Arlington County Board Chairman Dorothy T. Grotos told County Manager W. Vernon Ford yesterday he must work more cooperatively with the newly installed Republican majority on the board or quit.

Gortos' move came at the end of a 10-hour board meeting and appeared to catch Ford and the four other board members by surprise.

"We constantly find (Ford) in an adversary role than that of a helpful lietenant," Grotos said, reading from a three-page statement. "It is imperative that . . .there be complete cooperation between the county manager and the board. (Ford) should always defer to the will of the majority on any given issue, or resign."

Immediately after Grotos' statement, the board went into a 30-minute, closed-door session without Ford, a 20-year employe of Arlington and its county manager since 1976.

Grotos said after the meeting that she did not urge her colleagues to fire Ford, who earns $52,000 a year.

"Everyone was upset that they did't know what I was going to do before," she said. "I hope I got some of their attention. I think the county manager should be above politics and if he can't be, he should resign."

Grotos said she was disturbed by remarks attributed to Ford in a local newspaper. In the article, Ford was quoted as saying that the newly installed Repubiican-backed majority that Grotos heads had failed to establish clear priorities for the county, other than to promise reductions in the property tax rate.

For months county staff members have complained privately that although the board ordered a selective hiring freeze, it failed to specify where cuts should be made and pledged that employes would not be laid off.

In an interview yesterday, Ford, 48, repeated that statement. "The board had not taken explicit actions in certain cases," he said.

Ford said he had received other job offers, but said he had no immediate plans to resign. "I'd probably be better off without this job," he said. "I could do better in the private sector, certainly financially, and in terms of the time commitment." CAPTION: Picture, W. VERNON FORD . . . 20-year employe