantaneous lock oup if it were possible.

    Could this be a function also of the proportioning valve? i.e. to make sure that on nearly zero traction surfaces one set of wheels locks before the other set, thereby activating the ABS....

    You said, "nearly impossible" That means that it can be done, and there is a potential for an unexpected, Non ABS panic stop. Hmmm... I would think that the lawsuit potential there is too great.
    There has to be a back up for this.
     
  5. chibo

    chibo

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    Guess it's done differently on an 80 then.
    Yes, very possible.
     
  6. DanKunz

    DanKunz SILVER Star

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    The birfield and rear hubs have metallic pick-ups for the ABS sensors to magnetically detect. They (the pick-ups) will rotate with wheel turns. I suspect the amount of pedal pressure dictates the braking force, and once past a certain spec + wheels still turning the ABS fires...regardless of speed.
     
  7. e9999

    e9999 You want to do what...? Moderator

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    I don't know of an accelerometer for the ABS system.

    I never heard there was any besides for the air bags.
     
  8. Nemo

    Nemo

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    Under the center console piece for lack of words there is a deceleration sensor. I believe it detects actuall movement of the truck as well. I am not sure how it works, but it knows you are still moving. Not sure how accurate it is but if I understand that piece right that is what it does. I don't have my FSM right now so I can't remember what it is call, but it is something like a decelerametor :)
     
  9. -Spike-

    -Spike-

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    It's for the airbags.

    -Spike
     
  10. -Spike-

    -Spike-

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    The hydraulic braking system is an imperfect thing. Due to variables such as longer brake lines to some brakes, the proportioning valve, pad and rotor wear, etc, it would be nearly impossible to lock up all four tires at exactly the same instant, regardless of tire traction. Therefore, one or more wheels will lock, the ABS system activates on those, then another locks, ABS deals with it, etc, until all 4 wheels are doing the same thing.

    On top of that, the ABS is meant to assist the driver. As you could not sue the tire manufacturer in the event your tire loses traction, neither could the ABS manufacturer be held liable if the ABS wasn't able to deal with an extreme situation, nor an airbag manufacturer if someone died in a nasty collision. It's designed to help, but to think it will perform flawlessly in every situation would be silly.

    -Spike
     
  11. KliersLC

    KliersLC

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    That does make sense, but the tire and airbag analogies are a bit of a stretch. A vehicle manufacturer that marketed a vehicel with ABS, and then had a vehicle wreck due to non operation of the ABS without mechanical failure, would be liable. Just as firestone was liable for their tire fiasco, and an airbag company/MFG would be if they purported to have airbags, and an otherwise functioning system did not deploy when needed.

    Thanks for the explanation, it makes more sense now.:bounce:

    Dan
     
  12. knorrena

    knorrena

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    Here is another possiblility. The abs never locks all 4 wheels at once. Because at least one wheel is moving it can sense that the vehicle is moving and the relative speed the wheel is moving. I say relative because there is a slip condition and the wheel may be spinning at a different speed than the vehicle. I suspect that the pulses the abs emits are also staged such that no two wheels on the same axle lock up as well so that there is no fast hard loading/unloading of the diff and drive train.

    Just my crazy mind spinning freely again. No lsd, abs, or lockers up there.

    Karl
     
  13. firetruck41

    firetruck41

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    Here is some info from the New Features Guide:
    abs1.jpg abs2.jpg abs3.jpg
     
  14. Nemo

    Nemo

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    Deceleration sensor. That was the name of it....

    "I don't have my FSM right now so I can't remember what it is call, but it is something like a decelerametor"
     
  15. RavenTai

    RavenTai

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    There also has to be some logic in there, if the ABS system see's you are going 30MPH and then half a millisecond later it reads 0MPH it could safely assume that the tires are locked up.
     
  16. -Spike-

    -Spike-

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    If you'd like to believe that, you're welcome to it. :D

    Systems like this don't 'assume' anything. If there's no differential in wheel speeds, the ABS will do nothing.

    -Spike
     
  17. firetruck41

    firetruck41

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    I don't think he means literally assume. Maybe some logic like, if vehicle speed is greater than 10mph, and speed changes to 0 in less than xxx milliseconds, restart ABS pulse and begin ABS cycle again... or maybe something like, if deceleration sensor shows x deceleration and mph show 0, pulse ABS and restart ABS cycle... :)
     
  18. RavenTai

    RavenTai

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    from a Toyota training manual


    there is a chart for this part where it makes more sense.


    interesting bit about the deceleration sensor