Frank M. Johnson Jr., the federal judge chosen by President Carter to be director of the FBI, has been a middle-of-the-roader in cases where police have been charged with abusing their power.

Dealing with cases of alleged police excesses in obtaining confessions or gathering evidence. Johnson has broken no new legal ground and shown no decided preferences, a study of his opinions indicates.

"You can't say he was always pro-police or that he always tried to protect the individual," said Larry Yackle, an associate law professor at the University of Alabama. "He just seems to weigh and balance the facts."

Johnson, whom Carter has nominated to succeed FBI Director Clarence M. Kelly, is best known as the U.S. district judge in Alabama who blazed new legal trails in such areas as race relations and voting rights.

It was he who applied the Supreme Court's 1954 school desegregation decision to city buses. It was he who wrote (before the Supreme Court did) that legislative districts should be apportioned so that everyone's vote would count equally. It was he who ordered integration of parks, libraries, museums, schools - and the University of Alabama, which propelled Gov. George C. Wallace to make his last-ditch stand in the schoolhouse door.

But in questions of whether police have violated individual rights, Johnson has carefully picked his way through juridical thickers - sometimes ruling for the police, sometimes against them.

These rulings are of interest because the FBI itself has been accused of violating individual rights through electronic surveillance and burglaries. It should be noted, however, that most of Johnson's rulings came before the Supreme Court severely restricted police in making searches and obtaining confessions, and that the FBI has an excellent reputation for telling suspects of their rights.

In sentencing convicted criminals, Johnson has been called a hardliner. But Bobby Segal, a Montgomery lawyer who was a law clerk to Johnson in 1971 and 1972, says, "If I were a criminal and I had some legal points - some mitigating factors - on my side I'd want to go before Judge Johnson."

Segal adds, however, "If I deserved a heavy sentence, I wouldn't want to go before him."

A computer check of more than 250 Johnson opinions since 1960 shows wide flutuations in his decisions on major constitutional issues involving police.

For instance, in 1965 he threw out the murder confession of a 17-year-old youth who had not been told of his right to have a lawyer. But the next year Johnson upheld the murder confession of a high school youth with an 1Q of 83 who had not been advised of his right to counsel.

In 1968 the judge said it was legally permissible for state narcotics officers to search a dormitory room at Troy State University. The search led to a student's suspension because the officers found marijuana in the room. But two years later Johnson ruled invalid a police search of a dorm room at the same university that had led to a student's conviction for marijuana possession.

The computer check indicated that since 1960 Johnson has decided four confession cases, all involving trials that occured before the Supreme Court's June, 1966. Miranda v. Arizona decision that a suspect in police custody must be warned of his right to silence and to counsel.

In 1964 Johnson ordered the release of a black man sentenced to death for raping a white woman because the trial judge had not allowed his lawyer to offer evidence - outside the presence of the jury - about the suspect's confession.

The defendant contended police had beaten him, put him in the back of a patrol wagon with a police dog and had threatened to sic the dog on him before he confessed.

In a low-key opinion devoted mostly to citing federal and Alabama precedents. Johnson held that the state court's refusal to hear evidence without the jury violated the defendant's due process rights.

Johnson decided the 1965 case of the 17-year-old confessed to a Mississippi police officer that he had killed an Alabama policeman, partly by relying on a Supreme Court ruling of the previous year.

That decision, Escobedo v. Illinois, required police to advise a person of his or her rights once they began to focus on the person as a suspect. "There is no question in this case that [the defendant]was not represented by counsel." Johnson wrote. "he had not been offered counsel. He had not been informed and was never informed that he could have counsel, and he did not knowingly waive the assistance of counsel."

The judge noted that the 17-year-old was able "to read and write a little." However, the next year Johnson said he was not persuaded that Billy Don Franklin Boulden, a "dull-normal" youth with an IQ of 83, was coerced into confessing to a murder.

Johnson acknowledged in his ruling that shortly after the murder. Boulden was taken to the secene "and retained there in an Alabama highway patrol car in the presence of a number of hostile persons." But he said there was no evident that police protection of him was inadequate.

