A U.S. Commerce Department ruling that Japanese companies are dumping car telephones in the United States comes just as Washington has started a concerted push for greater access to the Japanese market for U.S.-produced car phones.

U.S. officials in Tokyo deny there is any link between the two events. But one American suggested that the dumping decision might strengthen the U.S. hand in negotiations on sales of car phones and other radio equipment which began in Tokyo this week.

The Commerce Department issued a preliminary ruling Wednesday that Japanese cellular phones and subassemblies were being sold in the United States at unfairly low prices. A hearing on the case, which was initiated by Motorola, Inc., will be held July 23.

Japanese manufacturers reacted to the ruling with dismay. "Our price is reasonable," said a spokesman for Oki Electric Industries, one of Japan's largest exporters of radio phones to the United States.

One Japanese executive pointed out that Motorola is the only foreign company that is authorized to sell car phones in Japan. "We knew that Motorola wanted to try to make a fuss," he said. Motorola officials in Tokyo could not be reached for comment.

This week's meetings mark the start of phase two of what have been long and often vitriolic talks between the United States and Japan on telecommunications sales here. The United States considers them a barometer of Japan's commitment to open its market and help reduce last year's $37-billion trade surplus with the United States.

Phase one, now largely completed, focused on computer network services and conventional telephone equipment. The door was opened to foreign companies for new sales in these fields by Japan's decision to strip the national phone company, Nippon Telegraph and Telephone, of its monopoly and open the field to competition April 1. But that had limited effect on rules for radio-based equipment like car phones.

The Commerce Department's finding, if upheld and countervailing duties imposed, will be a serious blow to the Japanese industry, which is heavily dependent on exports.

The Japanese are thriving in the U.S. market for car phones. Nine Japanese companies now account for more than half of equipment sales there. But in Washington's view, U.S. sales in Japan are hampered by outmoded laws, regulation and bureaucracy.

Earlier this week, the American negotiating team, led by outgoing Commerce Undersecretary Lionel Olmer, a former Motorola executive, made these proposals to the Japanese side: The government should consider car phone technical systems other than those now being used by NTT. The United States apparently hopes Japan will approve a variation of a type known as AMPS (Advanced Mobile Phone Service), which is the U.S. standard.

* Japan should simplify standards for new equipment and speed up the approval process. Current Japanese standards are more specific than U.S. ones. "American manufacturers can sell only a very small part of their product line in the Japanese market," one American official here said, because they do not meet Japanese standards.

* U.S. suppliers should not be required to reveal certain technical data about their equipment to a Japanese radio testing association. Doing so might compromise proprietary information, U.S. officials say, because the association's members and financial backers are Japanese manufacturers.

* The Japanese government should require its police, the state-owned Japanese National Railways and other government agencies to open their bidding to foreign companies.

Both Japanese and United States officials have said the talks are making progress but are focusing on difficult issues.

The Japanese government has agreed in principle to many of these requests, both sides say. "The Japanese position is basically that we are willing to simplify, but we have not settled on any concrete measures yet," said an official at the Japanese Ministry of Posts and Telecommunications.