DETROIT (AP) _ Ford Motor Co. will begin equipping its cars and trucks with a new computer-driven system that adjusts how air bags expand to better protect the driver and front-seat passenger from injury.

The system involves sensors that instantly determine the severity of a frontal crash, consider the front occupants' seating position and weight, and determine if they are belted. It then adjusts the explosive power of the air bags and tightness of the seat belts.

Ford planned to announce the system today at the North American International Auto Show. Elements of the system will begin appearing on Ford cars and light trucks beginning with year 2000 models that will arrive in showrooms later this year.

The Advanced Restraints System first will be the 2000 Taurus family sedan. Within three years, all elements of the system should be on all new Ford vehicles sold in the United States.

The move addresses concerns about the threat air bags pose to unbelted occupants and small children and adults in minor accidents. Though air bags have saved thousands of lives, they also have been blamed in the deaths of dozens of people, mostly children and small women.

``We are moving beyond mechanical devices for occupant safety and are now adding to the mix a fully integrated, computer-driven system that `thinks' about and responds to different conditions,'' said Neil Ressler, chief technical officer.

The system will work with two-stage air bags that will expand at full force in higher speed crashes and more slowly in lower-speed wrecks. They are designed not to inflate in minor fender-benders. If the driver is closer to the air bag, as determined by seat position, the air bag will inflate more slowly. It also will adjust the front passenger's air bag depending on the passenger's weight _ more so for the heavy, less for small children.

The seat weight sensor will not initially be included in the 2000 Taurus, however.

In severe crashes, the system will tighten the seat belts initially, but as the force on the occupant's chest increases a sensor will give the belt some slack to avoid crushing the chest.

``Every accident is as unique as the individuals involved,'' said Helen Petrauskas, vice president of environmental and safety engineering. ``Bearing that in mind, Ford has worked hard to develop a safety system that can provide people of different sizes, facing various types of frontal crash situations a restraints system that automatically tailors itself based on certain crash conditions.''

Ford plans to offer the system at no extra cost.