A new Arlington County manager, to replace the ousted W. Vernon Ford, probably will not be selected until May, according to Stephen H. Detwiler, chairman of the Arlington County Board.

But the board is moving closer to that decision and this week may select a consulting firm--expected to be Pequod Associates of Boston--to help screen the anticipated hundreds of applicants for the $60,000-plus position, Detwiler said.

The board is a month behind its original timetable, he said, because members have had too little time to get together to discuss the results of their October meeting with representatives of the Wharton School of Finance and Commerce of the University of Pennsylvania.

At that meeting in Philadelphia, attended also by a consultant from Pequod Associates, board members set out the responsibilities of the job and the personal qualities they want in the new manager. A report on the results of the board's sessions with Wharton experts should be made public soon, Detwiler said.

"The job description (for the new manager) is virtually done," Detwiler said. "But there are still some refinements to come before we have public hearings" on the job description and selection process.

The Pequod Associates' proposal on how it could assist in the manager selection process is being "fine-tuned," Detwiler said, on such points as the time to be spent on the nationwide search for candidates, the screening process and the assistance given the board with the final candidate interviews.

Detwiler has said he thinks the board will interview the final eight or 10 applicants.