ABS doesn't work when it's cold out

I have a 1996 Ford Ranger 4x4, my ABS light stays on all of the time when it is cold out. The brakes lock up when the light is on but when it warms up and the light goes off the brakes work fine. I've taken it to a mechanic and he could not find anything on the computer that was wrong, and he could not clear it. Can anyone help me out on this one?