1997 Ford Ranger ABS light is on all the time

The ABS light on the control panel is on. When I turn the key to ignition, the light goes on as it should do, and when the engine starts it goes off, but after fews seconds it goes on again, and stay on all the time when driving, the brakes still working normal. What that can be? Just the ABS sensor? Thank you very much, it is my first time on this web site and a friend told me it works well.