How do it Know? (1 Viewer)

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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:
 
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
 
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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
 
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


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

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

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.
 
My ABS works on ice and snow at speeds well below 10-12 mph
Guess it's done differently on an 80 then.
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....
Yes, very possible.
 
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.
 
I don't know of an accelerometer for the ABS system.

I never heard there was any besides for the air bags.
 
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 :)
 
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 :)

It's for the airbags.

-Spike
 
... 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.

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
 
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

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

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
 
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
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... :)
 
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



from a Toyota training manual


In a panic braking situation, the wheel speed sensors detect any
sudden changes in wheel speed.
The ABS ECU calculates the
rotational speed of the wheels and the change in their speed, then
calculates the vehicle speed. The ECU then judges the slip ratio of each
wheel and instructs the actuator to provide the optimum braking
pressure to each wheel.

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

The ECU continuously receives wheel speed signals from the speed
sensors and deceleration sensor. By calculating the speed and
deceleration of each wheel, the ECU estimates the vehicle speed. When
the brake pedal is depressed, the hydraulic pressure in each disc brake
cylinder begins to increase and the wheel speed begins to decrease. If
any of the wheels are near a lock-up condition the ECU goes into
pressure hold mode to stop the increase of hydraulic pressure in the
disc brake cylinder of that wheel.
SECTION A
The ECU sets the solenoid valves to the pressure reduction mode based
on wheel speed, thus reducing the hydraulic pressure in the disc brake
cylinder.
After the pressure drops, the ECU switches the solenoid valves to the
Holding Mode then monitors the change in wheel speed.
If the ECU judges that the hydraulic pressure needs to be reduced
further, it will return to reduction mode.
SECTION B
When the hydraulic pressure inside the disc brake cylinder decreases
(section A), the hydraulic pressure applied to the wheel falls. This
allows the wheel that was locking up to speed up. However, if the
hydraulic pressure is held down, the braking force acting on the wheel
will become too low. To prevent this, the ECU sets the solenoid valves
to the pressure increase mode and holding mode alternately as the
wheel which was locking up, recovers speed.
SECTION C
As the hydraulic pressure is gradually increased in the brake cylinder
by the ECU actuator (section B), the wheel tends to lock up again. In
response, the ECU again switches the solenoid valves to the pressure
reduction mode to reduce the hydraulic pressure inside the disc brake
cylinder.
SECTION D
Since the hydraulic pressure in the brake cylinder is decreased again
(section C), the ECU starts to increase the pressure again as in section B.
The cycle of Hold, Reduce and Increase is repeated many times until
the wheels are no longer outside the 30% slip ratio.


interesting bit about the deceleration sensor

The deceleration sensor is used on some systems to provide input to the
ABS ECU about the vehicle's rate of deceleration to improve braking
performance. In a typical ABS system, the ECU compares individual
speed sensors to determine the speed of the vehicle and rate of wheel
deceleration. The deceleration sensor is used on all full-time 4WD
vehicles equipped with ABS to determine deceleration, as the front and
rear axles are connected through the transfer case and present unique
braking characteristics.
 

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