The Federal Trade Commission has rejected a request by the Ford Motor Co. that the agency end its probe of auto industry competitition, but at the same time it has decided to cut back significantly its request for Ford documents.

Ford had asked the commission to quash a July 10, 1978 subpoena for its records, arguing that the information sought would not provide any information useful in evaluating the present and future makeup of the industry, and would cost the company $100 million to provide.

Further, Ford argued, the subpoena was too general and would impose an unreasonable burden on Ford in order to comply.

Ford is the only one of the four major automakers to appeal to the commission; the others have chosen to fight the FTC probe in federal court. They have challenged the agency's authority in bringing the case, which is focusing on the structure and competitive nature of the entire industry.

The commission action deleting about one-half of the documents requested in its subpoena applied only to Ford.

FTC sources close to the probe say they believe the industry's rather rapid change is more, not less, of a reason to continue its probe. They say, for example, that it will be important to see how the industry evolves into what it will become in the 1980s.

In rejecting Ford's request, the commission contended that it was not persuaded that the burden on Ford to cooperate is as large as the company contendes it is.

FTC staffers noted that the level of concentration in the auto industry is extremely high, and that the top three domestic firms, which held 79.6 percent of total U.S. sales in 1977, are the same three that led the industry in 1940.

The primary issue of the probe is whether the present structure and performance of the auto industry are the result of legal competitive practices, or whether they are the product of anticompetitive practices that violate the statutes enforced by the commision.

In a statement released late yesterday, Ford called the FTC's decision to deny Ford's motion "clearly not in the public interest."

"It is disappointing," Ford vice president and general counsel Henry Nolte Jr. said, "That the commission has approved, with some modification, a massive subpoena calling for historical data which have little relevance to the dynamic changes currently taking place in the industry in response to laws and regulations administered by other government agencies."

Nolte called the entire FTC probe "unwarranted," and a "waste of the government's and the company's resources."

Ford said it was exploring possbile court action aimed at terminating the investigation.