P.M. BRIEFING : U.S. Upgrades Seat-Belt Probe

Federal engineers are stepping up an investigation of reported seat belt failures in 1984 and 1985 Ford Mustang automobiles, the government reported today.

The National Highway Traffic Safety Administration said it has upgraded the probe from a preliminary evaluation--an examination of paper work--to an engineering analysis, in which engineers will study the possibly defective parts.

Agency spokesman Tim Hurd said there had been 21 complaints of plastic components in the seat belt buckles breaking so that the belt could not be latched. An estimated 273,500 of the vehicles are on the road.

The agency also announced it is similarly upgrading its investigation of reported cracks in the floors of Ward 54 school buses, manufactured from 1983 to 1990 by the American Transportation Co. Hurd said such cracks had been reported in 11 fleets totaling 103 buses.

The agency also dropped an investigation of reported rollover propensities of Ford Bronco IIs made between 1984 and 1989, saying it found no unreasonable risk.