Drivers of an estimated 9 million Ford Motor Co. cars and trucks with automatic transmissions will be told to "be careful" by a federal agency today.

The National Highway Traffic Safety Administration, which has been considering recalling the vehicles - built between 1973 and 1978 - has indicated that an apparent transmission defect might have led to six deaths and 39 injuries in at least 114 accidents over the past five years.

Today's announcement, warning consumers that the transmissions can have a defect causing them to slip out of park and into reverse, will be both a consumer alert and a request for consumers who have had such problems to report them to NHTSA.

NHTSA sources said the agency was close to ordering a recall on the cars - it would be the largest recall in history - but because the agency is convinced that Ford would challenge the action in court, it wants to build up as strong a case as possible about the alleged defect.

"Because there are deaths and accidents and injuries, and they all seem to have a relationship to the automatic transmission it appears that there is an alleged defect," said NHTSA official Lynn Bradford earlier this year.

Sources at the agency say government tests have shown that in some cases Ford cars with engines running, can jump from park to reverse and move even if the car has the emergency brake on. "The shift overrides the emergency brake in some cases," the source said.

One consumer group, the Center for Auto Safety, chided NHTSA last month for taking so long to reach a decision. That investigation began 10 months ago.

"To delay a defect finding any longer will only lead to more deaths and serious injuries in these vehicles," said Clarence Ditlow, head of the consumer group.

Although Ditlow originally charged that every Ford Motor Co. car manufactured with either the C-6 or FMX automatic transmission since 1966 - some 19 million vehicles - NHTSA has since limited its investigation to vehicles manufactured after 1973.

NHTSA staffers have briefed administrator Joan Claybrook at least twice on their investigation, but sources in the agency say she has sent the investigators "back to the drawing board" until they can come up with "an airtight case that will hold up in court."

Earlier this year, Ford said that its own investigation of the problem "has not revealed that a defect exists in either of the transmissions in question."