The ABS computer reads the speeds at each wheel. There are two ways that the ABS functions. While NOT braking, it continually reads the speeds of the wheels. If one wheel speed is much more different than the other 3 (or 2 are different from 2), then, it'll disable the entire system, thinking that one of the ABS sensors is not working. This is so that you don't have ABS coming in while braking if there is a fault in the sensors or other components. In your case, your rear wheels are signalling a slower speed than your front wheels. (In other words your front wheels are spinning much faster than your rear wheels at the same vehicle speed). While braking, the ABS takes inputs from the sensors and adjusts braking accordingly if it senses a wheel has locked.

When the ABS system disables, the light should come on, indicating a fault.

At high speeds, the rotational speed differential between the rear and front wheels are exaccerbated (as well as the fact that if you have rusted teeth, missing teeth, or other problems on the gear wheel) the ABS sensor is not able to read the speeds correctly. This is why some people have the ABS light come on at high speeds. Usually cleaning the sensor and gear wheel will solve those problems.

When you shut off the car, and restart, the ABS is not working until you get over 5 mph or so. Then, it still reads the speeds correctly, but up until that point at which the computer says, "no good, your speed differentials between wheels is too great while not braking, thus there must be something wrong in the speed reading," you still have properly working ABS.

Of note: One thing I've noticed is that if you're spinning your rear wheels at a seriously faster speed than the fronts are movin (say if you're in the snow, going 10-15 mph, and you're speedo says you're doing 30mph), the ABS light does NOT come on. I guess there must be some programming that comes in that says if both rear wheels (or one in the case if you don't have LSD) are spinning faster than the fronts, (WHILE NOT ON THE BRAKES), then it won't register faults.

Try to get another set of rear tires, and see if you still have the problem. If you do, we'll have to trouble shoot some more.

I have different sized tires on front and back (using all 16x8 rims). Up front I have 225/50 and in the rear I have 235/60. Would this cause the ABS computer to error out and stop working? As you may recall, my ABS was not working after driving for a while, but if I reset the ignition, the ABS starts functioning again.

Mucho gracias dudes,
Scottie