U.S. auto parts makers, complaining about being shut out of new assembly lines for Japanese cars in this country, mounted a campaign in Tokyo last week to get Japanese car makers to buy more of their products.

Their major goal is to win supply parts orders for the growing number of Japanese cars being assembled in the United States, estimated to reach between 1 million and 1.5 million by 1990.

The Japanese firms that are the traditional suppliers have the domestic market sewed up because of their long-term relationships with Japanese automobile companies and are now following the manufacturers to this country.

"We don't believe we are being given an equal opportunity to compete with Japanese auto suppliers," Assistant Secretary of Commerce H. P. Goldfield said in an interview here yesterday.

Goldfield led the U.S. delegation of 18 executives from a dozen companies. He said the U.S. auto parts makers want a chance to compete for sales in the United States, Japan and in other countries where Japanese cars are sold.

The Commerce Department said U.S. auto parts account for no more than 1 percent of cars produced in Japan and 20 percent of cars made in Japanese assembly plants in the United States. U.S. officials would like auto parts to be included in a new round of intensive market-opening trade talks with Japan. But administration sources said the Japanese industry is fighting it.

The trade deficit with Japan in cars and auto parts was $24 billion last year, about half of the entire deficit with Japan.

Frustrated for years by a lack of progress in winning sales from Japanese car makers, industry officials feel 1986 might provide their big chance -- especially if the Reagan administration keeps pressing the issue.

"U.S. government clout opens the door for aggressively sold U.S. world class products. Ultimately, we expect solid gains in all markets where Japanese cars are built or sold," said Julian Morris, president of the Automotive Parts and Accessories Association.

"This is going to be a good year for auto parts," said Richard Waterman, director of international marketing for Champion Spark Plugs. "I don't know how good, but I know from my meetings with Japanese during trade shows that they do have a great deal of pressure on them from their government and corporations" to buy American-made parts and accessories, he said.

He said it took "a great many years" for Champion to sell its products as original equipment on made-in-Japan cars. They succeeded, though, and U.S.-made plugs are being installed in engines produced in Japan. Some of these engines might be shipped to the United States for use in Japanese cars assembled here.

"The success we have now is primarily a result of sticking with it," Waterman said. "We are hopeful of increasing our share if the dollar makes our pricing more attractive."

The U.S. industry has thought government negotiations had secured it a niche in Japan before, only to see its hopes dashed. In 1980, the industry thought it had a commitment from Japan to buy $300 million worth of American parts in 1981, with yearly increases to follow.

But after the Reagan administration pressured Japan to limit its auto sales in the United States, the Japanese reneged on their pledge, industry representatives and U.S. trade officials said.

"The Japanese seemed more receptive this time," Goldfield said. "We are not going to let up our pressure." According to Goldfield, the Japanese didn't raise complaints about the quality of American-made parts, as they did in 1980. This time, Goldfield said he was prepared for the Japanese complaints; he had with him a long list of U.S. recall orders for quality and safety defects in Japanese-made cars.