The director of the Department of Transportation's Office of Civil Rights has resigned in the midst of an investigation into his efforts to solicit funds from DOT contractors on behalf of a private organization he headed.

Wesley A. Plummer left his $63,800-a-year job March 18, about six weeks afterr investigators discovered he had used government secretaries and postage to mail letters seeking contributions for the National Consortium of Civil Rights Transportation Officials. Plummer founded the consortium while serving at DOT.

Plummer is the second administration civil rights official to leave office under fire this year. In February, Isidoro Rodriguez was fired as minority affairs director of the Agriculture Department after writing a memo proposing to junk minority hiring goals and relax civil rights enforcement policies.

Plummer's letters, sent out over his name on department stationery in early January, asked for 5,000 contributions from more than two dozen transportation firms, including several that receive DOT contracts.

In some of his mailings, Plummer listed Drew Lewis, then transportation secretary, as an honorary co-chairman of the consortium, apparently without Lewis' knowledge.

"Drew hadn't known anything about this organization at all," said John Fowler, former DOT general counsel who now works for Lewis at Warner-Amex Cable Communications Inc. in New York.

When the mailings were discovered, Fowler ordered Plummer to pay back the government $293 for the cost of the mailings and to send out "clarifying letters" explaining that Lewis had nothing to do with the organization.

The mailings, however, triggered an investigation by the inspector general's office that has since been broadened to include allegationa that Plummer misused government travel funds and awarded contracts to friends and political supporters. A department official declined comment last week on the continuing investigation.

Plummer, who had earlier described the probe as a "character assassination," said in an interview this week that "we used some of our staff who typed the letters."

"In restrospect . . . I would probably do it again," he said. "I saw it as a transportation function. I saw it as a function of my office."

A department spokesman said the activities raised questions about potential conflicts of interest if a government official were affiliated with an organization whose members are regulated by the official's department.

Meanwhile, Plummer resigned last week as president of the consortium, saying he did not believe government post-employment restrictions would allow him to be associated with an organization involved in "public sector transportation interests."

The consortium was founded by Plummer last December to "promote effective civil rights programs in the transportation industry" and to help minority and women-owned businesses get government contracts, according to its literature.

Through the initial mailings using Lewis' name, Plummer was able to attract executives from several nationwide firms, including Greyhound, Westinghouse Electric and Motorola, to serve on the board of directors.

Plummer said this week that he talked to Lewis about lending his name to the group, but added, "Drew is a friend and I would not dispute his saying he did not know. I'm sure there was some misunderstanding."

The accusations against him came from within his staff, Plummer said, because "I was the most outspoken black spokesman for the Reagan administration."

The probe into Plummer's activities is the latest chapter in a long history of turmoil with the DOT civil rights office, a 26-person, $1.2-million-a-year unit charged with investigating discrimination complaints on DOT-funded projects and promotion minority business involvement.

Five years ago, the former male deputy director of the office was indicted for punching and kicking the female director. He was acquitted after claiming self-defense.

Critics say staff dissention increased with the 1981 appointment of Plummer, 35, a former defensive back for the Denver Broncos and Republican member of the Harrisburg, Pa., City Council.

Plummer started his job by telling the staff that "he was given the authority to get rid of anybody he didn't want and that heads would roll. . . . Those were his words, 'heads would roll,'" said Mildred W. Goodman, Plummer's former deputy director, who is now the president of Blacks in Government, an organization of workers.

Plummer, an ordained minister who often held lunchtime prayer sessions in DOT headquarters, expressed open disdain for mainstream civil rights organizations, which he viewed as hostile to the voluntary approach of the Reagan administration, Goodman said.

"He made no bones about the fact that he didn't feel civil rights were necessary," she said.

Plummer, however, vigorously defended his record, saying he was able to accomplish far more for minorities through negotiation with industry rather than "confrontation" on affirmative action goals. "I feel very positive about the job we've done," he said.