At a complete stop the wheels lock, so when you skid how does the ABS know the car hasn't come to a complete stop yet? I think I've heard they now have optical sensors, right? But then how did they use to do it?
ABS question
Dec 16, 2003
4 Replies
According to the FSM on mine, it uses the vehicle speed sensor and just stops pulsing the ABS below 4-5 mph.
|At a complete stop the wheels lock, so when you skid how does the ABS |know the car hasn't come to a complete stop yet?
All 4 wheels rarely lock simultaneously, so part of the decision is a reference to relative speed of the other wheels. i.e., if one wheel wants to lock and the others are still turning, the one wheel is controlled to keep it from locking.
Rex in Fort Worth
"We were doing 30 mph, the brakes were applied, and a fraction of a second later we're stopped? Not possible, so we must be skidding. Apply the ABS!"
That, simplistically, is the way the ABS system is 'thinking'. It expects to see a reasonable rate of deceleration and all four wheels turning at the same speed.
Part of the algorithm in the ABS controller includes a pre-programmed vehicle-specific map of known deceleration rates. In a sense, the ABS controller knows how fast the car is capable of decelerating. For example, the controller knows the vehicle is incapable of decelerating from 70 mph to
0 mph in 0.5 seconds (unless you hit a solid barrier in which case it is unlikely there will be enough surviving ABS hardware to make a difference at that point).Roger
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