Ford Motor Co. isn't likely to ask William Barr to testify on the company's behalf again.

The veteran Naval Academy engineering instructor was an expert witness for the automaker in 1978 during two trials involving charges that people were injured when the automatic transmissions in Ford vehicles slipped from "park" to "reverse" because of design flaws.

Barr said he testified that Ford's transmission setup was basically the same as the other domestic and foreign automakers' and that it was the state of the art at that time.

Later that year, however, something happened that changed his mind. Now, he believes not only that a design flaw in automatic transmissions can permit them to slip from "park" to "reverse" under certain circumstances, but that he has the solution and that the automakers, therefore, should have seen the flaw and corrected it.

Barr has applied for three patents on ways of solving the problem in pre-1980 transmissions, and he expects a decision on one of them within the next six months. He hopes that Ford or another automaker will be interested.

The first transmission-related case in which he testified involved a Falls Church resident who claimed that her 1973 Lincoln Continental slipped from "park" to "reverse" and ran over her left leg while she was loading groceries into the car outside a Seven Corners shopping center in 1976. She was awarded $16,500 for her injuries and $350,000 settlement in a related suit against Ford.

Other transmission-slippage reports generated a three-year National Highway Traffic Safety Administration investigation that tallied 6,000 such accidents involving 98 fatalities and 1,710 injuries. The agency reached a settlement with Ford under which the automaker agreed to mail out millions of dashboard stickers to motorists warning them that "unexpected and possibly sudden vehicle movement may occur" if the vehicle is not parked properly.

Barr changed his mind about the controversy when he was flipping through the maintenance manual for his wife's Buick and happened to spot a cutaway view of an automatic transmission.

When a motorist moves the gear shift lever of a car with automatic transmission into "park" position, a part attached to the lever is supposed to slip into a detent, or slot, to hold it in position. This movement also is supposed to lock a protrusion in the transmission between the teeth of a gear to prevent the transmission from moving the car's wheels.

Barr recalls that he noticed a spring in the cutaway view, and it "immediately became apparent why it was there and that the spring obviously had to be compressed at times." The spring was supposed to make sure that the part locks between the teeth,, but "when compressed, it exerted a force which urged the whole shifting mechanism toward reverse position," according to Barr.

He believes that if the gear shift lever is not in the "park" slot and if the part in the transmission is atop one of the teeth of the gear instead of between them, the spring can force the parts in the wrong direction and the car can slip into reverse.

"I thought, well now, I might as well do something" with this insight, Barr recalls, but he lost interest in the problem when his wife became critically ill and he only resumed working on it late in 1980. He progressed from theory to drawings to parts made for him by a local machine shop and installed on parts of old automatic transmissions.

The parts basically reverse the forces that could cause the transmission to slip, according to Barr. He believes that they could be added to existing transmissions for approximately $10, once they are mass-produced. He has not, however, installed the parts on a working automatic transmission and tested them. Instead, he can hand-turn various parts of the partly disassembled transmissions to show how his additions work.

Besides the three patents on the transmission modifications, Barr has applied for a patent on a safety change to the hitching mechanism of a tractor trailer. If any of the four is granted, it will mark his first success. Earlier he attempted to patent a toothpaste cap, an auto battery clamp and improvements in farm machinery.

He received a bachelor of arts degree from St. John's College in Annapolis in 1948, a bachelor of science in mechanical engineering from the University of Virginia in 1948 and a master of science in mechanical engineering from Virginia Polytechnic Institute in 1951. He was a member of that school's faculty from 1948 through 1951.

Barr also has had an engineering consulting firm, Annapolis Engineering Associates, since 1968. He lives on Gibson Island, Md.

The engineering instructor has been an expert witness in about 80 accident cases since 1966, appearing for the defendant in slightly more than half of them. In recent months, however, he has become involved in three auto-transmission cases on the plaintiff's side.