At the start of the police interrogation, Johnson wrote, "Boulden was informed of his constitutional rights with the exception of his right to counsel." The judge also found that Boulden was given food, permitted to smoke, and "was not mistreated in any way, nor was he threatened or otherwise coerced."

Such treatment during the arrest, which occurred before the Escobedo or Miranda decisions, met the then prevailing standard that all circumstances of an arrest should be considered in determining whether a confession was voluntary.

Johnson concluded that despite Boulden's low IQ the circumstances did not indicate "that his will was in fact overpowered or thathis confessions were other than voluntarily made."

The judge also found in 1966 that the "voluntariness" standard had been met in the murder confession of a 20-year-old man with a seventh-grade education who was not advised of his right to counsel but who was questioned only a short time after being told he did not have to make any statement.

Prof. Yackle noted that "it's very hard to determine voluntariness after the fact. Johnson had as much trouble with that standard as any other judge. That's why the Supreme Court set definite standards in Miranda."

In five cases of whether courts should admit incriminating evidence gained by police in searching a suspect's dwelling. Johnson found in four of them that the searches were reasonable and therefore constitutional.

However, the decided the four cases before the Supreme Court ruled in June, 1969, that in making an arrest police could search, without a warrant, only the person of the suspect and the immediate surroundings, not the next room or the whole house.

In 1964 Johnson upheld the admission of evidence that police obtained from hotel employees who had discovered it while cleaning the room of a robbery suspect 12 hours after the man was arrested.

The next year, while sitting on a panel of the Fifth U.S. Circuit Court of Appeals. Johnson validated a search by police who disgusted as telephone repairmen and armed with an arrest warrant, entered a man's apartment with his permission, arrested him and found two heroin pills and a gun on him.

The fact that the police gained entry by deception did not mean the arrest was illegal, said Johnson, noting the then prevailing rule that searches and seizures "incident to a lawful arrest" were themselves legal.

In 1968 he permitted the admission of evidence gained when two federal agents walked onto the porch of a known moonshiner, spotted four 5-gallon jugs of whisky in his nearby stationwagon and discovered a still in his work.

Johnson reasoned that the agents were lawfully on the premises because they expected the moonshiner to be home and thought he would become an informer for them. Since they had a right to be there, anything in plain view of the agents was liable to seizure, the judge said.

In the Troy State University marijuana seizure cases, Johnson held that the first search of a student's room was legal because university officials, who brought in state agents to conduct the search, had a right to maintain discipline and security. The student was suspended but not charged with a crime.

Johnson held that the second search, however, was "instigated and in the main executed by state police and narcotic bureau officials" for the purpose of making criminal arrests. Therefore, Johnson said, they had the burden of showing they had probable cause to believe a crime was being committed. Since they said they had information only from unidentified informers, they "failed completely" to show they had probable cause, he ruled.

The computer check turned up a 1958 decision involving the FBI, which is or soon will be familiar to each one of the bureau's 8,400 agents.

In that case, Penn v. United States, Johnson ruled for an appeals court panel that Alton Leroy Penn's conviction for violating the Universal Military Training and Service Act was unconstitutional because it was based on an FBI agent's hearsay testimony.

Penn, a Florida tomato picker, was arrested by the agents "in a jungle type area adjoining the tomato field." They charged him with failing to report for a physical exam, failing to submit to induction, and failing to keep his local draft board advised of his address.

Agent Harold K. Parson had testified in Penn's trial that when he and another FBI man arrived at the tomato field near the Everglades, they were told by other workers that Penn had gone into the jungle-type area.

Johnson wrote that it was not clear how long Penn had been gone before the agents arrived that neither of them had seen him leave the tomato field, and that parson's initial testimony that Penn "had fled" the field "was rank hearsay." The judge added, "The law clearly recognizes that juries may consider flight and draw an inference of guilt there from."

Because "Penn was substantially prejudiced by the admission of this testimony." Johnson wrote, his conviction on each of the three counts must be reversed.