1999 F 250 ABS light stays on?

On ANY vehicle where the ABS light stays on, it means a computer believes there is a problem with the anti-lock brake system.

Vehicles with ABS have sensors at the wheels to determine if they are rolling or stopped. If a wheel is stopped or "locked up" while the vehicle is moving, that means it's skidding. The sensor will signal the computer to reduce braking pressure on that wheel, ending the skid. In practice, this happens many times per second, resulting in a "pulsing" sensation at the brake pedal.

If one or more sensors isn't providing a proper signal (whether or not the vehicle is moving), the ABS computer will trigger the warning light. A problem with the computer itself (including lack of power) can also trigger the warning, and the first thing to check is whether the ABS circuit has a blown fuse.

However, the most common cause is either a bad sensor or damaged wiring from the sensor to the computer. This can be checked relatively easily by removing wheels and examining the sensors and wiring at each wheel. If the wiring is cracked or frayed, or the sensor is cracked or damaged, repairing the wiring or replacing the sensor will probably fix the problem.

If those checks don't detect the problem, the vehicle probably will have to be plugged into a dealership's computer to determine the exact cause.

While it's not a good idea to drive with non-working ABS, the warning light does NOT necessarily indicate a problem with the overall brake system -- it just means the anti-lock system may not be working. Otherwise, the brakes should operate normally, unless the brake warning light is on as well.