General Motors Corp. and Chrysler Corp. engineers said nearly two years ago that an alleged safety defect in the automatic transmissions of 26 million Ford cars and light trucks appeared to be unique to their competitor's design, contrary to statements by Ford executives.

The separate GM and Chrysler analyses were in internal memos that were released by Ralph Nader yesterday -- two days after the Department of Transportation formally notified Ford of the defect, which purportedly permits the transmissions to slip accidentally from the "park" position into reverse.

Independently, GM and Chrysler, concerned by adverse publicity about the Ford transmissions, undertook to find out if a similar problem could arise with their own units.

In a memo on Sept. 6, 1978, Chrysler executive engineer R. W. Steere set out his reasons for concluding that "the Chrysler transmission should be free of the problem."

Two weeks later, in a 12-page memo stamped "GM. Confidential" on every page, an unidentified GM engineer drew a similar conclusion about his company's Hydra-matics. "Reports of Ford transmissions spontaneously shifting park-to-reverse do not in any way suggest the possibility of Hydramatic transmission doing the same," he wrote.

By contrast, Ford Automotive Safety Director Roger Maugh, denying that there is a safety defect in the Ford transmissions, said that they are basically the same as those installed in millions of other American and imported cars.

Ford's counsel in the transmission case, former DOT secretary William T. Coleman Jr., denounced DOT's National Highway Traffic Safety Administration, telling the New York Times that interim findings of a defect were "shocking and irresponsible."

The agency, which acted after a three-year investigation to which it assigned "highest priority," set a public hearing for July 21. After that it will determine whether to make the interim findings final and to order a recall of the 16 million vehicles built between 1972 and 1980, when the transmissions were redesigned. The other 10 million, built between 1969 and 1972, are legally exempt because of a statute of limitations.

The 23,000 complaints in the NGTSA's files about the Ford transmissions include reports of 6,000 accidents, 98 fatalities and 1,710 injuries.

If a recall ultimately occurs, it would be the largest in history and cost Ford an estimated $200 million to correct the alleged defect -- at a time when the company is said to be facing substantial losses in addition to a 1980 first-quarter deficit of $164 million. s