A consumer group, claiming it has been "flooded with consumer complaints," has asked the Federal Trade Commission to help car owners who have experienced premature failures of certain General Motors automatic transmissions.

The Center for Auto Safety claims it has received in the last month alone more than 239 complaints of failures of Type 200 transmissions, originally designed for the subcompact Chevette, but used in late model down-sized versions of medium-sized cars such as the Chevrolet Malibu and Oldsmobile Cutlass.

In its letter to FTC chairman Michael Pertschuk-the second letter on this case in a month-center director Clarence Ditlow said the 200 transmission problem has led to 'more complaints in a short period of time . . . than any other defect since the Firestone 500 steel-belted radial."

Ditlow contends that GM used the problem transmission in 1976 and later compacts and 1977 and later down-sized intermediates. Complaints to the center of failure of the transmission in such cars now number over 350, he said. Publicity over the first letter to the FTC last month inspired the large number of complaints since that time, he added.

For its part, GM has denied any reports of widespread failures of the automatice 200 transmissions. "All transmissions in GM vehicles are selected by our engineers for use in a particular vehicle," a company statement said. "The weight of the car is matched to the power train, which includes the engine, transmission and differential."