A federal judge has ordered the Justice Department to let the public see all the documents that it showed to the defendants in negotiating a settlement of a 1969 antitrust case against the big four auto makers.

While U.S. District Court Judge Thomas F. Hogan's decision last Thursday is unlikely to affect that case, it is likely to influence future antitrust settlements.

Justice Department officials said they are still studying Hogan's opinion and have not yet decided whether to file an appeal. The department had argued that opening its negotiations to public scrutiny could interfere with present and future negotiations for consent degrees.

But Clarence Ditlow, whose Center for Auto Safety carried the Freedom of Information Act suit to the U.S. District Court here, said the decision "will expose any secret dealings that are going on.

"The public spotlight can only result in better settlements," he said.

At stake were 32 documents leading to a settlement in a 1969 antitrust suit charging that General Motors, Ford, Chrysler, American Motors and the Motor Vehicle Manufacturers Association conspired to eliminate competition in the development of anti-smog devices.

The documents--all of which were shown by Justice officials to the defendants--include draft settlement agreements, internal memos, notes discussing the agreements and correspondence between Justice and the defendants about the proposed settlement.

The suit was settled 10 months after it was filed by a consent decree between the automakers and Justice in which there was no admission of wrongdoing, but the defendants agreed to restrictions designed to stop anticompetitive activities.

In his decision, Judge Hogan rejected Justice Department arguments that the documents were exempt from Freedom of Information Act requests either because they are provisional internal memos that were part of the department's decision-making process or that they are investigation records whose disclosure would hurt on-going enforcement activities.

Instead, he went along with the Center for Auto Safety arguments that the documents had lost their confidential character when the Justice Department showed them to the automakers.

"While these documents may at one time have been used for internal advisory purposes and would therefore be protected," the judge said in his 35-page opinion, "when the Department of Justice elected to use them as tools in their negotiations, they lost their internal status and their qualification for exemption."