The Civil Aeronautics Board by a 41 vote, has ordered Allegheny Airlines to provide air service to Sullivan County, N.Y., for a three-year period.

Allegheny didn't want to serve the county - the area of the Catskill Mountains - but the board concluded the area was entitled to temporary test of its traffic generating capability and that the board has the power to force the carrier to do it.

The board said it hoped Allegheny would "enthusiastically" provide reliable service during the test period. In the past, the board has generally been liberal in not forcing carriers to continue serving pints they didn't want to serve.

Former chairman John E. Robson was the lone dissenter to the board's decision: the vote was taken Friday on his last day. He said the prospects for profitable operations by Allegheny is poor, noting that the little traffic generated by jet air service offered by Mohawk in 1969 and 1970 failed to support the board's conclusion that compulsory jet service was needed or would be successful.

CAB member II Tenny Johnson concurred generally in the board's action but said he would have limited the test period to two years.