Ending months of industry speculation, Volkswagen has disclosed that it plans to build a second automobile assembly plant in the United States.

James McClernon, president of Volkswagen of America Inc., said Wednesday the plant will be modeled after the factory the West German automaker opened in April 1978 just south of Pittsburgh. That factory manufactures the gasoline-powered Rabbit sold in the United States.

Interviewed at his Warren, Mich., office, McClernon said a study is under way to pick a site for the new factory and he added that there are about a dozen states still under consideration. He said the site would be chosen within about six months, and he expects the factory to begin building cars in the fall of 1982. He would not say which models would be built there, but hinted that it might not be the Rabbit, the company's best-selling auto.

McClernon also confirmed that VW would build an engine manufacturing plant in either Canada or in the United States that would provide engines for cars built in the United States. He said it probably will not be part of the newest assembly facility but that no final decision has been made.

Volkswagen, credited with starting the small car boom in America when it began importing its bettle model in 1949, is the only foreign automaker with an assembly plant in the United States. Work on the Westmoreland, Pa., factory began in late 1976 after Volkswagen purchased the new but unused factory shell from Chrysler Corp. VW has invested about $350 million in the factory thus far, McClernon said, and the plant employs about 4,300 workers.

He said the new factory will be about the same size, and will initially build some 800 vehicles a day. Production at the Pennsylvania facility began at 800 cars a day and is soon to be increased to 1,040 per day in the coming model year. McClernon said VM is looking at existing assembly plants, including those owned by Chrysler Corp., as possible VM factories, but would not provide specific details until a site is chosen.

The West German automaker said it intends to spend the equivalent of $3.1 billion over the next three years expanding its factories around the world and building new ones. Although figures have not been disclosed, the firm said it plans to give "special attention" to the North American market.