The Montgomery County Council voted yesterday to conduct its own investigation of allegations that Planning Board nominee John P. Hewitt failed to dismantle a racially segregated work area at a county facility while he was parks director.

The move delaying action on Hewitt's nomination to the influential board was a setback for County Executive Sidney Kramer, who chose him for the post and has stood behind the Silver Spring Republican during the three-week-old controversy.

Kramer had rejected calls by both the Montgomery County NAACP and the county Civic Federation, Montgomery's largest citizens alliance, to launch an independent investigation of charges that a segregated dressing area existed until 1974 at the Meadowbrook maintenance yard in Chevy Chase.

Hewitt, parks director from 1957 to 1971, has steadfastly denied that he knew of the existence of the area. He has told the council that he learned of the situation, subject of a 1974 complaint to the federal Equal Employment Opportunity Commission, only in the last two months.

Hewitt, reached yesterday at his Silver Spring real estate office, said he welcomed the investigation because he is confident of the outcome and "it might satisfy a lot of people." He said he had no intention of withdrawing from consideration for the $12,600-a-year post because "it is now a matter of principle."

Hewitt's denial that he knew the segregated facility existed was cited by council member Isiah Leggett as he successfully urged the council not to act until the charges were resolved. Leggett, the only black council member, pointed to a letter received by the council this week from Francis G. Mathews, a parks employe since 1968 who is black and one of those who filed the complaint.

Mathews said he "personally saw" Hewitt in the changing room "many times" and it would have been impossible for him not to know that there were segregated areas.

Leggett said he was troubled by that contradiction and the council had an obligation to determine just what happened. Council member Bruce Adams, who has been in the forefront of council opposition to Hewitt, also said the letter raises new questions about Hewitt's credibility.

Roscoe R. Nix, president of the county NAACP, which raised the allegations and urged Kramer to withdraw Hewitt's nomination, said "the council acquitted itself well today." He said, however, that he was distressed that Council President Rose Crenca and Vice President Michael L. Subin had opposed delaying the vote.

Crenca said she felt council members had had enough time and information to make up their minds. Subin said that although he had concerns, he didn't want to delay the matter.

Montgomery County prides itself on its civil rights reputation, Nix said, adding that it was time to prove it. "If Mr. Hewitt is not guilty, the investigation will show it," he said.

Kramer, in a personal appeal to the council, had urged quick action in confirming Hewitt and Olney civic activist Carol G. Henry in what are the first Planning Board appointments by the county executive. Henry, a Democrat, received the unanimous confirmation of the council yesterday.

The council scheduled a special meeting July 23 to act on the appointment.