Hey ABSters,

>>The ABS uses various methods to effectively make you
>>take your foot off the brake for very short periods.
>>One way is to pulse a very high pressure onto the line
>>for a short time which pushes your foot off the pedal.

>I don't think that's how it's done. If you put very high pressure on the
>line you will also apply this pressure to the brakes, and they would
>brake even harder.
>But that's not the issue here. I'm not interested
>in the mechanic part of this,

Just in case someone else might be.
The push you feel is the pressure that is being released from the
wheel that is at a different speed than the others and is being
reintrodeced upstream of the wheels fluid cutoff device, which is
on the same side of the pressure line as your foot is. There is
most likely a cutoff solinoid on the line to each wheel and is
closed when pressure is about to be released so pressure can
be held on the other wheels.


>what I'm interested in is the electronic part.
>How will the ABS system sense speed when the wheels
>are locked up?

>I'm not sure it does.

I think you are right, if all the wheels are locked up then the
vechile is, probably, stationary, because if the ABS is doing its
job any one wheel will not ever be locked up alone. Sure it might
chirp on dry tarseal but not because its locked up but because its
doing 50 kmh for a split second and the car is doing 100 kmh. To
my knowledge, up until 2 years back I was a Toyota mechanic,
no Toyota has any other speed sensors for the BRAKE SYSTEM
other than at the wheels to tell rotating speed and maybe it might
use the Vechile Speed Sensor which is usually either at the gear
box end of the speedo cable/pickup or inside the car in the
speedo dash assembly. No accelerometer used in the brake
system, yes they are used else where.


>And if this is correct, it is possible that the system could be fooled.
>If a person brakes in panic and the road is *very* slippery the ABS will
>start functioning. The speed is reduced and falls below the limit of
>which the ABS will deactivate (and the wheels are locked).

I think the speed limit for ABS cutout is very close to 0 kmh. If it
was disabled at say 5 kmh then as soon as the car dropped below
this, under heavy braking, the wheels would lock, and I haven't seen
an ABS car leave black marks on the road or heard a wheel skid
under heavy braking just as it comes to a stop.

>By coincidence the road starts to decline more at this point, and the
>speed of the car increases again. But since the ABS was deactivated and
>the wheels locked it will not start functioning again until the brake
>is released manually so that the wheels get rolling again.

You are probable right here too, if I understood what you said.
It would be the same as if you hooked a tow rope up to the
car in front and stood hard on the brakes. If the tow car could
pull you forward I'd expect your wheels to stay locked until the
brakes were released then on reapplying them the ABS should
start functioning again.

>Ok, I know this is a bit weird, but I'm interested in knowing if this
>is true or not.

[sniped from another posting]

>Here's the scenario:
>You're off road, climbing a steep hill. You lose traction and the wheels
>start to spin like mad. At this point, the ABS controller thinks you're
>travelling quite fast.
>You slam your foot on the brakes, and the wheels stop spinning. The ABS
>controller thinks 'Hey up, the wheels have locked', and releases the
>brakes again. You go careering backwards down the hill.

I can't work out how they got around that problem.

Dean.

(-;     Forget about World Peace . . . Visualize Using Your Turn Signal!

;-)