A top-ranking Alexandria police offical has been suspended pending the outcome of allegations that he covered up a 1975 probe of a complaint of sexual misconduct by former city prosecutor William L. Cowhig.

Capt. Norman E. Grimm, 48, head of the department's Patrol Division and one of the highest ranking commanders on the force, was relieved of his duties last week by Police Chief Charles T. Strobel while an internal police investigation continues.

Grimm, a 26-year veteran and former head of the Criminal Investigaion Division, will continue to draw his $31,351 annual salary.

Deputy Chief Arlen Justice, confirming the suspension, said yesterday the allegations charges of misconduct ever made against a top-level police official on the force.

One source close to the investigation said yesterday that Grimm is expected to leave the force permanently. "It will be a question of whether he is fired or asked to resign," the source said.

Strobel, reached at a police seminar in San Diego, Calif., said he suspended Grimm on March 1. He declined to comment further.

Grimm was unavailable for comment yesterday. Robert Osborne, a police polygraph examiner who was reportedly told by Grimm to keep the Cowhig report confidential, refused to speak with a reporter.

Grimm at the time was head of CID and Osborne's immediate superior. Osborne still is on the force.

Grimm's suspension comes three weeks after Arlingon attorney Claude M. Hilton was appointed as special prosecutor to investigate the allegations against Cowhig and the alleged police cover-up.

In 1975, the wife of a defendant in a drug case accused Cowhig of soliciting and act of oral sodomy from her in return for a recommendation of leniency for her husband.

The woman, Sherry Chenault, then 19, said she met Cowhig in his third-floor City Hall office and performed the act.

Although Cowhig reportedly passed a lie detector test, which he asked to take, sources said Cowhig told Osborne in a pretest interview that the sexual activity had occurred.

That information was contained in a confidential police report on the results of the polygraph test, which allegedly was suppressed at Grimm's order.

Cowhig reportedly then told assistant prosecutor Thomas Rawles Jones who was handling the drug case, that he had been cleared and the matter was dropped. Chenault's husband was convicted and did not receive a recommendation for a lighter sentence.

Chief Strobel has said that criminal charges could be brought against those police officials involved. John Holihan, who was police chief at the time, has denied any knowledge of the confidential Cowhig report.

The report resurfaced last July when Osborne informed Strobel of the matter during a secret police investigation of Cowhig's role in connection with the city's bingo scandal.

Cowhig resigned Feb. 23 after being acquitted of two bingo-related felony charges.Special bingo prosecutor Edward J. White agreed to drop a third felony charge in exchange for Cowhig's resignation.

Cowhig vehemently has denied that any sexual activity took place and said Sherry Chenault had tried to seduce him during their meeting in Cowhig's office.

Grimm joined the police force in 1953 and was promoted to lieutenant in 1965. He was named captain in 1969 and three years later became chief of detectives as commander of the Criminal Investigations Division. Named head of the Support Services Division in 1976, Grimm later took over the Patrol Division. He was one of five captains.

Grimm is the son of a former Alexandria policeman, and once served as president of the Policeman's Association.

His career was described by one colleague yesterday as "distinguished" and "above average."