The D.C. Board of Elections and Ethics announced last night that it has found no evidence of wrongdoing in the financing of Mayor Walter E. Washington's 1974 election campaign.

The board said it had ended its 5-months "preliminary inquiry" into allegations, reported by The Washington Post, that secret cash payments were made during Washington's 1974 campaign and that the payments were not reported, as required by law. The investigation "failed to produce substantial probative evidence" of wrongdoing, the board said.

Two of the board's three members, Jeanus Parks and Grayson McGuire, voted to dismiss the investigation, according to Winfred R. Mundle, the board's general counsel. Board chairwoman Shari B. Kharasch abstained "That's the end of it," Mundle siad in announcing the decision last night.

The Washington Post reported last December that, according to several persons in the mayor's 1974 campaign, cash was secretly used to pay salaries for campaign workers and not reported, as legally required.

The sources said, according to The Post's report, that at least $1,400 was given in $100 bills to campaign coordinator John Dean by Julian R. Dugas, a close friend of the mayor. Dean then used the cash to pay the salaries of campaing workers, The Post reported.

The cash payments, The Post reported, were made at a line when managers of the mayor's well-financed election campaign feared they might violate a $200,000 campaign spending limit imposed by Congress. Knowledgeable sources said that, if they had been properly reported, the cash payments combined with unpaid debts incurred in the mayor's campaign would have raised his campaign spending above the $200,000 limit.

In addition, the city's election law, enacted by Congress, required that all campaign expenditures of $10 or more be reported and itemized in regular campaign and reports to the Board of Election and Ethics.

In an interview in December, the Mayor said he had to knowledge of use of a secret cash fund in his campaign. "I know nothing about a fund. I didn't have one. Never had one. Never knew about one. Now there's pretty straight," he said.

Asked early last evening for comment on the board's decision and her abstention, election board chairman Kharasch said she would prepare a statement for release later. Mundle, the board's general counsel, said the board had failed to receive testimony from any person who claimed to have witnessed secret campaign cash payments in the mayor's 1974 campaign.

The Post reported in February that, according to knowledgeable sources, city investigators had completed interviews with several key workers in the mayor's 1974 campaign without asking about some of the alleged secret cash payments thought to be the focus of the investigation.

The sources said, according to The Post's report, that some persons interviewed by city campaign finance director Carl McIntyre had first-hand knowledge of the alleged transacitons and were prepared to discuss them. The only questions asked were general ones, The Post reported.