Volkswagen of America has increased production at its Pennsylvania factory to 1,000 cars a day and will decide in the next three months whether to build a second assembly plant in the United States, President James W. McLernon said.

Demand for small cars has taxed capacity of VW's Westmoreland, Pa., plant "to the limit," forcing the company to begin searching for a second plant site, McLernon said in a year-end statement last week.

He predicted the company will sell 325,000 Volkswagens in the United States in 1980, at least 200,000 of them Rabbits made in Pennsylvania.

VW of America's sales target calls for an 8.3 percent increase in VW sales and an 11 percent jump in imports of Porsche and Audi cars, to 65,000 units.

Opening of a second U.S. plant for VW has been expected since last summer when the Volkswagenwerk AG in Germany announced a 43 billion capital spending program, which it said would place "special emphasis" on North America.

Volkswagen built its first U.S. assembly line in a factory that was started by Chrysler Corp. but abandoned before the machinery was in place.

Auto industry sources say VW is more likely to build its second factory from scratch than to buy an existing plant. Although Chrysler has other facilities that are likely to be closed down and offered for sale, they are old and would be difficult to adapt to VW production.

In addition to its Pennsylvania assembly plant, VW of America has a metal stamping plant in Charleston, W.Va., that turns out body parts for American-built Rabbits.

Both the gasoline and diesel engines for American Rabbits are now made in Germany. Volkswagen officials have said the company's U.S. output would have to be stepped up considerably before the firm could afford to invest in a U.S. engine plant.

The Pennsylvania VW assembly plant began producing about 400 cars a day, added a second shift to double output and recently stepped up production to 1,000 units.

Besides building two- and four-door Rabbits, the Pennsylvania plant now is beginning to turn out a small pickup based on the Rabbit.