>From: Walter Fooshee <wfooshee@csi.com>
>It's my understanding, and this is from a write-up in a magazine
> a while back, that GM's ABS is not electronic, but mechanical.
Sorry to disagree, but all GM vehicles use electronic ABS.
>Somehow this is released if the wheel stops, and a valve opens and
>bleeds pressure off the piston. When the wheel starts rolling, the
>differential reapplies and braking resumes.
The valves and the pressures are the result of computer control.
> There is no wheel speed sensor, no computer, no hydraulic servos.
Well, yes, there are wheel speed sensors, a computer, and hydraulic
servos.
> It's cheap, and by the letter of the definition, it is ABS, but
> it's very slow in response compared to electronic systems, easily
> demonstrated by driving just about anyone else's ABS-equipped
> car. It's cheap enough, though, to put on everything they make,
> which GM thinks is a marketing bonanza.
I won't disagree with the idea of GM being cheap! :) I mean GM
building vehicles as inexpensively as they can. Anybody else would,
too, wouldn't they?
All ABS works basically the same. When the computer senses wheel
lock-up is about to take place, it will not let the driver apply
more pressure to that wheel (hold pressure). If the wheel speeds up
then driver control is returned. If the wheel continues toward
lock-up after hold, then the pressure is released by accumulators
and/or pumps (release pressure). Kind of like that all fuel
injection works basically the same; some work better than others.
It can cycle between hold, release, and nothing up to 15 times per
sec. Does it always help in an autocross? No. Is it for experienced
drivers that can threshold brake in a panic situation? No. It's for
drivers that have no idea what to do except turn the key and go.
Also, it doesn't matter if it's a 3-channel or 4-channel, they ALL
operate the rear brakes together. The difference is for front-rear
brake systems (3-channel) and diagonal-split systems (4-channel).
If someone knows of an example where all four wheels are operated
independently, I'd like to know.
Mumbles
BTW, I've taken a few GM ABS classes at the GM Training Center.
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